Synthesis, Properties Characterization and Applications of Various Organobismuth Compounds

Organobismuth chemistry was emphasized in this review article due to the low price, low toxicity and low radioactivity characteristics of bismuth. As an environmentally-friendly class of organometallic compounds, different types of organobismuth compounds have been used in organic synthesis, catalysis, materials, etc. The synthesis and property characterization of many organobismuth compounds had been summarized. This review article also presented a survey of various applications of organobismuth compounds in organic transformations, as reagents or catalysts. The reactivity, reaction pathways and mechanisms of reactions with organobismuths were discussed. Less common and limiting aspects of organobismuth compounds were also briefly mentioned.


Introduction
Organometallic compounds such as organoboron, organotin, organosilicon, organoantimony, organolead, etc. have been widely applied in organic synthesis in the past several decades [1][2][3]. A number of reports [4][5][6][7][8] on these compounds have mentioned their positive role in a variety of synthetic transformations or polymerizations, acting as reagents or catalysts. Bismuth is relatively low-cost, easily available, and it presents the lowest toxicity among the heavy non-radioactive main group elements [9,10]. These attractive features, combined with the Green Chemistry principles, have made many inorganic OPEN ACCESS and organic derivatives of bismuth broadly and safely used in organic synthesis, catalysis, materials and even medicine [11][12][13][14][15][16]. Hence, organobismuth compounds have emerged as an important class of organometallic compounds that have seen increased applications in different areas over the last 20 years [8,17], as organobismuth chemistry has developed rapidly and some novel organobismuth compounds have been reported [18][19][20][21]. Many types of organobismuth compounds have been proven to be efficient reagents in C-, N-and O-arylation reactions [9], benzoylation reactions [22], cross-coupling reactions [10], etc. However, the organometallic chemistry of bismuth has developed relatively slowly compared to that of other main group metals due to the instability of the bismuth carbon bond [23,24]. Up till now, the exploration and information on the chemistry of organobismuth species has been relatively limited [25,26]. For example, application of organobismuth compounds as reagents to C-C bond formations was relatively scarce [1,10]. The poor σ-donor power and the reactive Bi-C bonds which were fissile in the presence of some metals resulted in few studies on bismuthine ligands [27].
Moreover, examples where organobismuth compounds acted as catalysts in organic synthesis were even less numerous [28].
In view of the fact that organobismuth compounds have been attracting growing attention to their utilization in various fields with promising novel reactivities, more work is needed in this field, including the design of new organobismuth compounds. Previous review articles written by Gilman et al. [29], Finet et al. [30], Suzuki et al. [31], Zhang et al. [32], Jiang et al. [33] were already devoted to the chemistry of organobismuth compounds. Hence, this review article was focused specifically on the chemistry of organobismuth compounds, presenting a summary on the synthesis, structural characterization, catalytic and reaction mechanisms and reaction pathways of new organobismuth compounds. The applications of different types of organobismuth compounds in organic synthesis, catalysis, materials science, etc. have also been summed up in this review article.

. Organobismuth(III) and Organobismuth(V) Complexes
Hassan and co-workers reported their efforts on the synthesis of bismuth complexes containing functional groups in the early exploration period of organobismuth chemistry [25]. Two types of organobismuth complexes, bismuth(III) complexes (BiAr 3 ) and bismuth(V) complexes (BiAr 3 L 2 ) where Ar was a phenyl derivative containing a donor functional group, were prepared and characterized.
The model complex Bi[p-C 6 H 4 (NMe 2 )] 3 (1) which was previously reported with poor yield by Gilman and Yablunky was newly synthesized by a modified procedure. Taking compound 1 for example, the typical preparation procedure of these compounds is depicted below in Scheme 1. Compound 1 was obtained with a pyramidal geometry which was similar to that of BiPh 3 . Several bismuth(V) derivatives of compounds 1 and Bi{p-C 6 H 4 -[CH 2 N(2-Py) 2 ]} 3 (2) were synthesized via chlorine oxidation of 1 and 2 and the subsequent substitution reactions. Among above newly synthesized complexes, compound 2 was the first example of a tertiary organobismuth(III) complex containing a chelating functional group while compound Bi{p-C 6 H 4 [CH 2 N(2-Py 2 )]} 3 (O 2 CCH 3 ) 2 (3) was the first example of a tertiary bismuth(V) complex containing the bidentate dipyridyl functional group. Scheme 1. Preparation procedure of Bi[p-C 6 H 4 (NMe 2 )] 3 (1) [25].

Mixed Halophenylbismuthates(III)
Mixed halophenylantimonates(III) with Lewis acid character had been successfully prepared by Sharma et al. [34], who in the same way synthesized mixed halophenylbismuthates(III) [35]. The formulae of these complexes could be written as Bu 4 N[PhBiX 2 Y] where X = Cl or Br, Y = Cl, Br or I, X ≠ Y. The geometry around bismuth in the Bu 4 N[PhBiCl 2 Br] anion (4) was square pyramidal and the molecule existed as a dimer. In the structure of compound 4, the phenyl group was at the axial position and the four halogen atoms formed the basal plane of the square pyramid for [PhBiCl 2 Br] − . The complexes were prepared by cocrystallization of 1:1 mixtures of Bu 4 NY and PhBiX 2 from anhydrous methanol where X ≠ Y. The crystals of compound 4 were obtained by solvent diffusion from 1:2 CH 2 Cl 2 -ethanol mixtures at −5 °C over a period of two days while the crystals of Bu 4 N[PhBiBr 2 I] (5) were obtained from slow evaporation of its dichloromethane solution.

Pentavalent Biphenyl-2,2'-ylenebismuth Derivatives
Pentacoordinate biphenylylbismuth derivatives, especially the diacetate, which could perform various types of aryl transfer reactions, were investigated by the Fedorov group [36]. The target compound which was identified as (biphenyl-2,2'-ylene)phenylbismuth diacetate (6) was obtained under mild conditions with a simple treatment of diacetate (Scheme 2) by a modification of the method of Wittig and Hellwinkel [37].

