Transition Metal Catalyzed Hiyama Cross-Coupling: Recent Methodology Developments and Synthetic Applications

Hiyama cross-coupling is a versatile reaction in synthetic organic chemistry for the construction of carbon–carbon bonds. It involves the coupling of organosilicons with organic halides using transition metal catalysts in good yields and high enantioselectivities. In recent years, hectic progress has been made by researchers toward the synthesis of diversified natural products and pharmaceutical drugs using the Hiyama coupling reaction. This review emphasizes the recent synthetic developments and applications of Hiyama cross-coupling.

Keeping the wide applications of the Hiyama coupling reaction in mind, here we have provided a comprehensive compilation of recent methodologies of the Hiyama coupling reaction.

Mechanistic Consideration
The mechanism of the Hiyama coupling reaction was proposed by Foubelo et al. in detail. The initial step of the mechanism involves oxidative addition of halide to Pd metal, resultantly converting palladium(0) to palladium(II). The transmetalation step leads to the splitting of the C-Si bond, and the cycle moves towards the construction of a new bond between carbon and palladium. In reductive elimination, the C-C bond is established, and a zero-valent state of palladium is achieved to restart a new cycle (Scheme 1) [49].
A facile and effective protocol for the synthesis of symmetrically and asymmetrically functionalized (E)-1,2-diarylethenes by sequential one-pot Hiyama-Heck reactions was disclosed by Gordillo et al. The reaction proceeded in aqueous and ligand-free conditions. The symmetric (E)-1,2-diarylethene 12 was synthesized via one-pot Hiyama vinylation and Heck arylation of triethoxy(vinyl)silane 10 and aryl bromide 11 using Pd(OAc)2 (0.3 mol%) and NaOH in 98% yield with selectivities (100:0), respectively (Scheme 3). For the synthesis of asymmetric (E)-1,2-diarylethene via one-pot Hiyama vinylation reaction, a different strateg A facile and effective protocol for the synthesis of symmetrically and asymmetrically functionalized (E)-1,2-diarylethenes by sequential one-pot Hiyama-Heck reactions was disclosed by Gordillo et al. The reaction proceeded in aqueous and ligand-free conditions. The symmetric (E)-1,2-diarylethene 12 was synthesized via one-pot Hiyama vinylation and Heck arylation of triethoxy(vinyl)silane 10 and aryl bromide 11 using Pd(OAc) 2 (0.3 mol%) and NaOH in 98% yield with selectivities (100:0), respectively (Scheme 3). For the synthesis of asymmetric (E)-1,2-diarylethene via one-pot Hiyama vinylation reaction, a different strategy was developed in which treatment of 3-bromopyridine 13 with triethoxy(vinyl)silane 10 by using Pd(OAc) 2 aqueous NaOH and PEG followed by the Heck The combination of palladium acetate with XPhos exhibits high efficacy in Hiyama cross-coupling of aryl mesylates and arylsilanes to generate corresponding biaryl derivatives in a 40-97% yield range. Wu and coworkers developed a general and efficient synthetic route for synthesizing the substituted biaryl compounds by palladium-catalyzed Hiyama cross-coupling reaction of unactivated aryl mesylates with arylsilanes. Various substituted aryl mesylates containing either electron-rich or electron-poor groups furnished the corresponding biaryl products in 40-97% yields. Maximum yield (97%) was attained in the case of coupling of methoxy substituted aryl mesylate 15 with triethoxy(phenyl)silane 16 using 4 mol% of Pd(OAc) 2 as a catalyst, 10 mol% of XPhos 17 as ligand, and 2.0 equivalents of tetra-n-butylammonium fluoride (TBAF) as an additive in THF/t-BuOH mixture at 90 • C (Scheme 5) [55].
lanes. Various substituted aryl mesylates containing either electron-rich or electron-poor groups furnished onding biaryl products in 40-97% yields. Maximum yield (97%) was attained in the case of coupling of ubstituted aryl mesylate 15 with triethoxy(phenyl)silane 16 using 4 mol% of Pd(OAc)2 as a catalyst, 10 Phos 17 as ligand, and 2.0 equivalents of tetra-n-butylammonium fluoride (TBAF) as an additive in H mixture at 90 °C (Scheme 5) [55]. Molander and Iannazzo outlined an impressive synthetic approach toward the synthesis of biaryls and heterobiaryl derivatives by Hiyama cross-coupling reactions of aryltrifluorosilanes with aryl and heteroaryl chlorides using a palladium catalyst. Aryl chlorides, bearing both electron-donating and withdrawing substituents, afforded corresponding der Scheme 5. Use of Pd(OAc) 2 and XPhos 17 in coupling of aryl mesylate 15 and triethoxy(phenyl)silane 16.
Molander and Iannazzo outlined an impressive synthetic approach toward the synthesis of biaryls and heterobiaryl derivatives by Hiyama cross-coupling reactions of aryltrifluorosilanes with aryl and heteroaryl chlorides using a palladium catalyst. Aryl chlorides, bearing both electron-donating and withdrawing substituents, afforded corresponding derivatives in 70-98% yields. Phenyltrifluorosilane 19 was allowed to couple with methyl 3-chlorobenzoate 20 in the presence of 2.5 mol% palladium acetate and 5 mol% XPhos 17 using TBAF (2.5 equiv) as fluoride activator and t-BuOH as solvent. The reaction proceeded at 60 • C, providing the targeted product 21 in 98% yield (Scheme 6). The coupling of a wide range of substituted heteroaryl chlorides with aryltrifluorosilanes provided the desired products in good to excellent yields (71-94%) [56]. Arenediazonium salts are the most reactive and efficient electrophiles for palladium-catalyzed synthetic organic chemistry [57][58][59][60][61][62]. Considering their importance, Qi and coworkers achieved the Hiyama coupling of dimethoxydiphenylsilane with mono or disubstituted arenediazonium tetrafluoroborate salts in a 65-89% yield range under mild reaction con Arenediazonium salts are the most reactive and efficient electrophiles for palladiumcatalyzed synthetic organic chemistry [57][58][59][60][61][62]. Considering their importance, Qi and coworkers achieved the Hiyama coupling of dimethoxydiphenylsilane with mono or disubstituted arenediazonium tetrafluoroborate salts in a 65-89% yield range under mild reaction conditions. The dimethoxydiphenylsilane 22 was allowed to couple with bromo substituted arenediazonium tetrafluoroborate salt 23 using 5 mol% Pd(OAc) 2 in methanol at room temperature for 6 h to obtain the targeted bromo substituted biaryl derivative 24 in 89% yield (Scheme 7). The cross-coupling of 4-methylbenzenediazonium tetrafluoroborate salt with various organosilanes yielded the desired cross-coupling products in 78-87% yields [63]. ditions. The dimethoxydiphenylsilane 22 was allowed to couple with bromo substituted arenediazonium tetrafluoroborate salt 23 using 5 mol% Pd(OAc)2 in methanol at room temperature for 6 h to obtain the targeted bromo substituted biaryl derivative 24 in 89% yield (Scheme 7). The cross-coupling of 4-methylbenzenediazonium tetrafluoroborate salt with various organosilanes yielded the desired cross-coupling products in 78-87% yields [63]. Yuen et al. carried out a facile synthetic protocol for the formation of biaryl derivatives by Hiyama coupling reaction of aryl and heteroaryl chlorides with phenyl trimethoxysilane catalyzed by 0.2 mol% Pd(OAc)2/26 to attain the cross-coupling products in moderate to good yield range (44-99%). The phenyl(trimethoxy)silane 8 was coupled with aryl chlorides 25 and 28 under H2O and solventless conditions, respectively, to obtain the desired products 27 and 29 in 97% and 99% yields, respectively. The reaction proceeded smoothly by using a 1:4 mixture of highly efficient Pd(OAc)2/26 and TBAF . 3H2O as the base at 110 °C under an N2 atmosphere (Scheme 8). The aryl chlorides having electron-withdrawing groups (F, CF3, CO2Me, and CN) afforded a 47-99% yield range. Under solvent-free conditions, the reaction of heteroaryl chlorides and alkenyl chlorides with aryl trialkoxysilanes gave a 63-99% yield range, while in the case of H2O, a 36-97% yield range was attained. On the other hand, a 54-81% yield range was observed in the case of heteroaryl trialkoxysilanes [64]. Yuen et al. carried out a facile synthetic protocol for the formation of biaryl derivatives by Hiyama coupling reaction of aryl and heteroaryl chlorides with phenyl trimethoxysilane catalyzed by 0.2 mol% Pd(OAc) 2 /26 to attain the cross-coupling products in moderate to good yield range (44-99%). The phenyl(trimethoxy)silane 8 was coupled with aryl chlorides 25 and 28 under H 2 O and solventless conditions, respectively, to obtain the desired products 27 and 29 in 97% and 99% yields, respectively. The reaction proceeded smoothly by using a 1:4 mixture of highly efficient Pd(OAc) 2 /26 and TBAF·3H 2 O as the base at 110 • C under an N 2 atmosphere (Scheme 8). The aryl chlorides having electronwithdrawing groups (F, CF 3 , CO 2 Me, and CN) afforded a 47-99% yield range. Under solvent-free conditions, the reaction of heteroaryl chlorides and alkenyl chlorides with aryl trialkoxysilanes gave a 63-99% yield range, while in the case of H 2 O, a 36-97% yield range was attained. On the other hand, a 54-81% yield range was observed in the case of heteroaryl trialkoxysilanes [64]. Azetidines are distinctly comprised of four-membered azaheterocycle motifs due to possessing numerous chemical properties and ring strain [65,66]. Arylazetidine scaffolds are significant building modules and exhibit a diverse range of biological activities [67][68][69][70][71][72]. A convenient approach toward the synthesis of a variety of 3-arylazetidines derivatives through Hiyama coupling of 3-iodoazetidines with arylsilanes in mild reaction Azetidines are distinctly comprised of four-membered azaheterocycle motifs due to possessing numerous chemical properties and ring strain [65,66]. Arylazetidine scaffolds are significant building modules and exhibit a diverse range of biological activities [67][68][69][70][71][72]. A convenient approach toward the synthesis of a variety of 3-arylazetidines derivatives through Hiyama coupling of 3-iodoazetidines with arylsilanes in mild reaction conditions was reported by Zou and coworkers. Triethoxy(phenyl)silane with electron-donating and -withdrawing substituents afforded the products moderate to excellent yields (30-88%). 1-Boc-3-iodoazetidine 30 was allowed to couple with 4-methylphenyltriethoxysilane 31 in the presence of Pd(OAc) 2 catalyst (5 mol%), phosphine ligand (10 mol%) 32, tetra-nbutylammonium fluoride (TBAF in THF) and dioxane under argon atmosphere to obtain the targeted product 33 in an excellent (88%) yield along with 34 in 99:1 ratio, respectively (Scheme 9). The substrate scope of heterocycloalkyl iodides was observed under optimized conditions. The desired coupling products were obtained in a 33-85% yield range [73]. Dihetaryl disulfides possessing pyrimidine rings exhibit diversified biological a pharmacological activities [74][75][76], such as antifungal, antibacterial, and calcium-chan modulation [77]. An interesting and modular strategy for the formation of carbon-carb bonds to produce potent biologically active compounds through palladium-catalyz copper-promoted Hiyama cross-coupling reaction has been reported by Liu et al. D hetaryl Scheme 9. Synthesis of 3-arylazetidine derivative 33 from 3-iodoazetidine 30.
