Boron and Silicon-Substituted 1,3-Dienes and Dienophiles and Their Use in Diels-Alder Reactions

Boron and silicon-substituted 1,3-dienes and boron and silicon-substituted alkenes and alkynes have been known for years and the last 10 years have seen a number of new reports of their preparation and use in Diels-Alder reactions. This review first covers boron-substituted dienes and dienophiles and then moves on to discuss silicon-substituted dienes and dienophiles.


Introduction
This review article covers recent reports of boron and silicon-substituted 1,3-dienes and dienophiles and their use in Diels-Alder reactions. Reviews on boron diene/Diels-Alder chemistry were published in 2017 [1] and 2014 [2] so this review will cover that topic from 2016-2020. Silyl-substituted 1,3-dienes and their Diels-Alder reactions were reviewed in early 2011 [3] so this review will cover that chemistry from 2010-2020. The articles reviewed here came from one of four topical searches within the Science Citation Index. A search using keywords boron and diene between 2016 and 2020 yielded 51 references, which were subsequently checked for reported organic chemistry. A second search over the same period using keywords boron and Diels-Alder yielded 76 references. Similar searches over 2010-2020 using silicon and diene and silicon and Diels-Alder yielded 101 and 137 references respectively, which were all checked for diene preparation and/or reports of Diels-Alder chemistry. In terms of review format, boron and silicon dienes and dienophiles and their Diels-Alder reactions will be presented chronologically from oldest to most recent reports within the topical areas of boron dienes, boron dienophiles, silicon dienes, and silicon dienophiles.

Boron Dienes Other Than Boroles
Erker and co-workers reported hydroboration of a phosphorus-substituted enyne (1) to produce a proposed boron-substituted 1,3-diene (3) which presumably cyclized to the isolated borata-dienes (4) (Scheme 1) [4]. While this chemistry has a proposed boron diene intermediate (3), the intramolecular diene cyclization proposed presumably precludes any diene trapping of this intermediate via Diels-Alder chemistry, but furthermore, running the reaction in the presence of a dienophile would provide that answer. Lastly in recent boron diene chemistry, Pellegrinet and co-workers reported preparation of 2and 3-boron-substituted furans (27) and found that while the 3-boron-substituted furans reacted well with maleic anhydride (28), the 2-boron-substituted furans failed to do so (Scheme 8) [12]. Theoretical calculations on these two types of reactions were also performed and the failure of the 2-boron-substituted dienes to react was largely attributed to steric distortion experienced with the dienophile when boron was in the 2 position. Lastly in recent boron diene chemistry, Pellegrinet and co-workers reported preparation of 2and 3-boron-substituted furans (27) and found that while the 3-boron-substituted furans reacted well with maleic anhydride (28), the 2-boron-substituted furans failed to do so (Scheme 8) [12]. Theoretical calculations on these two types of reactions were also performed and the failure of the 2-boronsubstituted dienes to react was largely attributed to steric distortion experienced with the dienophile when boron was in the 2 position. Scheme 8. Diels-Alder reactions of boron-substituted furans.

Boroles
Boroles are anti-aromatic heterocycles that one can consider as a special kind of boronsubstituted diene, and they continue to be an active area of research. They are broken out here into their own separate category within boron diene chemistry.
Wrackmeyer and Khan have worked in this area for a number of years and authored a microreview that appeared in 2016 (Scheme 9) [13]. The synthetic chemistry covered involves 1,1carboboration of diynes (30) to yield boroles (31), siloles, and stannoles. The general reaction used to make boroles is shown below. Martin and co-workers also published a review on boroles in 2016 but this review covers Diels-Alder and ring expansion reactions of boroles (Scheme 10) [14]. In general, in the absence of Lewis bases, boroles 32 dimerize to 33 or react with other potential dienophiles via Diels-Alder chemistry and in the presence of Lewis bases boroles generally undergo ring expansion 36.