Scheme 2.
Reactions and conditions for the synthesis of phenylbiphenyl-2,2'-ylenebismuth diacetates (6)  The (relatively stable) structure including two aryl groups in a heterocyclic bismuth substructure was prepared based on the concept of ligand coupling in heteroaromatic compounds. Structures of this type might enhance the reactivity of biphenylylbismuth derivatives in reactions with different nucleophiles under different conditions. It was reported that the nature of the two extra ligands in pentavalent triarylbismuth derivatives determined the type of reactivity [38].
A similar synthetic route (Scheme 3) had already been applied in the synthesis of triarylbismuth derivatives as early as 1999 [38]. Allylation of the phenolic group, metalation with butyllithium followed by treatment with bismuth chloride afforded the trivalent compound 7. Scheme 3. Synthetic route for triarylbismuth derivatives [38]. Although tris(2-methoxyphenyl)bismuth was oxidized to the corresponding diacetate in relatively good yields by iodobenzene diacetate under neutral conditions, compound 7 was recovered unaffected. Oxidation of the trivalent bismuth compound 7 was accomplished by reaction with sodium perborate in acetic acid, which led to the diacetate 8 in 63% yield. These organobismuths were found to be efficient reagents in the three major types of arylation reactions (C-, N-and O-arylation).

Ladder-Type Organobismuth Compounds
In order to construct new organobismuth compounds by taking the advantage of the combination of multiple inter-and intramolecular Bi-O interactions [39], a novel structure which presented a ladder-type was achieved by Uchiyama and co-workers [40]. Three 1,3,2,4-dioxadibismetane rings successfully comprised this special framework by both inter-and intramolecular Bi-O interactions.
These novel compounds were prepared by the procedure shown in Scheme 4. By using a method similar to the synthesis of benzoxabismole [41], the benzoxabismole R f Bi(C 6 H 4 -4-CH 3 ) (R f = -C 6 H 3 -5-C(CH 3 ) 3 -2-C(CF 3 ) 2 O-) (9) with the Martin ligand was prepared first. After that, the dibismuth oxide (R f Bi) 2 O 10 was obtained from 9 on standing in CDCl 3 -D 2 O or C 6 D 6 -D 2 O in an NMR tube. By analysis of the GC-MS and 1 H-NMR spectra of the reaction mixture, researchers concluded that hydrolysis of 6-tert-butyl-1-p-tolylbenzoxabismole (9) followed by self-condensation of hydroxybismuthine 11 resulted in the corresponding dibismuth oxide 10.

t-Bu
Bi In Scheme 4, it could be seen that two independent molecules of 10 constituted a dimeric structure, and in each independent molecule of 10, one oxygen atom of the Martin ligand coordinated to the bismuth atom intramolecularly to form Bi-O bonds. X-ray crystallographic analysis showed that the Bi-O intramolecular distances were somewhat longer than the Bi-O intermolecular distances. The strong Bi-O intramolecular coordination was the first example of such intramolecular coordination of the oxygen atom of the Martin ligand. These intramolecular Bi-O interactions constructed two 1,3,2,4-dioxadibismetane rings on both sides of the four-membered ring, eventually providing a ladder type framework.
Scheme 5. Synthesis of (acylimino)triaryl-λ 5 -bismuthanes [42]. The structure and reactivity comparisons were made between (acylimino)triaryl-λ 5 -bismuthane 12 and a series of (acylimino)pnictoranes. Structural observation revealed that the Bi=N bond of 12 possessed a highly polarized single-bond character, probably due to the differences in orbital size and electronegativity between the bismuth and nitrogen atoms. Thermal stabilities of 12 relied on the ortho-substituted aryl ligands and the electron-withdrawing N-substituents which also afforded kinetic stabilization to the reactive Bi=N bond. Chemical behavior investigations indicated that compound 12 possessed remarkable oxidizing and nitrene-transfer abilities and nucleophilicity, showing potential utility in organic synthesis. This might be attributed to the good leaving ability of the bismuthonio group and the highly polarized character of the Bi=N bonding as well.

Fluorotetraphenylbismuth
Tetraphenylbismuth(V) compounds were previously reported to be useful reagents for organic synthesis [45]. However, these compounds were instable because of their weakly coordinating (less nucleophilic) anions. It was reported that fluorine-containing bismuth compounds such as pentafluorophenylbismuth(V) derivatives [46] were strong arylating reagents. It is worth mention that fluorotetraphenylbismuth (14) was the first synthesized thermally stable example, taking advantage of its unique amphiphilic property of possessing both nucleophilic (fluorine atom) and electrophilic (phenyl group) moieties on the central bismuth [47]. It was found to be efficient reagent for regioselective α-phenylation. Fluorotetraphenylbismuth (14) was obtained through the reactions depicted in Scheme 7. In the molecular structure of compound 14, the bismuth center adopts a distorted trigonal bipyramidal geometry with three ipso carbons at the equatorial sites and one ipso carbon and fluorine atom at the apical sites.

Triarylbismuth(V) Di(N-p-toluenesulfonyl)aminoacetates
Yu et al. [16] synthesized a series of triarylbismuth(V) di(N-p-toluenesulfonyl)aminoacetates to study their cytotoxicity and the influence of amino acid ligands at bismuth on their antitumor activity. Compounds 15-18 were synthesized under mild conditions and were formulated as (4-CH 3 C 6 H 4 SO 2 NHCH 2 CO 2 ) 2 BiAr 3 (Ar = Ph, 4-CH 3 C 6 H 4 , 4-ClC 6 H 4 , 4-BrC 6 H 4 ). Ar 3 BiCO 3 (0.5 mmol) was added to a boiling solution of N-p-toluenesulfonylaminoacetic acid (1 mmol) in acetone (50 mL). Subsequently, the mixture was refluxed for 4 h, cooled and filtered. The starting materials such as N-p-toluenesulfonylaminoacetic acid and Ar 3 BiCO 3 were prepared beforehand by the methods reported in the prior literature [48,49]. The solid which was obtained was recrystallized from CH 2 Cl 2 -hexane, to give stable and colorless crystalline solids. The crystal structure of compound 17 showed the bismuth atom existing in distorted trigonal bipyramidal geometry. The bismuth moiety and the nature of the aryl played a positive role on the cytotoxicity of the compounds.