Dihetaryl disulfides possessing pyrimidine rings exhibit diversified biological and pharmacological activities [74][75][76], such as antifungal, antibacterial, and calcium-channel modulation [77]. An interesting and modular strategy for the formation of carbon-carbon bonds to produce potent biologically active compounds through palladium-catalyzed copper-promoted Hiyama cross-coupling reaction has been reported by Liu et al. Dihetaryl disulfides containing electron-rich and electron-poor substituents in the para-positions of benzene rings afforded corresponding products in moderate yield range. A maximum yield (78%) of 36 was observed in the case of cross-coupling of dihetaryl disulfide 35 with trimethoxy(phenyl)silane 8 using 3 mol% Pd(OAc) 2 catalyst, an efficient activator copper (I) thiophene-2-carboxylate (CuTC), TBAF and 6 mol% PCy 3 ligand using THF as the only solvent (Scheme 10) [78]. disulfides containing electron-rich and electron-poor substituents in the para-positions of benzene rings afforded corresponding products in moderate yield range. A maximum yield (78%) of 36 was observed in the case of cross-coupling of dihetaryl disulfide 35 with trimethoxy(phenyl)silane 8 using 3 mol% Pd(OAc)2 catalyst, an efficient activator copper (I) thiophene-2-carboxylate (CuTC), TBAF and 6 mol% PCy3 ligand using THF as the only solvent (Scheme 10) [78]. The palladium NNC-pincer complex was found to be an efficient catalyst precursor for the generation of monomeric palladium(0) species. The applications of palladium NNC-pincer complex as a catalyst in the allylic arylation of allyl acetates with sodium tetraarylboronates [79] and in the Heck reaction of aryl halides with activated alkenes [80]  The palladium NNC-pincer complex was found to be an efficient catalyst precursor for the generation of monomeric palladium(0) species. The applications of palladium NNC-pincer complex as a catalyst in the allylic arylation of allyl acetates with sodium tetraarylboronates [79] and in the Heck reaction of aryl halides with activated alkenes [80] gained tremendous importance in recent years. Keeping its efficiency under consideration, Ichii et al. synthesized corresponding biaryls via palladium NNC-pincer complex catalyzed Hiyama coupling reactions of aryl bromides with aryl(trialkoxy)silanes. Bromobenzenes having electron-donating and electron-withdrawing substituents resulted in a moderate to excellent yield range (39-99%) of biaryl products. The substituted aryl bromide 11 was allowed to couple with phenyl trimethoxysilane 8 using 5 mol ppm of palladium complex 37. KF was selected as a suitable base for the corresponding reaction. The maximum yield (99%) of biaryl derivative 38 was observed using propylene glycol as an effective solvent (Scheme 11) [81]. Amide is the versatile and valuable functionality widely utilized as a building block in biologically important compounds such as proteins and enzymes due to the sturdy nature of the amide C-N bond [82,83]. Highlighting the significance of the amide C-N bond, Idris and Lee devised a route for the Pd-catalyzed transformation of amides to corresponding aryl ketones. A number of different substituted N-acylglutarimides were reacted with phenyl t Scheme 11. Aryl bromide 11 and phenyl trimethoxysilane 8 coupling reaction using palladium NNC-pincer complex as catalyst.
Amide is the versatile and valuable functionality widely utilized as a building block in biologically important compounds such as proteins and enzymes due to the sturdy nature of the amide C-N bond [82,83]. Highlighting the significance of the amide C-N bond, Idris and Lee devised a route for the Pd-catalyzed transformation of amides to corresponding aryl ketones. A number of different substituted N-acylglutarimides were reacted with phenyl triethoxysilanes to give targeted products in (0-98%) yield. The highest yield (98%) was observed by coupling of N-4-fluorobenzoylglutarimide 39 and phenyltriethoxysilane 16 using palladium acetate (2 mol%), PCy 3 (4 mol%) in 1,4-dioxane/H 2 O, Et 3 N . 3HF and LiOAc at 90 • C for 6 h (Scheme 12). N-Acylglutarimide bearing sterically bulky tert-butyl group provided no desired coupling product. The coupling of substituted N-acylglutarimides with a broad range of arylsiloxanes afforded corresponding coupled products up to 93% yield [84]. The trapping of σ-alkylpalladium intermediate using arylsilanes under palladium-catalyzed Domino Heck/Hiyama coupling was disclosed by Wu and coworkers. This approach demonstrated the broad substrate scope and compatibility with various functional groups. Different aryl-tethered activated or unactivated alkenes were treated with substituted arylsilanes by means of palladium acetate catalyst in MeCN at 80 °C under argon atmospheric conditions. Consequently, the corresponding products were obtained in 62-88% and 53-81% yields, respectively. Excellent results were observed in the case of coupling of The trapping of σ-alkylpalladium intermediate using arylsilanes under palladiumcatalyzed Domino Heck/Hiyama coupling was disclosed by Wu and coworkers. This approach demonstrated the broad substrate scope and compatibility with various functional groups. Different aryl-tethered activated or unactivated alkenes were treated with substituted arylsilanes by means of palladium acetate catalyst in MeCN at 80 • C under argon atmospheric conditions. Consequently, the corresponding products were obtained in 62-88% and 53-81% yields, respectively. Excellent results were observed in the case of coupling of N-(2-iodo-4-methoxyphenyl)-N-methylmethacrylamide 41 and methoxy substituted phenyl triethoxysilane 42 using an effective ligand PPh 3 (10 mol%) and Bu 4 NF giving 88% yield to furnish the respective product 43 (Scheme 13). This methodology was employed to yield the ezetimibe, a cholesterol absorption inhibitor, analogs using N-(2-iodophenyl)-N-methylmethacrylamide and ezetimibe-derived arylsilane [85]. The azaindoline framework is a nitrogen-bearing heterocycle that exists in several complex compounds and exhibits numerous biological and pharmaceutical attributes [86]. Keeping these characteristics in mind, Ye et al. disclosed the formation of azaindoline derivatives, which was carried out by the Domino Heck cyclization/Hiyama coupling protocol. A number of functionalized azaindoline derivatives were achieved in 46-85% yields. A series of ligands (PPh3, P(2-furyl)3, XPhos, SPhos, P(4-MeOC6H4)3 were screened, and the electron-rich P(4-MeOC6H4)3 was declared as a suitable ligand for the corresponding rea The azaindoline framework is a nitrogen-bearing heterocycle that exists in several complex compounds and exhibits numerous biological and pharmaceutical attributes [86]. Keeping these characteristics in mind, Ye et al. disclosed the formation of azaindoline derivatives, which was carried out by the Domino Heck cyclization/Hiyama coupling protocol. A number of functionalized azaindoline derivatives were achieved in 46-85% yields. A series of ligands (PPh 3 , P(2-furyl) 3 , XPhos, SPhos, P(4-MeOC 6 H 4 ) 3 were screened, and the electron-rich P(4-MeOC 6 H 4 ) 3 was declared as a suitable ligand for the corresponding reaction. The reaction of Bn-protected aminopyridine 44 and 4-methoxy substituted phenyltriethoxysilane 42 was progressed in 1,4-dioxane using 5 mol% Pd(OAc) 2 catalyst and 2.0 equivalents of Bu 4 NF. The targeted coupling product 45 was furnished in 85% yield by maintaining the temperature at 80 • C (Scheme 14). Excellent results (46-85%) were obtained in the case of electron-donating (phenyl, OMe, CH 3 ) and -withdrawing (F, CF 3 , Cl) substituents. Moreover, heteroarylsilanes, including 2-thienylsilane and 3-thienylsilane, provided access to targeted products in 77% and 82% yields, respectively [87]. d in 1,4-dioxane using 5 mol% Pd(OAc)2 catalyst and 2.0 equivalents of Bu4NF. The targeted coupling 5 was furnished in 85% yield by maintaining the temperature at 80°C (Scheme 14). Excellent results (46-85%) ined in the case of electron-donating (phenyl, OMe, CH3) and -withdrawing (F, CF3, Cl) substituents.
, heteroarylsilanes, including 2-thienylsilane and 3-thienylsilane, provided access to targeted products in 2% yields, respectively [87]. Scheme 14. Pd(OAc)2 catalyzed formation of (hetero)aryl-functionalized azaindoline-derived compounds. Organosilanes have attracted the attention of research groups due to their assorted benefits (such as low cost, ease of availability, nontoxic byproducts, and stability) a compared to the other organometallic precursors (organoboron, organostannane, or ganozinc, organotin). Unsuccessful attempts to couple 2-trimethylsilylpyridine wit 4-iodoanisole diverted the attention of researchers towards sustainable liquids, i.e chloropyridyltrimethylsilanes for Hiyama cross-coupling reaction. Chloropyridyltrime thylsilanes could be afforded either by halogen or hydrogen lithium exchange of chlo Organosilanes have attracted the attention of research groups due to their assorted benefits (such as low cost, ease of availability, nontoxic byproducts, and stability) as compared to the other organometallic precursors (organoboron, organostannane, organozinc, organotin). Unsuccessful attempts to couple 2-trimethylsilylpyridine with 4-iodoanisole diverted the attention of researchers towards sustainable liquids, i.e., chloropyridyltrimethylsilanes for Hiyama cross-coupling reaction. Chloropyridyltrimethylsilanes could be afforded either by halogen or hydrogen lithium exchange of chloropyridines followed by the reaction with chlorotrimethylsilane. Pierrat et al., in 2005, disclosed the Hiyama cross-coupling of chlorosubstituted pyridyltrimethylsilanes with aryl halides. 1-Fluoro-4-iodobenzene 52 was allowed to react with chloro substituted pyridyltrimethylsilanes 53 using 5% PdCl 2 (PPh 3 ) 2 , 10% PPh 3 , and copper iodide (1 equiv) to carry out the coupling in DMF solvent. The reaction was conducted using two equivalents of tetra-n-butylammonium fluoride (TBAF in THF) as a suitable base at room temperature for 12 h, affording the targeted biaryl product 54 in 95% yield (Scheme 17) [89]. The phosphine-free palladium-catalyzed synthesis of para-substituted biaryl scaffold using air-insensitive PdCl2/hydrazone ligand was reported by Mino and coworkers. Several functionalized substituted biaryls were obtained in a 50-90% yield range via the reaction of aryl bromides with different siloxanes. An excellent result (90%) was attained by the reaction of phenyl bromide 55 with para-substituted phenyl triethoxysilane 56 in the presence of air-stable ph The phosphine-free palladium-catalyzed synthesis of para-substituted biaryl scaffold using air-insensitive PdCl 2 /hydrazone ligand was reported by Mino and coworkers. Several functionalized substituted biaryls were obtained in a 50-90% yield range via the reaction of aryl bromides with different siloxanes. An excellent result (90%) was attained by the reaction of phenyl bromide 55 with para-substituted phenyl triethoxysilane 56 in the presence of air-stable phosphine-free PdCl 2 /hydrazone ligand 57 using TBAF in THF under argon atmosphere. After screening a variety of solvents (DMSO, dioxane, t-BuOH, toluene, and DMF), toluene was selected as a suitable solvent for the corresponding reaction (Scheme 18) [90]. Multicomponent assembly protocol fascinates synthetic chemists due to its efficacy in the synthesis of molecular complexity and is acceptable for diversity-oriented synthesis [91,92]. An efficient approach for the synthesis of multisubstituted unsymmetrical biaryls was reported by Akai and coworkers in 2008. The reaction of substituted 8-TBDMS-1-naphthols with a wide Scheme 18. PdCl 2 catalyzed Hiyama cross-coupling using phosphine-free hydrazone ligand.

Palladium Chloride as Catalyst
Multicomponent assembly protocol fascinates synthetic chemists due to its efficacy in the synthesis of molecular complexity and is acceptable for diversity-oriented synthesis [91,92]. An efficient approach for the synthesis of multisubstituted unsymmetrical biaryls was reported by Akai and coworkers in 2008. The reaction of substituted 8-TBDMS-1-naphthols with a wide range of aryl iodides afforded the biaryl derivatives moderate to excellent yields (46-81%). The fluoride-free Hiyama coupling of 59 was accomplished with 4-cyanoiodobenzene 60 giving a multisubstituted asymmetrical biaryl through the formation of pentacoordinate silicate 61 as an intermediate. [(allyl)PdCl] 2 , AsPh 3 , Cs 2 CO 3 , and dimethoxyethane (DME) were selected as an appropriate catalyst, ligand, base, and solvent, respectively, to afford the maximum yield (81%) of targeted asymmetrical biaryl derivative 62 under argon atmosphere (Scheme 19) [93]. Pincer-type palladium complexes are intriguing due to their promising reactivity and stability. Inés et al. synthesized the non-symmetric PCN pincer-type palladium complex by the reduction of 1-(3-nitrophenyl)pyrazole to amine, which upon treatment with ClPPh2 provided an unreliable and air-sensitive ligand. The ligand further reacted with Pd(COD)Cl2 to yield a targeted non-symmetric pincer-type complex that efficiently catalyzed the Hiyama cross-coupling reaction in eco-friendly reaction media. Excellent results were achieved using two methods in the presence of an efficient catalyst. The first pathway was concerned with the reaction of phenyl trimethoxysilane 8 with 11 catalyzed by 2 mol% 63 using NaOH i Pincer-type palladium complexes are intriguing due to their promising reactivity and stability. Inés et al. synthesized the non-symmetric PCN pincer-type palladium complex by the reduction of 1-(3-nitrophenyl)pyrazole to amine, which upon treatment with ClPPh 2 provided an unreliable and air-sensitive ligand. The ligand further reacted with Pd(COD)Cl 2 to yield a targeted non-symmetric pincer-type complex that efficiently catalyzed the Hiyama cross-coupling reaction in eco-friendly reaction media. Excellent results were achieved using two methods in the presence of an efficient catalyst. The first pathway was concerned with the reaction of phenyl trimethoxysilane 8 with 11 catalyzed by 2 mol% 63 using NaOH in H 2 O at 140 • C for 3 h to obtain the corresponding biaryl derivative 38 in 82% yield. The alternative developed method involved the coupling of phenyl trimethoxysilane 8 with 11 in the presence of 4 mol% of catalyst 63 using n-Bu 4 NF and o-xylene at 80 • C for 4 h to afford the required biaryl product 38 in 61% yield (Scheme 20) [94]. Aryl imidazol-1-ylsulfonates are the electron-deficient substrates that promote the palladium-catalyzed Hiyama and Sonogashira cross-coupling under copper-free conditions, as reported by Williams and coworkers. Several functionalized biaryl derivatives were obtained by the reaction of aryl imidazylates with (2-hydroxymethylphenyl) dimethyl silane (HOMSi) reagent in 72-99% yields. The 2-naphthyl imidazol-1-ylsulfonate 67 was coupled with 4-met Aryl imidazol-1-ylsulfonates are the electron-deficient substrates that promote the palladium-catalyzed Hiyama and Sonogashira cross-coupling under copper-free conditions, as reported by Williams and coworkers. Several functionalized biaryl derivatives were obtained by the reaction of aryl imidazylates with (2-hydroxymethylphenyl) dimethyl silane (HOMSi) reagent in 72-99% yields. The 2-naphthyl imidazol-1-ylsulfonate 67 was coupled with 4-methoxyphenyl HOMSi reagent 68 in the presence of Pd(dppf)Cl 2 (0.05 equiv) in dry DMSO with K 2 CO 3 at 65 • C giving the cross-coupling product 69 in 99% yield (Scheme 22) [96]. A research group by Hughes also reported a facile and convenient methodology for the palladium-catalyzed synthesis of biphenyls in 2011 using the HOMSi ® reagent. In this methodology, the aryl bromides and iodides were proved to be more efficient substrates to couple with the HOMSi ® reagent in DMF solvent. Excellent (>98%) conversion was achieved by the reaction of A research group by Hughes also reported a facile and convenient methodology for the palladium-catalyzed synthesis of biphenyls in 2011 using the HOMSi ® reagent. In this methodology, the aryl bromides and iodides were proved to be more efficient substrates to couple with the HOMSi ® reagent in DMF solvent. Excellent (>98%) conversion was achieved by the reaction of HOMSi ® reagent 71 with 72 using PdCl 2 as the catalyst, 73 as ligand using CuI, and K 2 CO 3 as the base at 80 • C for 15 h. DMF was selected as the optimal solvent by observing the results with THF, MeOH, 1,4-dioxane, and DMSO (Scheme 23). Compatibility with functional groups, reactions under fluoride-free conditions, and recycling of organosilicon byproducts are the salient features of the HOMSi ® reagent [97]. Stilbenes and their hydroxylated derivatives are of tremendous interest as they exhibit diversified biological activities such as fungistatic, antibacterial and anticancer [98][99][100][101]. Due to their extended conjugation system, stilbenes have been utilized to assemble electronic and optoelectrical devices, i.e., solar cells, LEDs, and dye lasers [102,103]. Resveratrol and combretastatin A-4 are a few bioactive derivatives of stilbene that can be obtained naturally from the extraction of plants. An effective catalytic system was established for the stereoselective formation of substituted E-stilbenes using Heck cross-coupling reaction while studying the catalytic activity of Pd complexes with H-spirophosphane ligands. Hiyama coupling reaction emerged to be a more effective route to synthesize substituted E-stilbenes. Keeping these considerations in mind, Skarżyńska  Stilbenes and their hydroxylated derivatives are of tremendous interest as they exhibit diversified biological activities such as fungistatic, antibacterial and anticancer [98][99][100][101]. Due to their extended conjugation system, stilbenes have been utilized to assemble electronic and optoelectrical devices, i.e., solar cells, LEDs, and dye lasers [102,103]. Resveratrol and combretastatin A-4 are a few bioactive derivatives of stilbene that can be obtained naturally from the extraction of plants. An effective catalytic system was established for the stereoselective formation of substituted E-stilbenes using Heck cross-coupling reaction while studying the catalytic activity of Pd complexes with H-spirophosphane ligands. Hiyama coupling reaction emerged to be a more effective route to synthesize substituted E-stilbenes. Keeping these considerations in mind, Skarżyńska and coworkers, in 2011, proposed an efficient synthesis of E-stilbenes via Hiyama coupling reactions of substituted styrylsilanes with aryl halides catalyzed by an efficient and stereoselective precatalyst. The styrylsilanes possessing either electron-deficient groups or para-substituted electron-rich groups resulted in an excellent yield range (87-99%). A 100% conversion of iodobenzene and 99% yield of bromo substituted E-stilbene 77 was obtained by the addition of 75 to iodobenzene 76 using palladium complex with H-spirophosphorane ligand [PdCl 2 P(OCH 2 Cme 2 NH)OCH 2 Cme 2 NH 2 ] as precatalyst in the presence of [ n Bu 4 N]F additive. The reaction progressed in tetrahydrofuran solvent at 60 • C for 12 h (Scheme 24) [104]. Chang et al. reported a facile protocol for synthesizing the biaryl ketones via carbonylative Hiyama coupling reaction. In this regard, the phenyl triethoxysilane 16 was allowed to react with 4-bromophenyl iodide 80 using PdCl 2 (MeCN) 2 catalyst and cesium fluoride (CsF) promoter to afford the corresponding ketone 81 in 94% yield (Scheme 26). The reaction temperature was maintained at 80 • C. Different solvents (DMSO, toluene, DMF, CH 3 CN, and 1,4-dioxane) were screened, and NMP (N-methyl-2-pyrrolidone) was selected as the optimal solvent for the corresponding reaction. Aryl halides having electronwithdrawing (-Cl, -F, -NO 2 , CN) and electron-donating substituent (-CH 3 ) gave moderate to excellent yields (68-94%) [106]. Diarylmethanes have attained huge interest as they exist in biologically active natural compounds and drugs [107,108]. For example, avrainvilleol, a marine natural product, possesses a diarylmethane motif and exhibits antibacterial [40] and antioxidant [41] activities. Several drugs containing diarylmethanes demonstrate a wide range of biological activities such as segontin (used to cure coronary heart disease) [35], bifemelane, tolpropamine, and piritrexim act as an antidepressant, antiallergic and anticancer agents, respectively [36][37][38][39]. Functionalized diarylmethane derivatives were achieved by coupling a variety of benzylic ammonium salts with phenyl trimethoxysilane, as elaborated by Zhao and coworkers in 2019. Methoxy substituted benzyltrimethylammonium salt 82 was treated with phenyl trimethoxysilane 8 to acquire the desired diarylmethane derivative 83 in 92% yi Scheme 26. Synthesis of ketones by using carbonylative Hiyama cross-coupling.
Diarylmethanes have attained huge interest as they exist in biologically active natural compounds and drugs [107,108]. For example, avrainvilleol, a marine natural product, possesses a diarylmethane motif and exhibits antibacterial [40] and antioxidant [41] activities. Several drugs containing diarylmethanes demonstrate a wide range of biological activities such as segontin (used to cure coronary heart disease) [35], bifemelane, tolpropamine, and piritrexim act as an antidepressant, antiallergic and anticancer agents, respectively [36][37][38][39]. Functionalized diarylmethane derivatives were achieved by coupling a variety of benzylic ammonium salts with phenyl trimethoxysilane, as elaborated by Zhao and coworkers in 2019. Methoxy substituted benzyltrimethylammonium salt 82 was treated with phenyl trimethoxysilane 8 to acquire the desired diarylmethane derivative 83 in 92% yield. The reaction proceeded well at 120 • C in the presence of 5 mol% PdCl 2 (CH 3 CN) 2 catalyst, 20 mol% PPh 2 Cy ligands, and TBAF in EtOH (Scheme 27). In the case of aryl trimethoxysilanes, the substrate containing electron-donating groups afforded corresponding diarylmethane derivatives in (57-97%) yield range. Moreover, the substrate-bearing heterocycles, including thiophene and furan, gave coupling products a 94% yield [109]. Synthesis of vinylsilanes is a very promising task in research due to their spacious applications in organic syntheses, such as the Hiyama coupling reaction [110]. Wisthoff  Synthesis of vinylsilanes is a very promising task in research due to their spacious applications in organic syntheses, such as the Hiyama coupling reaction [110]. Wisthoff et al. designed a facile protocol to accomplish the synthesis of cisor trans-tetrasubstituted vinylsilanes via carbosilylation of three components, including symmetrical alkynes, alkyl zinc iodides, and iodosilanes using a palladium catalyst. The authors also reported conditions for tetrasubstituted vinylsilanes via the Hiyama coupling reaction that facilitated the synthesis of tetrasubstituted alkenes. The cisor trans-tetrasubstituted vinylsilanes were synthesized in a 32-97% yield range by the addition of iodosilane to the solution of alkyl zinc iodide and alkyne using (Ph 3 P) 2 PdCl 2 (2 mol%) catalyst in dioxane. Triethylamine (Et 3 N) was reported as a suitable base for this reaction. Tetrasubstituted vinylsilanes underwent Hiyama cross-coupling reactions to achieve the stereodefined tetrasubstituted alkenes. The reaction of 84 with 1-bromo-3-methybenzene 46 was conducted in the presence of KOSiMe 3 , and 18-crown-6 in THF at 65 • C, followed by the treatment with 2.5 mol% [(allyl)PdCl 2 ] catalyst and 5 mol% SPhos ligands at 65 • C afforded the desired geometrically defined tetrasubstituted alkene 85 in 62% yield (Scheme 28). The effect of ligands on the selectivity of alkyl-substituted tetrasubstituted vinylsilanes was also observed. It was found that the use of DrewPhos ligand 86 ( Figure 2) gave results in (54-92%) yield of alkyl-substituted tetrasubstituted vinylsilanes with excellent syn-selectivity while using JessePhos 87 ( Figure 2) as ligand resulted in 61-92% yield range of alkyl-substituted tetrasubstituted vinylsilanes with excellent anti-selectivity [111]. γ-Valerolactone-based tetrabutylphosphonium 4-ethoxyvalerate are effective and versatile biomass-derived ionic liquids and have been extensively utilized in synthesis protocols due to their tunability. In order to elaborate on the effect of ionic liquid, Orha et al. outlined the efficient synthesis of substituted biaryl derivatives by the addition of iodoaromatic substrates to triethoxyphenylsilane. In this methodology, reactions of several aryl iodides were perf  γ-Valerolactone-based tetrabutylphosphonium 4-ethoxyvalerate are effective and versatile biomass-derived ionic liquids and have been extensively utilized in synthesis protocols due to their tunability. In order to elaborate on the effect of ionic liquid, Orha et al. outlined the efficient synthesis of substituted biaryl derivatives by the addition of iodoaromatic substrates to triethoxyphenylsilane. In this methodology, reactions of several aryl iodides were performed with triethoxyphenylsilane to give numerous substituted biaryl structures in good yields (45-72%