Cyclic Organobismuth(III) Chlorides and their Triphenylgermylpropionate Derivatives
Zhang et al. synthesized six cyclic hypervalent organobismuth (III) chlorides and triphenylgermylpropionates containing a nitrogen or sulfur atom as intramolecular coordination atom [15]. These cyclic organobismuth compounds showed good antiproliferative activities. The synthetic routes for organobismuth chloride 19 and organobismuth triphenylgermylpropionate 20 are shown in Scheme 8. Compound 20 was obtained through the reaction of 19 with triphenylgermylpropionic acid in the presence of NaOH (as neutralizing agent) and THF-H 2 O (as solvent) under the protection of a N 2 atmosphere. Scheme 9 presents the synthetic routes for compounds 21 that bear a nitrogen atom. Scheme 8. Synthetic route for organobismuth chloride 19 and organobismuth triphenylgermylpropionate 20 [15]. The eight-membered tetrahydroazabismocine rings were revealed to be highly flexible from the results of single-crystal X-ray analysis. The way the substituted groups acted on the Bi, S or N atom determined the Bi-S or Bi-N bond lengths in the corresponding thiabismocine or azabismocine derivatives. Moreover, the lengthening of Bi-N and Bi-S bond was directly affected by the replacement of Cl atom in azabismocine and thiabismocine with the triphenylgermylpropionic group. The substituents which were connected with the nitrogen atom also had an effect on the Bi-N bond length of azabismocine. Good antiproliferative activities might be related to a suitable coordination ability of Bi 3+ and the introduction of an organogermanium group.

Resin-Bound Triaryl Bismuthanes
Intensive research efforts had been devoted to the study of triorganylbismuth reagents in organic synthesis, and discovery of multidirectional linker strategies for increasing the flexibility in solid-phase organic synthesis (SPOS) had caught the attention of numerous researchers in recent years. Rasmussen and co-workers [50] considered that the combination of the versatile chemistry of triarylbismuthanes coupled with the advantages of solid-phase chemistry would provide a powerful tool for solid-phase and solution-phase synthesis. Thus, they first reported that resin-bound bismuth constituted a novel arylation reagent and its utilization as part of a multidirectional linker system in SPOS. Their method allowed a simple attachment of bismuthanes without polymerization starting from commercially available chloromethylpolystyrene. Furthermore, sufficient discrimination between the two rather similar Bi-sp 2 C bonds A and B which were shown in Figure 1 was achieved. That was a key problem. Cleavage of bond A would result in lower yield and contamination of the products, thus it was crucial to cleave bond B selectively. Scheme 10 reveals the synthetic route for the resin-bound triarylbismuthanes. Resin 23 was prepared by reaction of 4-iodophenol (22) with commercially available chloromethylpolystyrene. In order to improve the required discrimination between the two different types of Bi-sp 2 C bonds in the cleavage step, a phenoxy group was chosen as spacer for the bismuth atom. The resin-bound aryl Grignard compound 24 was prepared from resin 23 by iodomagnesium exchange using isopropyl-magnesium bromide. In order to obtain resin-bound bismuthanes, a suitable bismuth electrophile was needed to be coupled with resin 24. Ar 2 BiOTf was chosen out of three potential electrophilic bismuth reagents which were BiCl 3 , Ar 2 BiCl, and Ar 2 BiOTf. Three different resin-bound triaryl bismuthanes 25a-c were prepared from resin 24 with loadings from 0.9 to 1.1 mmol/g. Resins 25a-c were subsequently oxidized quantitatively with diacetoxy iodobenzene to the resin-bound triaryl bismuth(V) diacetates 26a-c. Another resin-bound triarylbismuthane 27 was also prepared (Scheme 11) for application in multistep solid-phase organic synthesis [51]. Novel chemoselective cross-coupling reactions with resin-bound triarylbismuthane gave substituted biphenyls in good yield by traceless and multidirectional cleavage of unsymmetrical biphenyls. In addition, both aryl groups of the bismuth linker could be utilized in product formation. Scheme 11. Synthetic route for the resin-bound triarylbismuthanes [51].

Cyclopropylbismuth
In consideration of the unique spatial and electronic properties of the cyclopropyl group, Gagnon et al. developed a new cyclopropylbismuth reagent which was capable of N-cyclopropylating cyclic amides, azoles, and related derivatives [52]. The cyclopropylbismuth compound was prepared in the form of colorless oil, however, compared to other alkylbismuth reagents it was found to be nonpyrophoric. The novel compound, named tricyclopropylbismuth (28a) was produced by the addition of cyclopropyl magnesium bromide to bismuth trichloride (Scheme 12). Subsequently the reaction mixture was transferred over brine and the resulted organic layer was evaporated under argon. Mass spectroscopy experiments detected not only 28a, but also other minor cyclopropylbismuth species such as 28b.

Tris(polymethoxyphenyl)bismuth Derivatives
Finet et al. investigated the synthesis of tris(polymethoxyphenyl)bismuth derivatives and their utilization in C-, N-and O-arylation reactions [9]. These bismuth compounds were prepared with electron-rich aryl groups (polymethoxyaryl fragments) which could be effectively transferred in arylation reactions, breaking the commonly accepted stereotype that organobismuth arylation agents were effective only for the transfer of aryl groups with electron-poor and electron-neutral substituents.
Synthesis of triarylbismuth diacetates 31 and triarylbismuth dichlorides 32 are shown in Scheme 13. The corresponding triarylbismuthanes 30 were produced by bromine-lithium exchange on compounds 29, and subsequently a transmetalation reaction with BiCl 3 followed. Treating the triarylbismuthanes with sodium perborate in acetic acid (c: method A) or by using iodobenzene diacetate as the oxidizing agent (d: method B) gave the triarylbismuth diacetates 31 in relatively good yields. Method A and method B were both utilized in preparing compounds 31, but method B was not efficient for the oxidation of some of the compounds 31.

Cationic Organobismuth Complex and Its Coordination Complexes
The weak Bi-C bonds of organobismuth compounds cause some instability and consequently limit their application to catalysis. Researchers had found that compounds which possessed stable cyclic frameworks were useful and recoverable reagents for specific organic reactions [53]. Based on these stable cyclic frameworks, Shimada and co-workers continued the research on cationic organobismuth complexes for catalysis [28]. Cationic  Under solid state conditions, one of the fluorine atoms of the anion in compound 34 weakly coordinates to the bismuth to form a 10-electron four-coordinate center. In order to study the coordination ability of compound 34, it was examined with various neutral donor molecules such as aldehydes, methanol, acetonitrile and dichloromethane. Moreover, it was found that the coordination of the donor molecules elongated the coordination distance between the intramolecular nitrogen and bismuth while the degree of elongation depended on their coordination strength. This adjustable property of the intramolecular nitrogen to bismuth coordination stabilizes the cationic complex and is potentially useful for catalysis.
Recently, Shimada et al. reported a unique reaction for obtaining new dibismuthanes possessing a 5,6,7,12-tetrahydrodibenz[c,f] [1,5]azabismocine framework [54]. The expected dibismuthanes 35 were synthesized by reaction of organobismuth oxides with organophosphorus compounds which possessed a P(=O)H group (Scheme 15). This new procedure was a highly efficient and simple way to prepare 35 in the form of stable crystals. The high reactivity of Bi-Bi bonds which was determined by X-ray analysis in dibismuthanes 35 made them become potentially useful precursors for various bismuth compounds as well as interesting reagents for organic synthesis.

Borate Ester Coordinated Organobismuth Compounds
Caires et al. [55] reported the first example of organobismuth compounds which contained organic boron groups. These organobismuth complexes were synthesized with a borate ester moiety because of its relative rarity as an oxygen donor in coordination compounds. These complexes were interestingly probed to have the reactivity of generating arynes at ambient temperature. The above complexes corresponded to the species ArBiCl 2 (37), Ar 3 Bi (38), ArPh 2 Bi (39), Ar 3 BiCl 2 (40), and ArPh 2 BiCl 2 (41), where Ar = 3-fluoro-2-pinacolatoboronphenyl. As shown in Scheme 16, compounds 37-39 were prepared by reacting Grignard reagent 36 with BiCl 3 or Ph 2 BiCl. Subsequently, compounds 40 and 41 were prepared by respective treatment of compounds 38 and 39 with an excess of freshly distilled sulfuryl chloride in benzene at ambient temperature. X-ray structures showed that all the complexes 37-41, except for 40, gave Bi-O bond distances under 3.0 Å in the solid state, which was between the covalent radii (2.16 Å) and van der Waals radii (3.9 Å). It was consistent with the literature precedent for a Bi-O bonding interaction [56]. Scheme 16. Preparation of borate ester coordinated organobismuth compounds [55].  [23]. These special compounds (Scheme 17) had increased stability due to the use of bulky arene ligands. It was reported that arene ligands not only provided steric protection but also stabilized organobismuth derivatives by intramolecular bismuth-arene π coordination [57]. By single-crystal X-ray diffraction and additional NMR spectroscopy analysis of its bismuth-arene π interactions, compound 47 revealed an unusual 2-fold bismuth-arene coordination. Compound 47 might thus be a suitable model compound for investigating the nature and strength of bismuth-arene π complexation.

Air-Stable Organobismuth Compounds
Most recently, Zhang and co-workers have synthesized a series of air-stable organobismuth compounds which were efficient catalysts for many organic reactions in aqueous media as well as in various organic solvents. The air-stable cationic organobismuth(III) compound [S(CH 2 C 6 H 4 ) 2  The molecular structure of compound 48 showed that the oxygen atom of the coordinating water occupied a vacant site of the cationic bismuth center, resulting in a distorted coordination geometry. The perchlorate anion was hydrogen bonded to the coordinated water molecule. The sulfur atom had two lone pairs of electrons: one coordinated with the bismuth center and the other one possibly acting as a Lewis base.
A strongly electron-withdrawing tetrafluoroborate (BF 4 ) was attached to the Bi atom of C 6 H 11 N(CH 2 C 6 H 4 ) 2 BiBF 4 , and this compound showed a Lewis acid strength of 3.3 < H 0 ≤ 4.8 which was stronger than that of C 6 H 11 N(CH 2 C 6 H 4 ) 2 Bi(OSO 2 C 8 F 17 ) which was previously synthesized [24]. The compound reported in reference [24] was an organobismuth(III) perfluorooctanesulfonate with the formula C 6 H 11 N(CH 2 C 6 H 4 ) 2  This might be attributed to the difference between the coordination parts (BF 4 ) and (OSO 2 C 8 F 17 ). Additionally, C 6 H 11 N(CH 2 C 6 H 4 ) 2 BiBF 4 was highly soluble in methanol and in aqueous solutions of common polar organic molecules. In a test of one year exposure in air, the organobismuth tetrafluoroborate remained as dry crystals and suffered no color change. With all these positive features, C 6 H 11 N(CH 2 C 6 H 4 ) 2 BiBF 4 was evaluated as a Lewis acid catalyst for the allylation of aldehydes and ketones with tetraallyltin.

Silyl-Substituted Bismuth Compounds
Since silyl groups could play a stabilizing role in bismuth compounds [61], Monakhov et al. synthesized a stable silyl-substituted dibismuthane and a disilylbismuth halide by redox/metathesis reactions of bismuth tribromide (BiBr 3 ) with the lithium silanide [Li(thf) 3 SiPh 2 tBu] in various ratios [62]. Stable silyl-substituted dibismuthane and disilylbismuth halides might be potential starting materials in synthetic reactions. As represented in Scheme 20, bismuthanes 52 and 53 were formed via two reaction pathways (I and II). Scheme 20. Two reaction pathways for synthesis of bismuthanes 52 and 53 [62]. The reaction pathway II gave compound 53 together with compound 52. However, main product compound 52 was the sole product in pathway I. Compound 52 displayed high thermostability under reflux at 100 °C for 3 h due to the relatively short Bi-Bi distance. X-ray structure determination showed that in compounds 52 the reactive Bi-Bi bond was effectively surrounded by the bulky tBuPh 2 Si groups providing steric protection of the bismuth centers.