Pd/C as Catalyst
A heterogeneous catalytic system proved to be an effective system over a homog neous catalytic system in the organic synthetic field regarding their stability, recove bility, and ease of handling [113]. Considering the efficacy of heterogeneous catalys Yanase et al., in 2011, reported the synthesis of biaryl derivatives via water-mediat Hiyama cross-coupling reaction using Pd/C, a heterogeneous catalyst [114] with el tron-deficient phosphine ligand [(4-FC6H4)3P]. Several functionalized biaryl derivativ were atta Scheme 29. Pd(PPh 3 ) 2 Cl 2 catalyzed coupling reaction using tetrabutylphosphonium 4-ethoxyvalerate as ionic liquid.

Pd/C as Catalyst
A heterogeneous catalytic system proved to be an effective system over a homogeneous catalytic system in the organic synthetic field regarding their stability, recoverability, and ease of handling [113]. Considering the efficacy of heterogeneous catalysts, Yanase et al., in 2011, reported the synthesis of biaryl derivatives via water-mediated Hiyama cross-coupling reaction using Pd/C, a heterogeneous catalyst [114] with electron-deficient phosphine ligand [(4-FC 6 H 4 ) 3 P]. Several functionalized biaryl derivatives were attained in a 47-90% yield range by cross-coupling of aryl halides with aryltrialkoxysilanes. The reaction was progressed at 120 • C using 0.5 mol% of 10% Pd/C catalyst, 1 mol% of tris(4-fluorophenyl)phosphine as ligand, TBAF . 3H 2 O as an activator, and 4.8% aqueous toluene. A 90% yield of targeted biaryl product 18 was obtained by coupling 3-methoxyphenylbromide 89 with phenyl triethoxysilane 16 (Scheme 30) [115]. Later on, in a 2013 research group by Monguchi, they developed a facile and efficient methodology to synthesize the biaryl derivatives in (47-90%) yield range using (4-C 6 H 4 ) 3 P (1 mol%) ligand [116]. The very first ligand-free Pd/C catalyzed Hiyama cross-coupling reaction fo construction of a variety of biphenyl derivatives was developed by Sajiki and cowo in 2012. A series of solvents (DMF, THF, CH3CN, EtOH, toluene) and fluoride so (TBAF, LiF, CsF, KF) were screened, and among them, toluene and TBAF . 3H2O we lected as a suitable solvent, and fluoride source for corresponding Pd/C cata Hiyama cross-co The very first ligand-free Pd/C catalyzed Hiyama cross-coupling reaction for the construction of a variety of biphenyl derivatives was developed by Sajiki and coworkers in 2012. A series of solvents (DMF, THF, CH 3 CN, EtOH, toluene) and fluoride sources (TBAF, LiF, CsF, KF) were screened, and among them, toluene and TBAF·3H 2 O were selected as a suitable solvent, and fluoride source for corresponding Pd/C catalyzed Hiyama cross-coupling reaction. Aryl bromide 11 was reacted with methoxy substituted phenyl triethoxysilane 42 to afford a maximum (90%) yield of desired biphenyl derivative 90. The reaction was refluxed for 24 h in toluene containing 0.5 mol% of 5% Pd/C catalyst, TBAF·3H 2 O as an activator, and acetic acid (Scheme 31) [117]. upling reaction. Aryl bromide 11 was reacted with methoxy substituted phenyl triethoxysilane 42 to afford a maximum (90%) yield of desired biphenyl derivative 90. The reaction was refluxed for 24 h in toluene containing 0.5 mol% of 5% Pd/C catalyst, TBAF . 3H2O as an activator, and acetic acid (Scheme 31) [117]. γ-Valerolactone (GVL), a safe and sustainable bio-based chemical obtained from lignocellulosic biomass [118], was found to be an efficient media for Hiyama coupling reactions. The use of highly effective and non-expensive heterogeneous catalyst Pd/C with γ-valerolactone competently promotes the eco-friendly Hiyama cross-coupling re action under practical and mild conditions without using any ligand or additive as re ported by Ismalaj et al. A number of aryl halides were treated with phenyl triethox ysilanes using 1M GVL Scheme 31. Synthesis of biphenyl derivative 90 by ligand-free Pd/C catalyzed Hiyama coupling reaction.
γ-Valerolactone (GVL), a safe and sustainable bio-based chemical obtained from lignocellulosic biomass [118], was found to be an efficient media for Hiyama coupling reactions. The use of highly effective and non-expensive heterogeneous catalyst Pd/C with γ-valerolactone competently promotes the eco-friendly Hiyama cross-coupling reaction under practical and mild conditions without using any ligand or additive as reported by Ismalaj et al. A number of aryl halides were treated with phenyl triethoxysilanes using 1 M GVL as an efficient biomass-based solvent for Hiyama coupling reaction. Tetrabutylammonium fluoride (TBAF) was found to be the best activator of organosilanes, while CsF and KF were not such efficient activators. An excellent result (94%) of desired biaryl product 92 was obtained by carrying out the reaction of bromobenzene 55 with phenyl triethoxysilane 16 using Pd/C (0.5 mol%) as catalyst in 1M GVL 91 solvent and TBAF for 24 h at 130 • C (Scheme 32) [119].

Pd/Fe3O4 as Catalyst
An efficient methodology for the synthesis of a diverse range of biaryl derivative via Hiyama cross-coupling reaction of aryl bromides with aryl siloxanes using a highl efficient, easily recoverable, and eco-friendly Pd/Fe3O4 catalytic system was proposed b Sreedhar and coworkers. Fe3O4 supported palladium catalyst shows superparamagneti behavior, and its catalytic activity remains unchanged after recycling five times. Th mechanisti Scheme 32. Reaction of bromobenzene 55 with phenyl triethoxysilane 16 in the presence of γ-valerolactone (GVL) solvent.

Pd/Fe 3 O 4 as Catalyst
An efficient methodology for the synthesis of a diverse range of biaryl derivatives via Hiyama cross-coupling reaction of aryl bromides with aryl siloxanes using a highly efficient, easily recoverable, and eco-friendly Pd/Fe 3 O 4 catalytic system was proposed by Sreedhar and coworkers. Fe 3 O 4 supported palladium catalyst shows superparamagnetic behavior, and its catalytic activity remains unchanged after recycling five times. The mechanistic studies showed that Pd/Fe 3 O 4 catalyzed Hiyama cross-coupling reaction proceeds through oxidative addition and transmetalation followed by reductive elimination [120]. The effect of the base is more pronounced in achieving the maximum yield. Sodium hydroxide (NaOH) was investigated as an appropriate base for this reaction. In this methodology, the substituted aryl bromide 93 was treated with aryl siloxanes 94 by using magnetically recoverable Pd/Fe 3 O 4 catalyst and NaOH in H 2 O at 90 • C to obtain the excellent yield (92%) of desired coupling product 95 (Scheme 33) [121].
c studies showed that Pd/Fe3O4 catalyzed Hiyama cross-coupling reaction proceeds through oxidative addition transmetalation followed by reductive elimination [120]. The effect of the base is more pronounced in achieving maximum yield. Sodium hydroxide (NaOH) was investigated as an appropriate base for this reaction. In methodology, the substituted aryl bromide 93 was treated with aryl siloxanes 94 by using magnetically recove Pd/Fe3O4 catalyst and N aOH in H2O at 90 °C to obtain the excellent yield (92%) of desired coupling product 95 (Scheme 33) [121]. Pd-Fe 3 O 4 Heterodimeric nanocrystals have been found to be effective and recyclable catalysts to achieve a successful Hiyama cross-coupling under ligand-free conditions. Keeping the effectiveness of catalyst in view, Lee et al. proposed a methodology for the synthesis of diversified biaryl derivatives. The protocol involved the treatment of 96 with phenyl trimethoxysilane 8 under ligand-free conditions. The reaction was proceeded in DMA (dimethylacetamide) using 1 mol% of Pd-Fe 3 O 4 heterodimeric nanocrystals as a catalyst, KF as a base, and tetra-n-butylammonium iodide (TBAI) as an additive at high temperature of 150 • C. Consequently, the desired biphenyl derivative 97 was afforded in 94% yield (Scheme 34). Electron-donating and -withdrawing groups (-Me, -OMe, -F, -CF 3 , -NO 2 ) bearing aryl halides provided the corresponding biaryl derivatives in an excellent yield range (70-94%) [122].

N-Heterocyclic Carbenes (NHCs) Palladium Complexes
NHCs are nucleophilic in nature as they exhibit σ-electron donating attributes. The application of N-heterocyclic carbene (NHC) ligands in transition metal-catalyzed cross-coupling acquired notable interest recently [123][124][125][126]. Peñafiel et al. synthesized [(NHC) 2 PdCl 2 ] complex through direct metalation of 0.2% hydroxy-functionalized imidazolium salt and 0.1% palladium acetate to efficiently catalyze the Hiyama cross-coupling of aryl chlorides and bromides with arylsiloxanes under microwave irradiation and fluoride free conditions. The fluoride-free Hiyama reaction of 4-bromoanisole 64 was carried out with phenyl trimethoxysilane 8 using [(NHC) 2 PdCl 2 ] complex 98 to catalyze the reaction under microwave irradiation (80 W, 120 • C) for 1 h. NaOH was screened to be a suitable base for the corresponding reaction to afford the targeted derivative 66 in 95% yield (Scheme 35). The coupling of aryl and heteroaryl bromides with siloxanes resulted in biaryl products in a 5-88% yield range. The required products were obtained in lower yields with aryl chlorides than with aryl bromides [127]. Another research group of Pastor and coworkers disclosed the efficient synthesis of biaryl derivatives using 0.2 or 0.5% of imidazolium salt in 2013 [128]. derivative 66 in 95% yield (Scheme 35). The coupling of aryl and heteroaryl bromides with siloxanes resulted in biaryl products in a 5-88% yield range. The required products were obtained in lower yields with aryl chlorides than with aryl bromides [127]. Another research group of Pastor and coworkers disclosed the efficient synthesis of biaryl derivatives using 0.2 or 0.5% of imidazolium salt in 2013 [128]. The synthesis of four linear dinuclear N-heterocyclic carbene palladium complexes was reported by Yang and Wang. The structures were characterized by NMR, FT-IR, and elemental analysis. The synthesis of NHC-palladium complexes was achieved by a one-pot reaction of imidazolium salts, PdCl2, and bidentate N-heterocycles in the presence of potassium carbonate (K2CO3) with the formula [PdCl2(NHC)]2(μ-L) (L = DABCO, pyrazine). The catalytic effect of N-heterocyclic carbene palladium complexes was investigated in the Hiyama coupling reaction of aryl trialkoxysilanes with aryl chlorides. Among four dinuclear N-heterocyclic carbenes, 100 expressed excellent catalytic activity in the corresponding Hiyama coupling reaction. Both electron-donating and electron-withdrawing groups substituted aryl chlorides afforded biphenyl products in (54-92%) yield range. The reaction of 99 with phenyl trimethoxysilane 8 was conducted at 120 °C for 5 h using 0.5 mol% of NHC-Pd 100, TBAF in toluene to achieve a 92% yield of corresponding product 9 (Scheme 36) [129]. The application of homoleptic chelating N-heterocyclic carbene palladium complexes immobilized within the pores of SBA-15/IL (NHC-Pd@SBA-15/IL) as heterogeneous catalyst in the Hiyama coupling reaction was reported by Rostamnia and coworkers. The desired biaryl derivatives were achieved in an excellent yield range (82-95%). The catalytic activity of the catalyst was analyzed by treating phenyl trimethoxysilane 8 with phenyl iodide 76 using 0.8 mol% (NHC-Pd@SBA-15/IL) 101. The reaction proceeded in H 2 O/dioxane (1:2) at 80 • C using Cs 2 CO 3 as a suitable base and TBAF that increased the catalytic activity of (NHC-Pd@SBA-15/IL). As a result, 92 was obtained in an excellent (95%) yield (Scheme 37). Chemoselectivity of (NHC-Pd@SBA-15/IL) for the Hiyama coupling reaction was analyzed by treating p-bromobenzaldehyde with phenyl trimethoxysilane under the optimized reaction conditions giving the corresponding biaryl product. Reusability of catalyst for five successful runs was observed [130]. A very interesting application of palladium PEPPSI (Pyridine Enhanced Precatalyst Preparation Stabilization and Initiation) complexes with non-bulky NHC ligands as a catalyst precursor in cross-coupling reactions was disclosed by Osińska et al. for the synthesizing the non-symmetric biaryl scaffolds. The application of PEPPSI complexes in Suzuki-Miyaura was investigated, in which their catalytic activity was clearly evidenced [132][133][134][135][136][137], while the use of PEPPSI complexes in the Hiyama coupling reaction is rare [138][139][140]. The PEPPSI complexes were synthesized by reacting a palladium dimer [Pd(bmim-y)X 2 ] 2 and N-ligand. Hiyama coupling worked efficiently in ethylene glycol, and 97% conversion was achieved in the case of coupling of 2-bromotoluene with phenyl trimethoxysilane using Pd(IPr)Cl 2 (Cl-py) 103 ( Figure 3) as a catalyst in ethylene glycol solvent. The reaction did not work efficiently in water. Pd(bmim)Br 2 (CN-py) complex 104 catalyzed Hiyama coupling of substituted bromobenzenes and chlorobenzenes with phenyl trimethoxysilane afforded cross-coupled products in (50-93%) yields using NaOH base and ethylene glycol as solvent at 110 • C for 24 h with the exception of 1-chloro-3-nitrobenzene and 4-chloro-2-nitrobenzene giving no results [141]. 3

Nanoparticles as Catalyst
The substantial application of transition metal nanoparticles in catalysis has remained an area of interest for research groups due to their harmless characteristic features, easy preparation methods, and large surface area [142,143]. Srimani et al. synthesized the palladium nanoparticles by the dropwise addition of metal acylate salt (CO) 5 W=C(CH 3 )O(−)NEt 4 (+) solution to the PEG-6000 containing an aqueous solution of K 2 PdCl 4 to catalyze the Hiyama coupling reactions [144]. It was noticed that the increased amount of PEG-6000 stabilizer decreases the size of nanoparticles. The substituted aryl bromides 105 were coupled with phenyl triethoxysilane 8 using palladium nanoparticles as catalyst 106 and an appropriate base NaOH in the air at 90 • C to achieve the desired biaryls 107 in 98% yield (Scheme 39). H 2 O was utilized as an effective solvent in place of THF or DME [145]. A simple, convenient, and rapid one-pot Hiyama coupling of aryl bromides or iodides with arylsilanes under fluoride-free conditions was reported by the research group of Ranu and coworkers. The corresponding reaction was catalyzed by using palladium nanoparticles that were generated in situ from sodium dodecyl sulfate (Na2PdCl4/SDS) in A simple, convenient, and rapid one-pot Hiyama coupling of aryl bromides or iodides with arylsilanes under fluoride-free conditions was reported by the research group of Ranu and coworkers. The corresponding reaction was catalyzed by using palladium nanoparticles that were generated in situ from sodium dodecyl sulfate (Na 2 PdCl 4 /SDS) in H 2 O. SDS stabilizes the formation of palladium nanoparticles. The reaction of phenyl bromide 55 with phenyl trimethoxysilane 8 was carried out using palladium nanoparticles that efficiently catalyzed the reaction. Excellent yield (96%) of desired biphenyl product 92 was attained using water as solvent. A series of bases (KOH, Na 2 CO 3 , NaOAc, and NaHCO 3 ) was screened, and NaOH was investigated as a suitable base for this reaction (Scheme 40). Environmentally friendly solvent, no utilization of ligand, and good yields are the salient features of this synthetic protocol [146].
x FOR PEER REVIEW 40 of 77 mation of palladium nanoparticles. The reaction of phenyl bromide 55 with phenyl trimethoxysilane 8 t using palladium nanoparticles that efficiently catalyzed the reaction. Excellent yield (96%) of desired uct 92 was attained using water as solvent. A series of bases (KOH, Na2CO3, NaOAc, and NaHCO3) was NaOH was investigated as a suitable base for this reaction (Scheme 40). Environmentally friendly lization of ligand, and good yields are the salient features of this synthetic protocol [146].