Other Special Organobismuth Compounds
Unsymmetrical tetraarylbismuthonium salts which possess four different aryl ligands of the type [Ar 1 Ar 2 Ar 3 Ar 4 Bi + ][X − ] (54) were synthesized via two methods involving organotin and organoboron reagents, respectively [63]. The tin method and boron method are shown in Scheme 21. The unsymmetrically substituted tetraarylbismuthonium salts were conveniently produced by the Lewis acid-promoted reaction of triarylbismuth difluorides with arylstannanes or arylboronic acids. It was proven that a polybismuth network could be constructed by a simple one-pot polymetalation methodology. In addition, synthesis of thermally and kinetically stabilized oligomeric bismuth compounds bearing more than two bismuth atoms was attractive. Matano et al. also synthesized for the first time new water-soluble non-ionic triarylbismuthanes by the combination of sulfonamide function and neutral hydroxy groups [18]. Compound 58 (Figure 2) was claimed to be fairly soluble in water. The first organobismuth rings (RBi) 3 and (RBi) 4 (R=(Me 3 Si) 2 CH) were reported by Breunig and co-workers [19]. These compounds were synthesized by reduction of RBiCl 2 with magnesium filings in THF at -35 o C. The structure of (RBi) 4 showed a folded four-membered bismuth ring. A cationic trinuclear organobismuth complex with an unprecedented coordination mode in its hydrotris (2-mercaptoimidazolyl) [66] were prepared with intramolecular N→Bi interactions of different strength. The intramolecular N→Bi interactions resulted in different coordination geometry in these compounds.
New bismuth(V) complexes with the formula of Bi V R 3 (O 2 CR') 2 ( Figure 3) have been synthesized by the reaction of BiR 3 Cl 2 with Ag(O 2 CR'), where R was an aromatic ligand and R' was a substituent which contained a hydroxyl group [67]. The bismuth atoms in these compounds adopted distorted trigonal-bipyramidal geometries and showed an unusual stereoselectivity towards the chiral ligand R*CO 2 − .  [68]. As a bismuth derivative of N, N-diethyldithiocarbamate, this compound was expected to have high biological activity. There are two different bismuth atoms in this compound, one is seven coordinated with distorted pentagonal bipyramidal and the other is eight coordinated with distorted dodecahedron geometry.

Applications of Organobismuth Reagents in Organic Synthesis
Organobismuth reagents, like many other organometallic reagents, have been well studied and demonstrated to be selective reagents or mild oxidants in organic synthesis [1,69]. For the sake of the goal of environmentally benign reactions, the use of non-toxic reagents has been identified as one of the best procedures in organic synthesis. Thus, a large number of organobismuth mediated reactions had been studied and the fascinating role of organobismuth reagents had been revealed [9,36,38,47]. Many organic reactions such as C-, N-and O-arylation reactions, cross-coupling reactions, polymer reactions, asymmetric synthetic reactions and other special reactions with organobismuth reagents, especially many novel reactions were summarized in the following sections. In the meantime, the reaction pathways or mechanisms of these organobismuth mediated reactions have also been discussed.  [9,38]. Previous reviews about various arylation reactions with organobismuth reagents were focused on both the bismuth(V) and bismuth(III) compounds. Both the pentavalent and trivalent states of bismuth were found to display oxidizing power [17], and the selectivity in arylation reactions was dependent on the oxidation state [8]. Besides, the chemoselectivity was strongly dependent upon the choice of the bismuth ligands and the reaction conditions [36].

Arylation Reactions
Arylbismuth compounds could react under extremely mild conditions and afford high yields of the desired product [8,70]. It was said that pentavalent triarylbismuth could be widely used under mild conditions and excellent yields of products could be achieved [71,72]. The main problem was that commonly only one aryl group of the triarylbismuth reagents could be eventually transferred [36]. Moreover, limitations in the method of preparing these reagents limited the functionality on one of the two organic groups in the coupling event [8]. In retrospect, the reported examples of pentavalent triarylbismuth derivatives showed much of researchers' efforts in exploring the reactivity of organo-bismuth reagents.
Pentavalent biphenyl-2,2'-ylenephenylbismuth diacetate [36] was reported to react with nucleophiles under basic conditions providing modest to good (79%) yields of the corresponding C-phenylated substrates, while this compound reacted with hydroxy or amino groups providing the products of O-or N-phenylation under copper catalysis. For biphenyl-2,2'-ylenephenylbismuth diacetate, two aryl groups were included in a heterocyclic bismuth substructure to make a stabilized form of these two ligands. Hence, the loss of two ligands would be avoided and result in the selective transfer of the third aryl group. Above results showed that insertion of two phenyl groups of triphenylbismuth into a cyclic system allowed the selective transfer of only the free phenyl group (Scheme 25) both in base-catalysed C-phenylation and in the copper-catalysed O-and N-phenylation reactions. However, insertion of the bismuth atom into the five membered ring led to a decrease of the reaction rates. R BiPh 3 Cl 2 is one of the well-known triarylbismuth(V) reagents capable of effecting many arylation reactions [73]. Koech et al. [74] reported a special method for the regiospecific α-arylation of enones and enals by using BiPh 3 Cl 2 reagents under nucleophilic catalysis conditions. The reactions which were described in Scheme 25 afforded the corresponding α-aryl enones and α-aryl enals in good to excellent yield (93%). The use of tributylphosphine was a complement to the related Pd-catalyzed methods for enolate arylation in several respects.
PBu 3 (20 mol%) Ar 3 BiCl 2 (100 mol%) i-Pr 2 NEt (100 mol%) CH 2 Cl 2 -t-BuOH (9:1) 25℃ a-f Ph 3 Bi(OAc) 2 is another type of common triarylbismuth(V) reagent that has found application in N-arylations with aminoindazoles [75]. Mono-O-phenylation of diols with Ph 3 Bi(OAc) 2 afforded the monophenyl ether in good yield [76]. Cu(II)-catalyzed O-phenylation of functionalized tertiary alcohols such as α-hydroxycarbonyl compounds with Ph 3 Bi(OAc) 2 or tetraphenylbismuth compounds (Ph 4 BiF) provided tert-alkyl phenyl ethers in good yields [77]. With regard to tetraphenylbismuth compounds (Ph 4 BiX), these were representative reagents for the phenylation of various organic molecules [47]. The first report on fluorotetraphenylbismuth (Ph 4 BiF) which was published by Ooi et al. [47] demonstrated its applications for the efficient α-phenylation of ketones and esters. Based on the utilization of the inherent basicity of the fluorine atom, Ph 4 BiF was thermally stable and maintained its amphiphilic property of possessing both nucleophilic and electrophilic moieties within a molecule. Therefore, the problem that tetraphenylbismuth(V) compounds with a strongly coordinating (highly nucleophilic) anion decomposed rapidly could be solved.
Electronic factors were not the major limiting feature for the usefulness of the bismuth-mediated arylation reactions. Finet et al. recently investigated the reactivity of a series of tris(polymethoxyphenyl)bismuth derivatives in typical examples of C-, N-and O-arylation reactions [9]. The results indicated that bismuth compounds which bore electron-rich aryl groups were valuable and attractive reagents for the transfer of aryl fragments with electron-donating substituents in the arylation reactions (Schemes 27), affording good to high yields of the corresponding C-, N-and O-arylation products.