Scheme 40.
Palladium nanoparticles catalyzed Hiyama coupling reaction using NaOH as base.
Diarylmethanes have received appreciable attention from researchers due to their pharmacological activities [147][148][149][150][151]. They are regarded as distinct components of supramolecular structures, including catenanes, macrocycles, and rotaxanes [152][153][154][155]. Focusing on the significance of diarylmethanes, Sarkar and colleagues designed a successful protocol for the synthesis of diarylmethane derivatives using palladium nanoparticles in THF. Palladium nanoparticles were formed by the reaction of 4 mol% K2PdCl4 with PEG-600 at 70 °C. PEG-600 was used as a reducing and stabilizing agent for the preparation of nanoparticles [156]. Numerous diarylmethane derivatives were formed by Hiyama cross-coupling of benzyl halides with phenyl trialkoxysilanes in a 78-95% yield range. The 108 was coupled with phenyl trimethoxysilane 8 in the presence of 4 mol% K2PdCl4, PEG-600 using TBAF in tetrahydrofuran at 70 °C under argon environment to afford the desired coupling product 109 in 95% yield (Scheme 41). They applied the same methodology to synthesize diarylmethanes 111 in 95% yield using allyl halides 110 and phenyl trimethoxysilan Scheme 40. Palladium nanoparticles catalyzed Hiyama coupling reaction using NaOH as base.
Diarylmethanes have received appreciable attention from researchers due to their pharmacological activities [147][148][149][150][151]. They are regarded as distinct components of supramolecular structures, including catenanes, macrocycles, and rotaxanes [152][153][154][155]. Focusing on the significance of diarylmethanes, Sarkar and colleagues designed a successful protocol for the synthesis of diarylmethane derivatives using palladium nanoparticles in THF. Palladium nanoparticles were formed by the reaction of 4 mol% K 2 PdCl 4 with PEG-600 at 70 • C. PEG-600 was used as a reducing and stabilizing agent for the preparation of nanoparticles [156]. Numerous diarylmethane derivatives were formed by Hiyama cross-coupling of benzyl halides with phenyl trialkoxysilanes in a 78-95% yield range. The 108 was coupled with phenyl trimethoxysilane 8 in the presence of 4 mol% K 2 PdCl 4 , PEG-600 using TBAF in tetrahydrofuran at 70 • C under argon environment to afford the desired coupling product 109 in 95% yield (Scheme 41). They applied the same methodology to synthesize diarylmethanes 111 in 95% yield using allyl halides 110 and phenyl trimethoxysilane 8 as reacting substrates (Scheme 42). The naturally occurring 2,4-bis(4-hydroxybenzyl)phenol 112 was also synthesized by the same research group through multiple steps reactions starting from anisole using the same methodology ( Figure 4) [157].  Zhang et al. synthesized a silica-coated SiO 2 @Fe 3 O 4 -Pd catalyst that efficiently catalyzed the Hiyama coupling of 113 with phenyl trimethoxysilane 8. The catalytic efficacy of the supported catalyst remains even after 10 times of recycling. The reaction proceeded by using SiO 2 @Fe 3 O 4 supported palladium catalyst (0.5 mol%) using TBAF as a suitable base in THF at 60 • C under an N 2 atmosphere to obtain desired products 114 in 99% yield (Scheme 43). It was noticed that either electron-donating or electron-withdrawing groups containing aryl halides gave a good to excellent yield range (81-99%) [158].

EER REVIEW
43 of 77 2 atmosphere to obtain desired products 114 in 99% yield (Scheme 43). It was noticed that either electron-withdrawing groups containing aryl halides gave a good to excellent yield range Premi and Jain, in 2013, carried out the synthesis of substituted biphenyl derivatives, aromatic heterocycles, and substituted styrene derivatives by phosphane-free Hiyama cross-coupling reaction of aryl and heterocyclic halides with aryl and vinyltrimethoxysilane using palladium nanoparticles in ionic liquids. Palladium nanoparticles were generated by adding the solution of palladium acetate in acetonitrile to 3-(3-cyanopropyl)-1-methyl-1H-imidazol-3-ium hexafluorophosphate {[CN-bmim]PF6}, an ionic liquid, that stabilizes synthesis of palladium nanoparticles. It was observed that the electron-donating and -withdrawing substituents on arylhalides resulted in an excellent yield range (76-98%). The reaction of iodobenzene 76 with phenyl trimethoxysilane 8 u Premi and Jain, in 2013, carried out the synthesis of substituted biphenyl derivatives, aromatic heterocycles, and substituted styrene derivatives by phosphane-free Hiyama cross-coupling reaction of aryl and heterocyclic halides with aryl and vinyltrimethoxysilane using palladium nanoparticles in ionic liquids. Palladium nanoparticles were generated by adding the solution of palladium acetate in acetonitrile to 3-(3-cyanopropyl)-1-methyl-1H-imidazol-3-ium hexafluorophosphate {[CN-bmim]PF 6 }, an ionic liquid, that stabilizes synthesis of palladium nanoparticles. It was observed that the electron-donating and -withdrawing substituents on arylhalides resulted in an excellent yield range (76-98%). The reaction of iodobenzene 76 with phenyl trimethoxysilane 8 using 4 mol% Pd(OAc) 2 in 115 and CH 3 CN and 1-butyl-3-methylimidazolium fluoride [bmim]F, an organosilane activator, at 70-120 • C for 8 h to afford the desired biphenyl derivative in 98% yield (Scheme 44). The substituted styrene derivatives were also synthesized in (75-98%) yields by the Hiyama cross-coupling of a broad range of aryl iodides with vinyltrimethoxysilane catalyzed by Pd-NPs at 60-70 • C for 15-30 min [159]. Euphorbia thymifolia L. belongs to the family of Euphorbiaceae and is an apparently small and branched medicinal herb. It exhibits numerous pharmaceutical applications against dysentery, venereal diseases, and diarrhea. Focusing on the importance of Pd NPs, Nasrollahzadeh and colleagues developed a sustainable protocol for the formation of Pd NPs using an aqueous leaf extract of Euphorbia thymifolia L. due to its reducing and stabilizing abilities and observed its effectiveness in ligand-free Stille and Hiyama cross-coupling reactions in a green solvent. The Pd-NPs were distinguished by TEM, powder XRD, and UV-visible techniques. The catalytic efficacy of Pd NPs was evaluated for the Hiyama coupling reaction of 121 with phenyl trimethoxysilane 8 to afford the targeted biaryl produ Euphorbia thymifolia L. belongs to the family of Euphorbiaceae and is an apparently small and branched medicinal herb. It exhibits numerous pharmaceutical applications against dysentery, venereal diseases, and diarrhea. Focusing on the importance of Pd NPs, Nasrollahzadeh and colleagues developed a sustainable protocol for the formation of Pd NPs using an aqueous leaf extract of Euphorbia thymifolia L. due to its reducing and stabilizing abilities and observed its effectiveness in ligand-free Stille and Hiyama cross-coupling reactions in a green solvent. The Pd-NPs were distinguished by TEM, powder XRD, and UV-visible techniques. The catalytic efficacy of Pd NPs was evaluated for the Hiyama coupling reaction of 121 with phenyl trimethoxysilane 8 to afford the targeted biaryl products 122 in 96% yields (Scheme 48). The optimized reaction conditions (1 mol% of Pd NPs, NaOH in H 2 O as solvent at 90 • C, under air) were utilized for the corresponding reaction. Green solvent, high yields, substrate scope, cost-effectiveness, mild reaction conditions, and ease of handling are certain salient features of the corresponding methodology [164]. A homogeneous catalytic system is frequently used in metal-catalyzed C-C coupling reactions. However, due to some drawbacks of a homogeneous catalytic system, including availability, stability, difficult separation, and recycling, diverted the attention of researchers toward the synthesis of heterogeneous catalytic systems. Nanoscale palladium supported on zinc oxide was formed by a coprecipitation method and characterized by XRD, XPS, SEM, TEM, and thermogravimetric analysis. The application of palladium supported on zinc oxide nanoparticles as a novel heterogeneous catalyst for the formation of unsymmetrical biaryl derivatives by Suzuki-Miyaura and Hiyama coupling reaction was reported by Hosseini-Sarvari and colleagues in 2015. Suzuki-Miyaura cou Scheme 48. Pd-NPs catalyzed synthesis of biaryl derivatives 122 using green solvent. A homogeneous catalytic system is frequently used in metal-catalyzed C-C coupling reactions. However, due to some drawbacks of a homogeneous catalytic system, including availability, stability, difficult separation, and recycling, diverted the attention of researchers toward the synthesis of heterogeneous catalytic systems. Nanoscale palladium supported on zinc oxide was formed by a coprecipitation method and characterized by XRD, XPS, SEM, TEM, and thermogravimetric analysis. The application of palladium supported on zinc oxide nanoparticles as a novel heterogeneous catalyst for the formation of unsymmetrical biaryl derivatives by Suzuki-Miyaura and Hiyama coupling reaction was reported by Hosseini-Sarvari and colleagues in 2015. Suzuki-Miyaura coupling of aryl halides and aryl boronic acid catalyzed by Pd/ZnO nanoparticles afforded biaryl derivatives in an 84-97% yield range. Hiyama coupling reaction of iodobenzene 76 and phenyl trimethoxysilane 8 resulted in 96% yield of biaryl product 92. The reaction was catalyzed by Pd/ZnO nanoparticles using K 2 CO 3 as the base in ethylene glycol under an air atmosphere by maintaining a temperature of 100 • C (Scheme 49). Aryl iodides having electron-rich and poor groups gave the desired products in good to excellent yields. Electron deficient aryl bromides and chlorides resulted in biaryl products in short times (40-70 min) [165]. in 88% yield. The catalytic effect of PS-PdNPs on the Hiyama coupling reaction was observed, and it was noticed that the desired coupled product was not attained, but 4,4 -dimethylbiphenyl 124, the Ullmann coupling product, was afforded in 99% yield (Scheme 50). PS-PdONPs showed higher catalytic efficacy in contrast to PS-PdNPs. The catalyst was retrieved and successively subjected to four runs of cross-coupling reaction [166]. 88% yield. The catalytic effect of PS-PdNPs on the Hiyama coupling reaction was observed, and it was noticed that the desired coupled product was not attained, but 4,4'-dimethylbiphenyl 124, the Ullmann coupling product, was afforded in 99% yield (Scheme 50). PS-PdONPs showed higher catalytic efficacy in contrast to PS-PdNPs. The catalyst was retrieved and successively subjected to four runs of cross-coupling reaction [166]. The Heck and Hiyama coupling reactions clasp a pronounced emplacement due to their implementation in the synthesis of complex organic structures [167][168][169][170]. Focusing on the green methodology, Gaikwad et al. utilized Triton X-100, a non-ionic surfactant, as a stabilizer [171,172] for the generation of palladium nanoparticles and executed an appreciable protocol for the synthesis of symmetrical stilbenes via one-pot sequential Hiyama-Heck coupling reactions. For this purpose, the research group adopted arenediazonium salt Scheme 50. Synthesis of 4-methylbiphenyl 79 using PS-PdONPs as catalyst.
The Heck and Hiyama coupling reactions clasp a pronounced emplacement due to their implementation in the synthesis of complex organic structures [167][168][169][170]. Focusing on the green methodology, Gaikwad et al. utilized Triton X-100, a non-ionic surfactant, as a stabilizer [171,172] for the generation of palladium nanoparticles and executed an appreciable protocol for the synthesis of symmetrical stilbenes via one-pot sequential Hiyama-Heck coupling reactions. For this purpose, the research group adopted arenediazonium salt and triethoxy(vinyl)silane as substrates catalyzed by Triton X-100 stabilized palladium nanoparticles. The arenediazonium tetrafluoroborate 125 was allowed to couple with triethoxy(vinyl)silane 10 to afford the symmetrical trans-stilbene derivative 126 in excellent yield (95%) (Scheme 51). The potency of other surfactants, including cetyltrimethylammonium bromide (CTAB) and sodium dodecyl sulfate (SDS), was examined and found Triton X-100 an appropriate non-ionic surfactant for the respective reaction. The reaction was conducted at room temperature by using Triton X-100 (5 mol%) with Pd(OAc) 2 (2 mol%) in H 2 O [173]. Black pepper (piper nigrum) belongs to the Piperaceae family. Black pepper extract contains a variety of phytochemicals such as piperine, phenols, ethyl piperonyl cyanoacetate, N-isobutyl-tetradeca-2,4-dienamide that help to achieve the reduction of Pd(II) to Pd(0). It is used as an important component in traditional medicines [174]. The formation of green nanocatalyst (Pd NPs) using aqueous ethanolic extract of black pepper was carried out by Kandathil et al. in 2018, and its catalytic activity in the cyanation and Hiyama cross-coupling was observed. The cyanation of aryl halides was carried out in the presence of cyan Scheme 51. Synthesis of symmetrical stilbene derivative 126 using Triton X-100 stabilized Pd-NPs as catalyst.
Black pepper (piper nigrum) belongs to the Piperaceae family. Black pepper extract contains a variety of phytochemicals such as piperine, phenols, ethyl piperonyl cyanoacetate, N-isobutyl-tetradeca-2,4-dienamide that help to achieve the reduction of Pd(II) to Pd(0). It is used as an important component in traditional medicines [174]. The formation of green nanocatalyst (Pd NPs) using aqueous ethanolic extract of black pepper was carried out by Kandathil et al. in 2018, and its catalytic activity in the cyanation and Hiyama cross-coupling was observed. The cyanation of aryl halides was carried out in the presence of cyanating reagent K 4 Fe(CN) 6 . Either electron-donating or electron-withdrawing groups substituted on aryl halides gave moderate to excellent yields. The ligand-free Hiyama cross-coupling reaction of various aryl halides with aryl trimethoxysilane was carried out under fluoride-free conditions to afford the biphenyl derivatives in an 87-98% yield range. The reaction of iodobenzene 76 and phenyl trimethoxysilane 8 proceeded at 100 • C in the presence of NaOH, ethylene glycol, and Pd nanoparticles with 0.2 mol% Pd loading, which catalyzed the reaction efficiently, to obtain the cross-coupling product 92 in 98% yield (Scheme 52) [175].