R-Z-Ar
Arylation of hydrazines is attractive since hydrazine derivatives play important roles in the agrochemical and dye-stuff industries and a number of pharmaceuticals [78,79]. In order to design triprotected reagents for the stepwise synthesis of hydrazine derivatives, Tšubrik et al. had utilized pentavalent organobismuth reagents in the selective arylation of disubstituted hydrazines [78].
Compounds of the type R 1 NHNHCOR 2 could be selectively arylated under very mild conditions by using Ar 3 Bi(OAc) 2 and provide the product R 1 ArNNHCOR 2 in good yields (74-95%). Specifically, some of the arylation reactions could be accomplished even without a copper catalyst. In addition, pentavalent organobismuth reagents [Ar 3 Bi(OAc) 2 or Ph 3 Bi(OAc) 2 ] were compared with trivalent ones (Ar 3 Bi or Ph 3 Bi) in arylation of diversely substituted hydrazines (Scheme 28) [79]. It was found that tri-and pentavalent organobismuth reagents could complement each other with respect to arylation on nitrogen. Specifically speaking, the results showed apparent advantages of pentavalent over trivalent reagents in the case of mono-and disubstituted hydrazines. In contrast, trisubstituted hydrazines were more efficiently substituted by trivalent reagents.

Scheme 28.
Arylation of diversely substituted hydrazines using Ar 3 Bi or Ar 3 Bi(OAc) 2 [79]. Trivalent organobismuth reagents such as triarylbismuthanes (Ar 3 Bi) are available, easily handled reagents for the selective N-arylation of amino groups in functionalized aminobenzanilides under mild conditions (Scheme 29) [80]. The diarylamine products were obtained selectively in good yields (46%-94%). The drawback that only one of the aryl groups of the bismuth reagent was transferred to the substrate was unavoidable in this method. Special triarylbismuthanes 25 and triarylbismuth diacetates 26 were prepared by taking advantage of solid-phase chemistry [50]. The as-prepared resin-bound bismuths were utilized as resin-bound arylation reagents in O-, N-and C-arylations. The obtained products were of high diversity and were obtained in moderate to good yields. The multidirectional linker system allowed the synthesis of different scaffolds by the introduction of a wide range of diverse fragments in the final cleavage step (Scheme 30). Thus, resin-bound bismuth as a multidirectional linker system showed this advantage and also compensated for the lack of selectivity by the cleavage of aryl groups from the bismuth.

Cross-coupling Reactions
The metal-catalyzed cross-coupling reaction of organic electrophilic reagents with organometallic reagents is one of the most important synthetic methods for C-C bond formations, in particular for those containing sp 2 -or sp-C atoms [81][82][83]. Though many organometallic compounds such as organotin, organoboron and organozinc compounds had been successfully utilized in cross-coupling reactions, the search for new nucleophilic reagents that could serve in sub-stoichiometric amounts with respect to electrophilic partners was still underway [1]. It was gratifying that organobismuth compounds were demonstrated to be useful reagents for the cross-coupling reaction with various electrophilic reagents ( Table 2).
Organobismuth compounds such as triarylbismuth reagents are referred to as atom-efficient organometallic coupling partners for C-C bond formations. The reason was that triarylbismuth reagents could react with more than 1 equivalent of electrophilic coupling reagents in order to reduce the organometallic loadings for industrial scale preparations [87]. Thus, triarylbismuth reagents with three aryl groups on bismuth allowed the development of atom-economic coupling reactions. It was reported that moderate to good yields was achieved in the cross-coupling reaction of triarylbismuth compounds (BiAr 3 ) with aryl iodides under mild conditions [88].
It could be seen from Table 2 that organobismuth dialkoxides (Scheme 31) showed high reactivity in the cross-coupling reactions with aryl and vinyl triflates as well as with aryl bromides and iodides [84,81]. The researchers suggested the two possible catalytic cycles-Cycle A and cycle B-shown in Scheme 31. It was suggested that cycle A ocurred because of the weak Bi-C bond, and it was envisioned to readily add to Pd(0) species. Cycle B is similar to those which are generally accepted for the cross-coupling reactions of organotin and organoboron compounds. Scheme 31. Two possible catalytic cycles [84].
The tentative catalytic cycle (Scheme 32) which was proposed in reference [85] was expected to go through a π-allyl-palladium alkoxide intermediate (A). In this cycle, triarylbismuths were multicoupling organometallic nucleophiles with allylic carbonates for three C-C couplings. Besides, the product (E) was an unavoidable minor product by the homo-coupling of triarylbismuths [86]. Another mechanistic cycle (Scheme 33) which was proposed in reference [87] also referred to the formation of π-allyl palladium intermediate. The domino couplings of triarylbismuths with three equivalent of 1,1-dibromo-1-alkenes were very fast, affording high yields of alkynes in a short reaction time [86]. As depicted in Scheme 34, the proposed cycles indicated two domino coupling cycles, path A and path B. Both cycles involved an alkyne-Pd intermediate (R). It was worthwhile to mention that the Ar 2 BiBr or ArBiBr 2 species could participate in subsequent catalytic cycles during transmetalation.

Asymmetric Synthesis
Asymmetric synthesis is an important and useful tool in modern synthetic organic chemistry [89] and various organometallic reagents are involved in many asymmetric reactions [89,90]. Organobismuth reagents could act as basic reagents because of a pair of electrons in their highest s orbitals [91]. However, applications of organobismuths in asymmetric synthesis were relatively scarce till now [22]. Miyake et al. opened up new realms for organobismuth investigation [22,92]. Pd(II)-catalyzed kinetic resolution of racemic secondary alcohols via their enantioselective benzoylation by using CO and organobismuth(V) compound had been developed (Scheme 35) [22]. It was mentioned that although satisfactory enantioselectivity was not obtained (up to 48% ee), this reaction system seemed to be interesting from the viewpoint of both asymmetric synthesis and organobismuth chemistry.

Scheme 35.
Kinetic resolution of secondary alcohols [22].  Another highly enantioselective reaction (Scheme 37) was carried out by Sato et al. [89]. The aryl transfer reactions to aromatic aldehydes by using mixed triarylbismuthane and dimethylzinc reagents provided diarylmethanols in good yield (up to 97%). However, the active species of the mixed Ph 3 Bi/Me 2 Zn reagent is still unknown. It was difficult to know whether transmetallation products were formed or not. Otherwise a metal exchange reaction might happen between dialklyzinc and organobismuth reagents.

Scheme 37.
An enantioselective phenyl transfer reaction by using a mixed Ph 3

Other Special Reactions
A new synthetic route to dibenzo[b,d]pyran derivatives by using organobismuth-mediated one-pot reaction was developed [43]. As depicted in Scheme 38, triarylbismuth diacetate (59) resulted from the oxidation of Bi(III) derivative 13 and subsequent ortho-arylation reaction afforded product 60. Finally, a spontaneous intramolecular cyclisation led to the formation of compound 61. Though the yields of dibenzo[b,d]pyran derivatives were not so satisfactory compared with those possible using an organolead reagent, the environmentally friendly nature of the organobismuth variant should be noted. The synthesis of bismuth-containing polymers had been rarely reported, not to mention the construction of conjugated polymers containing the bismole skeleton in the main chain. Morisaki et al. [93] first synthesized a conjugated polymer ( Figure 4) containing a bismuth atom in the conjugated main chain by incorporating the bismuth atom into the cyclopentadiene skeleton (bismole skeleton). The as-prepared polymer was generated in 41% isolated yield and exhibited moderate bluish photoluminescence in solution. In addition, Pu et al. [94] have mentioned that some recently developed organobismuth compounds were effective mediators for the living radical polymerization of most N-vinylamides.  Maoecrystal V was a Traditional Chinese Medicine component with an unusual structure. The group of Krawczuk [95] recently described an enantioselective synthesis of the maoecrystal V carbon skeleton. Remarkably, one of the key transformations was the arylation of a 1,3-dicarbonyl compound with a triarylbismuth(V) dichloride species (Ar 3 BiCl 2 ).
The unique character of valency change from Bi V to Bi III allows pentavalent organobismuths to be effective reagents for O-phenylation of tertiary alcohols [96] and oxidative coupling reactions of carbonyl compounds [97], and also for synthesis of aryl tosylates [69]. In addition, two ligands on the pentavalent bismuth were coupled accompanied with reductive elimination to simultaneously generate trivalent bismuth. A novel ligand coupling reaction between an aryl group and a tosyloxy group on the pentavalent bismuth established by Sakurai et al. [69] afforded aryl tosylates in good to high yields. A plausible mechanism for the formation of the phenyl tosylate is shown in Scheme 39. Specially, the pentavalent bismuth ditosylates which were used in the reaction were heterocyclic bismuth compounds. Heterocyclic bismuth compounds were reported to be quite different from the noncyclic ones [98].

Scheme 39.
A plausible mechanism for the formation of the phenyl tosylate [69].

Applications of Organobismuth Catalysts in Organic Synthesis
A diversity of inorganic bismuth(III) compounds such as Bi(OTf) 3 , Bi(TFA) 3 , BiCl 3 , BiBr 3 , Bi(NO 3 ) 3 , etc. had been previously reported as effective catalysts in various organic reactions due to their Lewis acid character and the environmentally friendly nature of bismuth compounds [12,60,[99][100][101][102][103]. However, the unstable nature of Bi-C bonds of bismuth compounds has resulted in rare reports on the use of organobismuth compounds as catalysts [59]. In order to develop suitable organo-bismuth compounds for catalytic utilization, several advanced research works have reported about organobismuth catalysts. Thus, these works might offer some enlightenment to the development of organobismuth catalyzed reactions.