Pd(PPh3)4 as Catalyst
Cyclopropane motifs have gained huge interest due to their unique features and immense applications in chemical transformations [176,177]. These motifs exist in numerous biologically active natural products and synthetic drugs [178][179][180]. The contribution of the silanol group in the cyclopropanation and Hiyama-Denmark cross-coupling reaction was described by Beaulieu et al. Di

Pd(PPh 3 ) 4 as Catalyst
Cyclopropane motifs have gained huge interest due to their unique features and immense applications in chemical transformations [176,177]. These motifs exist in numerous biologically active natural products and synthetic drugs [178][179][180]. The contribution of the silanol group in the cyclopropanation and Hiyama-Denmark cross-coupling reaction was described by Beaulieu et al. Di-tert-butoxy(cyclopropyl)silanol serves as a substrate for Hiyama-Denmark cross-coupling reaction was synthesized by Simmons-Smith via cyclopropanation reaction of di-tert-butoxy(alkenyl)silanol. (Cyclopropyl)silanol 127 was treated with BF 3 . OEt 2 followed by the reaction with 128, 5 mol% Pd(PPh 3 ) 4 , and TBAF using THF solvent. The reaction worked well at 100 • C to afford the desired cross-coupling product 129 in 92% yield (Scheme 53) [181]. In 2015, the synthesis of unsymmetrical biaryl derivatives through the Hiyama cross-coupling protocol in the presence of Cu(I) and H2O was reported by Delpiccolo and coworkers. Cu(I) salts have been proven to improve the efficacy of palladium-catalyzed cross-coupling reactions [182]. A series of biaryl derivatives was achieved in a 62-98% yield ra Scheme 53. Hiyama-Denmark coupling of (cyclopropyl)silanol 127.
In 2015, the synthesis of unsymmetrical biaryl derivatives through the Hiyama crosscoupling protocol in the presence of Cu(I) and H 2 O was reported by Delpiccolo and coworkers. Cu(I) salts have been proven to improve the efficacy of palladium-catalyzed cross-coupling reactions [182]. A series of biaryl derivatives was achieved in a 62-98% yield range. Maximum yield (98%) of 131 was observed in the case of coupling of 130 with 31 using Pd(PPh 3 ) 4 (0.025 equiv.), TBAF as fluoride source and CuI to improve the catalytic reaction in THF (5% H 2 O). The reaction proceeded well at 80 • C for 18 h (Scheme 54) [183].

Copper Catalyzed Hiyama Cross-Coupling Reactions
Hiyama cross-coupling remained a valuable tool for the construction of carbon-carbon bonds in diversified natural products, pharmaceutical compounds [184][185][186][187][188][189], and complex structures. Hiyama coupling reaction is predominantly conducted with palladium catalysts. In 2013, Gurung et al. reported the first Cu I -catalyzed Hiyama cross-coupling reaction of aryl-and heteroaryl halides with aryl-and heteroaryltriethoxysilane in the presence or absence of PN-1 bidentate ligand. The electron-rich and electron-deficient groups substituted on aryl halides and aryltriethoxysilanes gave moderate to excellent yields. The coupling reaction of heteroaryl triethoxysilanes with aryl iodides in the presence of PN-1 ligand afforded the desired biaryl derivatives in a 40-74% yield range. An excellent result (94%) of 134 was obtained by the addition of 132 to 133 using 10 mol% CuI and CsF in the absence of PN-1 ligand 135. The reaction was conducted in DMF by maintaining the temperature at 120 • C for 24 h (Scheme 55). CsF stabilizes the formation of monomeric [CuAr] intermediate and acts as a fluoride source for the corresponding reaction [190]. source for the corresponding reaction [190].
Allylbenzenes have acquired an eminent interest in organic synthesis due to their pharmacological activities. The research group of Cornelissen carried out the synthesis to afford allylbenzene derivatives via copper-catalyzed Hiyama cross-coupling of vinylalkoxysilanes in the presence of an activating agent, TBAT (tetrabutylammonium difluorotriphe Scheme 55. Hiyama cross-coupling of aryl iodide with heteroaryl triethoxysilane using copper catalyst. Allylbenzenes have acquired an eminent interest in organic synthesis due to their pharmacological activities. The research group of Cornelissen carried out the synthesis to afford allylbenzene derivatives via copper-catalyzed Hiyama cross-coupling of vinylalkoxysilanes in the presence of an activating agent, TBAT (tetrabutylammonium difluorotriphenylsilicate) without using any ligand. Various allylbenzene derivatives were achieved in an 83-99% yield range by reaction of vinyl silane with substituted benzyl halides. The substrate 136 was allowed to couple with 137 to afford the desired product 138 in maximum (99%) yield. The optimization reaction conditions were Cu[MeCN] 4 PF 6 (10%), TBAT, MeCN solvent, 40 • C, 16 h (Scheme 56). The benzylation of various vinylsilanes gave coupling products in a 51-92% yield range. Moreover, Z-alkenes were synthesized using a copper-catalyzed Hiyama cross-coupling reaction. The substrate β-(Z)-vinylsilane 139 was coupled with 140 to acquire the desired Z-alkene 141 in 92% yield under optimized reaction conditions (Scheme 57) [191].

Nickel-Catalyzed Hiyama Cross-Coupling Reaction
Bimetallic nanoparticles have attained substantial attention as an efficient catalytic system for Hiyama cross-coupling reactions. Rothenberg and coworkers synthesized the core-shell Ni

Nickel-Catalyzed Hiyama Cross-Coupling Reaction
Bimetallic nanoparticles have attained substantial attention as an efficient catalytic system for Hiyama cross-coupling reactions. Rothenberg and coworkers synthesized the core-shell Ni-Pd nanoclusters by combining electrochemical and wet chemical methods that efficiently catalyzed Hiyama coupling reactions in contrast to bimetallic alloy clusters and monometallic clusters. Aryl iodides with electron-donating and electron-withdrawing substituents gave substituted biaryl derivatives in a 6->99% yield range. A competent reaction of haloaryls 142, phenyl trimethoxysilane 8, and 143 afforded the corresponding biaryl product 144 in good (>99%) yield along with homocoupling side-product (<2%). The reaction worked efficiently at 65 • C in the presence of core-shell Ni-Pd cluster (1 mol%) catalyst and tetrabutylammonium fluoride (TBAF) under an N 2 atmosphere. THF was utilized as the only solvent for the corresponding reaction (Scheme 58) [192]. The nickel/bathophenanthroline catalyzed Hiyama coupling reaction of unactivated secondary alkyl halides has been recently reported, but it was not found to be an efficient catalyst for activated secondary alkyl halides. The activity of amino alcohol ligand was also observed in the case of the Hiyama coupling reaction. Later, in 2007, the catalytic effect of nickel/norephed Scheme 58. Synthesis of biaryl derivatives 144 using Ni-Pd cluster as catalyst.
The nickel/bathophenanthroline catalyzed Hiyama coupling reaction of unactivated secondary alkyl halides has been recently reported, but it was not found to be an efficient catalyst for activated secondary alkyl halides. The activity of amino alcohol ligand was also observed in the case of the Hiyama coupling reaction. Later, in 2007, the catalytic effect of nickel/norephedrine in Hiyama coupling reactions of several unactivated alkyl halides was reported by Strotman et al. The cyclohexyl iodide 146 was treated with phenyl trifluorosilane 147 to achieve the desired cross-coupling product 148 in 94% yield. The reaction worked efficiently at 60 • C in the presence of 10% NiCl 2 .glyme with 15% norephedrine as a catalyst, using 12% lithium hexamethyldisilazide (LiHMDS) as the base, H 2 O and CsF in DMA (N,N-dimethylacetamide) (Scheme 59). In the case of nickel-catalyzed Hiyama coupling reactions of activated secondary alkyl halides, excellent results were obtained in a 60-92% yield range under the same reaction conditions mentioned above [193]. the case of nickel-catalyzed Hiyama coupling reactions of activated secondary alkyl halides, excellent ained in a 60-92% yield range under the same reaction conditions mentioned above [193]. Dai et al. reported a convenient protocol for Ni-catalyzed asymmetric Hiyama crosscoupling reactions of racemic α-bromo esters. On varying reaction conditions, several α-aryl esters were obtained in a low to excellent yield range (<2-92%) with enantioselectivities (13-99%), respectively. The phenylation of α-bromo ester 149 was carried out with phenyl trimethoxysilane 8 at room temperature using 10% NiCl 2 . glyme, 12% 150, and TBAT in dioxane yielded the corresponding coupling product 151 in 80% yield with 99% ee enantioselectivity (Scheme 60). The addition of substituted aryl silanes to α-bromo esters resulted in 64-76% yields of respective cross-coupling products and 87-94% ee enantioselective values, while α-bromo esters underwent alkenylation through catalytic asymmetric Hiyama coupling reactions afforded the coupling products in 66-72% yield range with 91-93% ee [194]. The environmentally benign Hiyama cross-coupling of tetrafluoroethylene (TFE) and perfluoroarenes via C-F bond activation was developed by Ogoshi and coworkers in 2014. An excellent yield (90%) of 154 was obtained when 152 was treated with 153 using 5 mol% [Ni 2 ( i Pr 2 lm) 4 (cod)] catalyst in tetrahydrofuran at 100 • C for 10 h (Scheme 61). The palladium-catalyzed base-free Hiyama coupling of tetrafluoroethylene with substituted aryl trimethoxysilane yielding α,β,β-trifluorostyrene derivatives in 40-94% yield range. The reaction was carried out using 2.5 mol% Pd 2 (dba) 3 (C 6 H 6 ) as the catalyst, 5 mol% PCyp 3, and 10 mol% FSi(OEt) 3 in THF at 100 • C [195]. tetrahydrofuran at 100 °C for 10 h (Scheme 61). The palladium-catalyzed base-free Hiyama coupling of tetrafluoroethylene with substituted aryl trimethoxysilane yielding α,β,β-trifluorostyrene derivatives in 40-94% yield range. The reaction was carried out using 2.5 mol% Pd2(dba)3(C6H6) as the catalyst, 5 mol% PCyp3, and 10 mol% FSi(OEt)3 in THF at 100 °C [195]. Wang and coworkers carried out the fluoroalkylation of arylsilanes via nickel-catalyzed Hiyama coupling reactions. The arylsilanes bearing electron-rich substituents underwent monofluoroalkylation smoothly to afford excellent yields (74-94%). The arylsilane 155 was coupled with 156 using a catalytic amount of Ni(dme)Cl2 (10 mol%), 157 (12 mol%) CsF as an activator to achieve the desired monofluoroalkylated product 158 in 94% yields. 1,4-Dioxane was the only solvent used for this reaction, and the temperature was Scheme 61. Base free nickel catalyzed Hiyama coupling reaction.
Wang and coworkers carried out the fluoroalkylation of arylsilanes via nickel-catalyzed Hiyama coupling reactions. The arylsilanes bearing electron-rich substituents underwent monofluoroalkylation smoothly to afford excellent yields (74-94%). The arylsilane 155 was coupled with 156 using a catalytic amount of Ni(dme)Cl 2 (10 mol%), 157 (12 mol%) CsF as an activator to achieve the desired monofluoroalkylated product 158 in 94% yields. 1,4-Dioxane was the only solvent used for this reaction, and the temperature was maintained at 80 • C (Scheme 62). Ezetimibe is a cholesterol absorption inhibiting drug [196]. The monofluoroalkylation of ezetimibe-derived arylsilane proceeded under optimized reaction conditions to afford the monofluoroalkylated ezetimibe 159 in 91% yield ( Figure 5) [197].  The palladium-catalyzed decarboxylative coupling reactions have been reported by many research groups in the past. The nickel-catalyzed decarboxylative Hiyama coupling reactions of alkynyl carboxylic acids with organosilanes were introduced for the first time by Raja et al. The reaction of 160 was carried out with phenyl triethoxysilane 16 using a catalytic amount of Ni(acac) 2 (10 mol%), affording the corresponding decarboxylative product 161 in 90% yield along with undesired homocoupling product 162 in low yield. The application of 1,10-phenanthroline as a ligand provided good results by using CsF as an activator of organosilanes and CuF 2 as an oxidant. The reaction worked well in DMSO at 120 • C for 12 h (Scheme 63). Various aryl alkynyl carboxylic acids were allowed to couple with substituted phenyl triethoxysilanes, resulting in decarboxylative products in a 74-90% yield range. A 90% yield of the desired decarboxylative product was also obtained in the case of p-chloro phenyl triethoxysilane. The symmetrical diarylacetylenes were synthesized in a 21-45% yield range under optimized reaction conditions. It was noticed that variation in reaction conditions (in the presence of TEMPO and the absence of CuF 2 , CsF, and Ni ligands) resulted in relatively low yields of corresponding products [198].
The palladium-catalyzed decarboxylative coupling reactions have been reported by many research groups in the past. The nickel-catalyzed decarboxylative Hiyama coupling reactions of alkynyl carboxylic acids with organosilanes were introduced for the first time by Raja et al. The reaction of 160 was carried out with phenyl triethoxysilane 16 using a catalytic amount of Ni(acac)2 (10 mol%), affording the corresponding decarboxylative product 161 in 90% yield along with undesired homocoupling product 162 in low yield. The application of 1,10-phenanthroline as a ligand provided good results by using CsF as an activator of organosilanes and CuF2 as an oxidant. The reaction worked well in DMSO at 120 °C for 12 h (Scheme 63). Various aryl alkynyl carboxylic acids were allowed to couple with substituted phenyl triethoxysilanes, resulting in decarboxylative products in a 74-90% yield range. A 90% yield of the desired decarboxylative product was also obtained in the case of p-chloro phenyl triethoxysilane. The symmetrical diarylacetylenes were synthesized in a 21-45% yield range under optimized reaction conditions. It was noticed that variation in reaction conditions (in the presence of TEMPO and the absence of CuF2, CsF, and Ni ligands) resulted in relatively low yields of corresponding products [198].

Applications of Hiyama Coupling Reactions for the Synthesis of Biologically Active Scaffolds
Retinoids, chemically related to vitamin A, have been the subject of tremendous endorsement due to their activities in biological processes, including cell growth, vision, embryonic development, immune response, and reproduction [199]. The transition metal-catalyzed Suzuki and Stille cross-coupling reactions have been extensively utilized to obtain retinoids. However, drawbacks to the above reactions, including low stability of organoboranes, toxicity, and high molecular weight of organostannanes, diverted the attention of researchers towards the highly efficient Hiyama coupling reaction. The total synthesis of retinoids was first outlined by Montenegro et al., employing the Hiyama coupling reaction as a key step. To the solution of organosilane reagents 164 and 167 in THF, TBAF was added, followed by the addition of trienyl iodide 163 and Pd2(dba)3 . CHCl3 to afford retinyl ethers 165 and 168, which underwent deprotection using TMSCl, H2O, and methanol to attain trans-retinol 166 and 11-cis-retinol 169 in 74% and 83% yields, respectively. The oxidation of 11-cis-retinol in the presence of BaMnO4 in dichloromethane solvent afforded the synthesis of 11-cis-retinal 170 in 90% yield (Scheme 64) [200].

Applications of Hiyama Coupling Reactions for the Synthesis of Biologically Active Scaffolds
Retinoids, chemically related to vitamin A, have been the subject of tremendous endorsement due to their activities in biological processes, including cell growth, vision, embryonic development, immune response, and reproduction [199]. The transition metalcatalyzed Suzuki and Stille cross-coupling reactions have been extensively utilized to obtain retinoids. However, drawbacks to the above reactions, including low stability of organoboranes, toxicity, and high molecular weight of organostannanes, diverted the attention of researchers towards the highly efficient Hiyama coupling reaction. The total synthesis of retinoids was first outlined by Montenegro et al., employing the Hiyama coupling reaction as a key step. To the solution of organosilane reagents 164 and 167 in THF, TBAF was added, followed by the addition of trienyl iodide 163 and Pd 2 (dba) 3 . CHCl 3 to afford retinyl ethers 165 and 168, which underwent deprotection using TMSCl, H 2 O, and methanol to attain trans-retinol 166 and 11-cis-retinol 169 in 74% and 83% yields, respectively. The oxidation of 11-cis-retinol in the presence of BaMnO 4 in dichloromethane solvent afforded the synthesis of 11-cis-retinal 170 in 90% yield (Scheme 64) [200]. Heliannuol A, isolated from sunflower Helianthus annuus [201][202][203][204][205][206], is regarded as the first member of the family of allelopathic [207,208] sesquiterpenoids holding benzoxocane moiety. Heliannuols have attained eminent attention from synthetic chemists due to their possessing unusual structures and biological activity. Vyvyan and coworkers described the synthesis of benzoxocane using trisubstituted Z-styrene derivatives synthesized by Hiyama coupling of oxasilacycloalkenes with aryl iodides. The substituted Z-styrene derivative, obtained by Hiyama coupling of 171 and 172 using Pd2(dba)3 (2-3 mol%) catalyst and tetrabutylammonium fluoride (TBAF) in THF solvent, underwent intramolecular Buchwald-Hartwig etherification using 10 mol% Pd2(dba)3 catalyst and 10 mol% Q-Phos ligands at 80 °C for 24 h. NaOt-Bu was selected as a suitable base in toluene for corresponding etherification reaction to afford the maximum (10%) yield of benzoxocane 174 along with 175 as the major product (Scheme 65). The hydrogenation of benzoxocane 174 using Pd/C in ethanol gave 176 in 44% yield [209]. Heliannuol A, isolated from sunflower Helianthus annuus [201][202][203][204][205][206], is regarded as the first member of the family of allelopathic [207,208] sesquiterpenoids holding benzoxocane moiety. Heliannuols have attained eminent attention from synthetic chemists due to their possessing unusual structures and biological activity. Vyvyan and coworkers described the synthesis of benzoxocane using trisubstituted Z-styrene derivatives synthesized by Hiyama coupling of oxasilacycloalkenes with aryl iodides. The substituted Z-styrene derivative, obtained by Hiyama coupling of 171 and 172 using Pd 2 (dba) 3 (2-3 mol%) catalyst and tetrabutylammonium fluoride (TBAF) in THF solvent, underwent intramolecular Buchwald-Hartwig etherification using 10 mol% Pd 2 (dba) 3 catalyst and 10 mol% Q-Phos ligands at 80 • C for 24 h. NaOt-Bu was selected as a suitable base in toluene for corresponding etherification reaction to afford the maximum (10%) yield of benzoxocane 174 along with 175 as the major product (Scheme 65). The hydrogenation of benzoxocane 174 using Pd/C in ethanol gave 176 in 44% yield [209]. The synthesis of biologically relevant benzofuran scaffolds by a convenient route is of huge interest because benzofuran compounds account for important bioactive molecules, including amiodarone [210], BNC105 [211], cytotoxic flavonoids [212], and natural products such as egonol [213,214], daphnodorin A and B [215] and moracin O and P [216,217]. The application of palladium(II) acyclic diaminocarbene (ADC) complexes in one-pot tandem Hiyama alkynylation/cyclization for synthesizing biologically relevant benzofuran derivatives w The synthesis of biologically relevant benzofuran scaffolds by a convenient route is of huge interest because benzofuran compounds account for important bioactive molecules, including amiodarone [210], BNC105 [211], cytotoxic flavonoids [212], and natural products such as egonol [213,214], daphnodorin A and B [215] and moracin O and P [216,217]. The application of palladium(II) acyclic diaminocarbene (ADC) complexes in one-pot tandem Hiyama alkynylation/cyclization for synthesizing biologically relevant benzofuran derivatives was disclosed by Singh et al. The palladium ADC complexes were synthesized via nucleophilic addition of secondary amines such as morpholine, pyrrolidine, and piperidine to metal precursor cis-{(2,4,6(CH 3 ) 3 C 6 H 2 )NC} 2 PdCl 2 at room temperature that efficiently catalyzed synthesis of benzofuran derivatives. Iodophenol and triethoxysilylalkynes underwent Hiyama alkynylation followed by cyclization catalyzed by Pd ADC complex to afford benzofuran derivatives in (14-57%) yield. Excellent result (57%) of benzofuran derivative 180 was obtained by Hiyama alkynylation/cyclization of 177 and 178 using 2 mol% palladium ADC complex 179 as catalyst and NaOH as the base. The reaction proceeded in the 4:2 mixture of 1,4-dioxane/H 2 O by maintaining the temperature at 80 • C for 4 h. The Hiyama cross-coupling of iodobenzene with triethoxysilylalkynes catalyzed by palladium ADC complex 179 gave alkyne derivatives in (35-76%) yield range using optimized reaction conditions (Scheme 66) [218]. Triflate derivatives have remained an area of interest for researchers due to their portfolio in catalytic transformation, i.e., one-pot tandem Heck alkynylation/cyclization reactions [219], and in biomedical applications as potent anticancer agents [220]. The utility of triflate derivatives of palladium acyclic diaminocarbene (ADC) complexes as Scheme 66. Synthesis of benzofuran derivative 180 via Hiyama alkynylation followed by cyclization.