Cationic Organobismuth Complexes
Cationic organometallic complexes of transition and main group metals such as Y, La, Sm, Ni, Pb, et al. [104] have been widely used as catalysts for polymerization and organic synthesis. However, for cationic organobismuth complexes, there were not examples of applications in catalysis until the study reported by Bao et al. [28]. Cationic 5,6,7,12-tetrahydrodibenz[c,f] [1,5]azabismocine with a weakly coordinating borate anion and its coordination complexes with some neutral donor molecules were synthesized. The preliminary evaluation of the catalytic performance of the cationic complex ([ t BuN(CH 2 C 6 H 4 ) 2 Bi] + [B(C 6 F 5 ) 4 ] − ) showed that this complex worked as a Lewis acid catalyst for allylation reaction of aldehydes with allylbismuth species and the Mukaiyama aldol reaction.
This breakthrough showed the potential of the use of cationic organobismuth complexes for catalysis and also highlighted the synthetic possibility of stable organobismuth compounds by adoption of stable cyclic frameworks [53,54]. To achieve this purpose, recent studies have brought forth new ideas in cationic organobismuth catalyzed reactions.
Air-stable cationic organobismuth(III) compound [S(CH 2 C 6 H 4 ) 2 Bi(OH 2 )] + [ClO 4 ] − with both Lewis acidity and basicity efficiently catalyzed the direct diastereoselective Mannich reaction in various solvents, including water [58]. There was difficulty in carrying out Mannich reactions over bifunctional catalysts which were composed of a Lewis acid and Lewis base. Moreover, the rarely reported examples with low yield, poor diastereoselectivity or environmentally unfriendly disadvantage were not enough. This cationic organobismuth(III) compound exhibited high activity, diastereoselectivity, stability and reusability compared with other bismuth catalysts, and its anti-selectivity was almost independent of the solvents. Especially, high efficiency (yield 98%) and stereoselectivity (syn:anti = 5:95) were obtained using water as the solvent. Thus, the as-prepared cationic organobismuth(III) compound highlighted its catalytic application in organic synthesis.
Another cationic organobismuth perfluorooctanesulfonate [S(CH 2 C 6 H 4 ) 2 Bi(OH 2 )] + [OSO 2 C 8 F 17 ] − (49) also showed Lewis acidity for the stereoselective synthesis of (E)-α,β-unsaturated ketones in water [59]. Experiments were carried out for one-pot synthesis of (E)-α,β-unsaturated ketones through the direct crossed condensation of aldehydes and ketones in various solvents, including water, providing the target products in good yields (up to 95%) without multistep transformations and product separation. High diastereoselectivity (anti/syn > 99/1) and mild conditions made the new synthetic process superior to the common processes. As shown in Scheme 40, the reactions probably took place through a Mannich-type mechanism. The sulfur atom in compound 49 could act as a weak Lewis base, and the high stereoselectivity of 49 might have some connection with that Lewis base. Another green synthesis of (E)-α,β-unsaturated ketones was catalyzed by a new bifunctional complex [S(CH 2 C 6 H 4 ) 2

Air-Stable Organobismuth Compounds
In order to obtain organometallic compounds that had potential uses in catalysis, as previously stated, a stable metal-carbon bond was required. On the other hand, proper counteranions were needed to stabilize the structures and to improve the catalytic activity. Stable cyclic frameworks could be adopted in the synthesis of organobismuth derivatives to get stable organobismuth compounds [28]. In addition, acidity and stability of organometallic (e.g., Sn, Ti, Zr, Hf) complexes could be enhanced by the incorporation of long-chain perfluoroalkylsulfonate and perfluoroarylsulfonate groups [24]. Referring to a preceding part of the text (Section 2.1), Zhang and co-workers have synthesized a series of air-stable organobismuth compounds which were proven to be excellent catalysts in organic reactions such as allylation reactions and Mannich reactions. The allylations of aldehydes and ketones with tetraallyltin were conducted by a Lewis acid catalyst which was identified as the hypervalent organobismuth(III) tetrafluoroborate C 6 H 11 N(CH 2 C 6 H 4 ) 2 BiBF 4 [60]. Reactions occurred smoothly in various solvents such as MeOH, C 2 H 5 OH, CH 3 CN, etc. and the presence of electron-withdrawing or electron-donating groups on the aldehydes had little effect on the reaction. The expected homoallylic alcohols were formed with excellent chemoselectivity and yields as high as 97%. Researchers have postulated the mechanism of the allylation reaction over C 6 H 11 N(CH 2 C 6 H 4 ) 2 BiBF 4 in aqueous methanol solution (Scheme 41). C 6 H 11 N(CH 2 C 6 H 4 ) 2 BiBF 4 was regenerated by the cleavage of the coordinate bond, and was ready for the next catalytic cycle. The bismuth center of this compound acted as a Lewis acid site and the electron-withdrawing group (BF 4 ) was strong. The one-pot three-component Mannich reactions of aldehydes, amines and ketones in water catalyzed by organobismuth(III) perfluorooctanesulfonate [C 6 H 11 N(CH 2 C 6 H 4 ) 2 Bi(OSO 2 C 8 F 17 )] [24] were proposed to ocurr according to the mechanism described in Scheme 42. It can be seen from this Scheme that the carbonyl oxygen atom of benzaldehyde coordinated with the bismuth atom of the catalyst (I) and was thus activated. As a result, an intermediate (II) was formed. Thus, the bismuth atom of the catalyst played an important role during the catalytic procedure.

Carbon Dioxide Fixation by Organobismuths
Chemical fixation of CO 2 is an attractive field and many metal compounds can be used for CO 2 fixation [106]. Yin and co-workers have reported the use of organobismuth oxides, hydroxides, and alkoxides for CO 2 conversion into useful chemicals as well as for CO 2 capture and separation [107]. As shown in Scheme 43, the CO 2 fixation by bismuth oxide was irreversible at room temperature. These compounds were synthesized with the 5,6,7,12-tetrahydrodibenz[c,f] [1,5]azabismocine framework. Other two new bismuth compounds which possess a sulfur-bridged bis(phenolato)ligand ( Figure 5) were used for solvent-free synthesis of propylene carbonate from CO 2 and propylene oxide by Yin and co-workers [108]. The new compounds were thermally stable and showed high catalytic activity for the coupling of CO 2 with propylene oxide.

Conclusions
In conclusion, as an environment-friendly class of organometallic compound, organobismuth compounds participated in new, simple, efficient and 'green' protocols for organic synthesis, catalytic processes and even synthesis of materials. During the last two decades, interest in organobismuth compounds and the applications of organobismuth compounds had already promoted their development with a great step forward, and some remarkable progress had been achieved. Nonetheless, there were still problems hindering more widespread applications of organobismuths. As described in the present review article, though arylation with organobismuths afforded high yields in the products, the aryl groups of the bismuth reagent could not be fully transferred and the synthesis of new reagents seemed unsatisfactory. Application of organobismuths in C-C bond formations and asymmetric synthesis remain a tough topic for future studies. Besides, many reaction mechanisms for some special reactions with organobismuths are still not clarified. Thus, the search for new and useful reactions with bismuth compounds, especially organobismuths, is expected to be time consuming but full of interest. Overall, the future is bright for these organobismuth compounds, and we look forward to the great progress in this area.