Miscellaneous
The Hiyama coupling of unactivated alkyl bromides and iodides in mild conditions was achieved by Lee et al. A series of cross-coupled products were achieved in a 65-81% yield range by Hiyama coupling of functionalized alkyl bromides with phenyl trimethoxysilanes using PdBr 2 catalyst, P(t-Bu) 2 Me and Bu 4 NF. The reaction was carried out in THF at room temperature, while in the case of [HP(t-Bu) 2 Me]BF 4 , 42-88% yields of crosscoupled products were achieved. The 183 was allowed to couple with 184 using 4% PdBr 2 /P(t-Bu) 2 Me and Bu 4 NF in the presence of THF solvent at room temperature to afford the desired coupling product 185 in 84% yield (Scheme 67). The same Hiyama coupling reaction also proved to be effective for functionalized alkyl iodides [222]. The Hiyama coupling of unactivated alkyl bromides and iodides in mild conditions was achieved by Lee et al. A series of cross-coupled products were achieved in a 65-81% yield range by Hiyama coupling of functionalized alkyl bromides with phenyl trimethoxysilanes using PdBr2 catalyst, P(t-Bu)2Me and Bu4NF. The reaction was carried out in THF at room temperature, while in the case of [HP(t-Bu)2Me]BF4, 42-88% yields of cross-coupled products were achieved. The 183 was allowed to couple with 184 using 4% PdBr2/P(t-Bu)2Me and Bu4NF in the presence of THF solvent at room temperature to afford the desired coupling product 185 in 84% yield (Scheme 67). The same Hiyama coupling reaction also proved to be effective for functionalized alkyl iodides [222]. Alacid and Nájera reported the NaOH promoted Hiyama coupling reactions of aryl halides with vinyltrialkoxysilanes using fluoride-free conditions. The Hiyama coupling of vinyltrimethoxysilane with aryl bromides and chlorides afforded styrene derivatives in 47-99% yields. The reaction of 4-bromo acetophenone 11 was carried out with vinyltrimethoxysilane 50 using 4-hydroxyacetophenone oxime-derived palladacycle (0.1 mol% Pd), 2.5 equivalent NaOH promotor, and 1 equivalent of tetra-n-butylammonium bromide (TBAB) additive under microwave irradiation for 10 min. In the case of styryltriethoxysilane, stereospecific coupling with aryl or vinyl bromides provided the corresponding stilbenes or dienes, respectively. Moreover, the undesirable polymerization of products is prevented by using mild reaction conditions in this protocol (Scheme 68) [223]. chlorides afforded styrene derivatives in 47-99% yields. The reaction of 4-bromo acetophenone 11 was carried out with vinyltrimethoxysilane 50 using 4-hydroxyacetophenone oxime-derived palladacycle (0.1 mol% Pd), 2.5 equivalent NaOH promotor, and 1 equivalent of tetra-n-butylammonium bromide (TBAB) additive under microwave irradiation for 10 min. In the case of styryltriethoxysilane, stereospecific coupling with aryl or vinyl bromides provided the corresponding stilbenes or dienes, respectively. Moreover, the undesirable polymerization of products is prevented by using mild reaction conditions in this protocol (Scheme 68) [223]. Bhaumik and coworkers synthesized the palladium containing periodic mesoporous organosilica (PMO) and analyzed their catalytic efficacy in Hiyama coupling, Sonogashira coupling, and cyanation reaction. Despite Sonogashira coupling and cyanation reaction, the focus was being placed on environment-friendly Hiyama coupling to achieve unsymmetrical biphenyls. The substituted benzonitriles and disubstituted alkynes were achieved in 68-95%, and 72-90% yield ranges, respectively. Hiyama coupling reaction proceeded under green conditions, using a Pd-containing (PMO) catalytic system giving 60-95% yield of the respective coupled products. The best results were obtained in the case of coupling reaction of p-iodonitrobenzene 7 with trimethoxy(vinyl)silane 50 to attain a 95% yield of the targeted product 187. The essential parameters utilized in the reaction comprised of Pd-LHMS-3 catalytic system and NaOH at 100 • C in water (Scheme 69). The excellent yields of products, reusability, and easy work-up made Pd-grafted PMO an effective catalyst for synthesizing benzonitrile derivatives, disubstituted alkynes, and unsymmetrical biphenyls [224].
ction proceeded under green conditions, using a Pd-containing (PMO) catalytic system giving espective coupled products. The best results were obtained in the case of coupling reaction of with trimethoxy(vinyl)silane 50 to attain a 95% yield of the targeted product 187. The essential the reaction comprised of Pd-LHMS-3 catalytic system and NaOH at 100 °C in water (Scheme lds of products, reusability, and easy work-up made Pd-grafted PMO an effective catalyst for trile derivatives, disubstituted alkynes, and unsymmetrical biphenyls [224]. The catalytic activity of thermally stable and oxygen insensitive dimeric ortho-palladated complex [Pd{C6H4(CH2N(CH2Ph)2)}(μ-Br)]2 homogeneous catalysts [225] in Hiyama coupling was investigated by Hajipour and colleagues. The substituted biphenyls were obtained in a 68-97% yield range along with homocoupling products. Maximum yield (97%) of 92 was observed in the case of coupling of aryl halides (bromides and iodides) 188 with phenyl tr The catalytic activity of thermally stable and oxygen insensitive dimeric ortho-palladated complex [Pd{C 6 H 4 (CH 2 N(CH 2 Ph) 2 )}(µ-Br)] 2 homogeneous catalysts [225] in Hiyama coupling was investigated by Hajipour and colleagues. The substituted biphenyls were obtained in a 68-97% yield range along with homocoupling products. Maximum yield (97%) of 92 was observed in the case of coupling of aryl halides (bromides and iodides) 188 with phenyl triethoxysilane 16 using efficient [Pd{C 6 H 4 (CH 2 N(CH 2 Ph) 2 )}(µ-Br)] 2 catalysts under microwave irradiation at 90 • C and 500W. Among different solvents (THF, DMF, Dioxane, EtOH, MeOH, p-Xylene), DMF was screened to be a microwave-active polar solvent and TBAF . 3H 2 O as an efficient additive to improve the yield within short times (Scheme 70) [226].
iethoxysilane 16 using efficient [Pd{C6H4(CH2N(CH2Ph)2)}(μ-Br)]2 catalysts under microwave irradiation at 90 °C and 500W. Among different solvents (THF, DMF, Dioxane, EtOH, MeOH, p-Xylene), DMF was screened to be a microwave-active polar solvent and TBAF . 3H2O as an efficient additive to improve the yield within short times (Scheme 70) [226]. Arylsulfonyl chlorides have been used for many years in manufacturing pesticides, dyes, drugs, polymers [227,228], etc., due to their affordability and ease of availability. A novel method for the formation of unsymmetrical biaryls using arylsulfonyl hydrazides by Hiyama cross-coupling was developed by Miao et al. The arylsulfonyl hydrazides containing electron-rich and -poor groups smoothly underwent cross-coupling with phenyl trimethoxysilane providing a series of biaryl derivatives in a 72-95% yield range. The coupling of 193 with phenyl trimethoxysilane 8 catalyzed by 5 mol% Pd(TFA) 2 gave the desired biaryl product 66 in 95% yield under desulfitative and denitrificative reaction. The reaction worked efficiently in DMI solvent using a TBAT (tetrabutylammoniumdifluorotriphenylsilicate) activator at 60 • C for 12 h under an O 2 atmosphere (Scheme 72). Between 74-94% of yields of desired cross-coupled products were obtained by the addition of phenylsulfonyl hydrazide with a wide range of aryl trimethoxysilanes under optimized conditions [230]. roduct 66 in 95% yield under desulfitative and denitrificative reaction. The reaction worked efficiently in DMI solvent using a TBAT (tetrabutylammoniumdifluorotriphenylsilicate) activator at 60 °C for 12 h under an O2 atmosphere (Scheme 72). Between 74-94% of yields of desired cross-coupled products were obtained by the addition of phenylsulfonyl hydrazide with a wide range of aryl trimethoxysilanes under optimized conditions [230]. In 2016, the catalysis of Suzuki-Miyaura and Hiyama-Denmark coupling of aryl sulfamates using (1-t Bu-indenyl)Pd(L)(Cl) precatalysts was reported by Hazari and coworkers. Hiyama-Denmark coupling of substituted aryl silanolates and aryl chlorides afforded the desired coupled products in maximum yields (85-97%). A total of 2.5 mol% Pd-P t Bu 3 precatalysts, toluene, 70 • C, and 4 h were the standard reaction conditions (Scheme 73). The same research group described the first Hiyama-Denmark coupling of aryl sulfamates using Pd-RuPhos precatalyst. Several additives (NaOMe, NaO t Bu, and RuPhos) were screened to improve the yields of Hiyama-Denmark reactions. An excellent result (91%) was obtained in the case of coupling of 197 with 198 in the presence of 5 mol% Pd-RuPhos precatalyst and 5 mol% RuPhos as an additive in toluene at 110 • C for 8 h (Scheme 74) [231]. The synthesis of valuable biarylboronates via photochemical gold-catalyzed Hiyama redox-neutral arylation of mechanistically preferential B,Si-bimetallic coupling reagents with substituted diazonium salts was reported by Hashmi and coworkers. The scope of Hiyama coupling with respect to boronic acid derivatives was investigated. It was observed that boronic acid The synthesis of valuable biarylboronates via photochemical gold-catalyzed Hiyama redox-neutral arylation of mechanistically preferential B,Si-bimetallic coupling reagents with substituted diazonium salts was reported by Hashmi and coworkers. The scope of Hiyama coupling with respect to boronic acid derivatives was investigated. It was observed that boronic acid derivatives (BMIDA, BPin, and Bnep) with substituted aryl silanes afforded desired cross-coupled products in reasonable (49-86%), (40-65%) and (42-60%) yield ranges, respectively. Excellent output (91%) of desired biofunctionalized biarylboronate derivative 202 was attained in case of coupling of B,Si-bimetallic reagent 201, and 4-CF 3 substituted aryldiazonium salt 200 using a 10 mol% Ph 3 PAuNTf 2 catalyst in acetonitrile solvent under irradiation of blue LEDs at room temperature (Scheme 75) [232]. A very interesting green procedure of Hiyama cross-coupling using a highly stable NCN-pincer palladium complex as catalyst was disclosed by Marset et al. The catalytic efficacy of the NCN-pincer palladium complex was assessed during the coupling of aryl iodides and bromides and phenyl trimethoxysilane. The aryl bromides and iodides with electron-donating or electron-withdrawing groups gave moderate to excellent yields. The NCN-pincer palladium complex catalyzed coupling of aryl iodides and bromides with phenyl trimethoxysilane in glycerol gave expected biaryl products in 40-99% yields while a 1:2 mixture of choline chloride:glycerol (ChCl:glycerol) facilitated the synthesis of targeted biaryl products in 1-85% yields. Maximum yield (99%) of biphenyl product 92 was obtained in the case of coupling phenyl iodide 76 with phenyl trimethoxysilane 8 using 1 mol% NCN-pincer palladium-catalyzed catalyst 203 in 0.5 M glycerol at 100 • C for 24 h. K 2 CO 3 was screened as a suitable base for the corresponding reaction (Scheme 76). Isomerization was observed in the case of reaction with allyl trimethoxysilane. A mixture of isomers was formed, affording a highly stable internal trans double bond containing a major product in 83% yield. The recyclability of catalysts and the usage of biorenewable solvents are the salient features of this methodology [233]. Sobhani and coworkers synthesized a hydrophilic heterogeneous cob supported on chitosan [mTEG-CS-Co-Schiff-base]. The catalyst was identifi TEM, FT-IR, TGA, ICP, FE-SEM, and XPS analyses, and its implication as a ous catalyst in Hiyama, Heck, Suzuki, and Hirao cross-coupling was invest application of the mTEG-CS-Co-Schiff base in fluoride-free Hiyama coupli under green reaction conditions was reported for the first time. A series of bi tives having electron releasing and electron-withdrawing groups on aryl h synthesized in a good to excellent yield range (80-98%). The coupling react benzene 76 with triethoxyphenylsilane 16 worked well under green conditio mol% mTEG-CS-Co-Schiff-base (catalyst) 204 and NaOH as a base. Conse excellent (98%) yield of desired biaryl product 92 was obtained (Scheme 77). tion of water as a green solvent, recovery, and scalability of catalyst for at le cessful runs without losing catalytic activity and inexpensive abundant cob make this reaction an environmentally and economically friendly protocol [23 Sobhani and coworkers synthesized a hydrophilic heterogeneous cobalt catalyst supported on chitosan [mTEG-CS-Co-Schiff-base]. The catalyst was identified by XRD, TEM, FT-IR, TGA, ICP, FE-SEM, and XPS analyses, and its implication as a heterogeneous catalyst in Hiyama, Heck, Suzuki, and Hirao cross-coupling was investigated. The application of the mTEG-CS-Co-Schiff base in fluoride-free Hiyama coupling reaction under green reaction conditions was reported for the first time. A series of biaryl derivatives having electron releasing and electron-withdrawing groups on aryl halides were synthesized in a good to excellent yield range (80-98%). The coupling reaction of iodobenzene 76 with triethoxyphenylsilane 16 worked well under green conditions using 0.5 mol% mTEG-CS-Co-Schiff-base (catalyst) 204 and NaOH as a base. Consequently, an excellent (98%) yield of desired biaryl product 92 was obtained (Scheme 77). The utilization of water as a green solvent, recovery, and scalability of catalyst for at least six successful runs without losing catalytic activity and inexpensive abundant cobalt catalyst make this reaction an environmentally and economically friendly protocol [234]. The comparison of catalysts reported in the section above indicates that among the palladium-catalyzed reaction, several Pd based catalysts afforded the products in quantitative yields (Table 1, entries 1, 2, 3 , 4, 5, 7, 11, 12, 22, 23, 24, 25, 40 and 41) [ [53][54][55][56]64,81,84,[95][96][97]104,141].

Conclusions
In conclusion, we have reviewed a large number of strategic approaches published about Hiyama cross-coupling reactions during the last 15 years. Moreover, this review article highlights all major advances in Hiyama coupling reaction regarding different catalytic systems N-heterocyclic carbene (NHC)-Pd complexes, palladium-supported nickel catalysts, copper iodide, nanoparticles as catalysts, etc.), the effect of different substituents on product yields, enantioselectivity, and applications in the synthesis of valuable scaffolds. Transition metal-catalyzed coupling reactions have been emerging as efficient and reliable methodologies to realize the synthesis of a range of products. Transition metal-catalyzed Hiyama reaction is also a coupling reaction yielding scaffolds with potential applications in pharmaceutical and chemical industries. Although a number of methodologies highlighting different transition metal-based catalysts at different catalyst loadings have been reported in recent years, the area needs to be explored more and unbox certain limitations. Several nanocatalysts have been developed that realize the synthesis more efficiently due to higher surface-to-volume ratio and contact ratio with substrates; however, more efficient catalysts with environmentally benign methodologies, mild reaction conditions, and broad substrate scope are desired. Along with the monometallic catalytic systems, the focus could be placed on the development of bi-metallic and tri-metallic catalysts. The nanocatalysts are needed to be explored to develop cheap, reliable, and most efficient catalysts. The substrate scope of the reaction should be extended for the use of aryl chloride along with aryl bromide/iodide as coupling partners. We believe that this review will be a significant contribution to increasing the research interest in the respective field.