Boroles
Boroles are anti-aromatic heterocycles that one can consider as a special kind of boron-substituted diene, and they continue to be an active area of research. They are broken out here into their own separate category within boron diene chemistry.
Wrackmeyer and Khan have worked in this area for a number of years and authored a microreview that appeared in 2016 (Scheme 9) [13]. The synthetic chemistry covered involves 1,1-carboboration of diynes (30) to yield boroles (31), siloles, and stannoles. The general reaction used to make boroles is shown below. Lastly in recent boron diene chemistry, Pellegrinet and co-workers reported preparation of 2and 3-boron-substituted furans (27) and found that while the 3-boron-substituted furans reacted well with maleic anhydride (28), the 2-boron-substituted furans failed to do so (Scheme 8) [12]. Theoretical calculations on these two types of reactions were also performed and the failure of the 2-boronsubstituted dienes to react was largely attributed to steric distortion experienced with the dienophile when boron was in the 2 position. Scheme 8. Diels-Alder reactions of boron-substituted furans.

Boroles
Boroles are anti-aromatic heterocycles that one can consider as a special kind of boronsubstituted diene, and they continue to be an active area of research. They are broken out here into their own separate category within boron diene chemistry.
Wrackmeyer and Khan have worked in this area for a number of years and authored a microreview that appeared in 2016 (Scheme 9) [13]. The synthetic chemistry covered involves 1,1carboboration of diynes (30) to yield boroles (31), siloles, and stannoles. The general reaction used to make boroles is shown below. Martin and co-workers also published a review on boroles in 2016 but this review covers Diels-Alder and ring expansion reactions of boroles (Scheme 10) [14]. In general, in the absence of Lewis bases, boroles 32 dimerize to 33 or react with other potential dienophiles via Diels-Alder chemistry and in the presence of Lewis bases boroles generally undergo ring expansion 36. Martin and co-workers also published a review on boroles in 2016 but this review covers Diels-Alder and ring expansion reactions of boroles (Scheme 10) [14]. In general, in the absence of Lewis bases, boroles 32 dimerize to 33 or react with other potential dienophiles via Diels-Alder chemistry and in the presence of Lewis bases boroles generally undergo ring expansion 36.
Lectka and co-workers reported both an experimental and theoretical analysis of a C-F bond directed Diels-Alder reaction of pentasubstituted boroles (Scheme 11) [15]. The pivotal experimental result was a competitive Diels-Alder reaction between phenyltetramethylborole (32) and the syn and anti fluoro isomers of a methanoisobenzofuran 1,3-dione (37 and 38). The B-F coordinated Diels-Alder adduct (39) was the exclusive product. Martin and Yruegas reported the ring expansion reaction of boroles outlined above in Scheme 10 as a route to 1,2-thiaborines (41) in 2016 (Scheme 12) [16]. Two thiaborines were reported and their UV maxima were significantly red shifted in relation to hexaphenylbenzene. Their nuclear independent chemical shift values indicated they were significantly more aromatic than the one previously reported example of this heterocycle which contained a nitrogen substituent on boron. Lectka and co-workers reported both an experimental and theoretical analysis of a C-F bond directed Diels-Alder reaction of pentasubstituted boroles (Scheme 11) [15]. The pivotal experimental result was a competitive Diels-Alder reaction between phenyltetramethylborole (32) and the syn and anti fluoro isomers of a methanoisobenzofuran 1,3-dione (37 and 38). The B-F coordinated Diels-Alder adduct (39) was the exclusive product.
Lectka and co-workers reported both an experimental and theoretical analysis of a C-F bond directed Diels-Alder reaction of pentasubstituted boroles (Scheme 11) [15]. The pivotal experimental result was a competitive Diels-Alder reaction between phenyltetramethylborole (32) and the syn and anti fluoro isomers of a methanoisobenzofuran 1,3-dione (37 and 38). The B-F coordinated Diels-Alder adduct (39) was the exclusive product. Martin and Yruegas reported the ring expansion reaction of boroles outlined above in Scheme 10 as a route to 1,2-thiaborines (41) in 2016 (Scheme 12) [16]. Two thiaborines were reported and their UV maxima were significantly red shifted in relation to hexaphenylbenzene. Their nuclear independent chemical shift values indicated they were significantly more aromatic than the one previously reported example of this heterocycle which contained a nitrogen substituent on boron. Martin and Yruegas reported the ring expansion reaction of boroles outlined above in Scheme 10 as a route to 1,2-thiaborines (41) in 2016 (Scheme 12) [16]. Two thiaborines were reported and their UV maxima were significantly red shifted in relation to hexaphenylbenzene. Their nuclear independent chemical shift values indicated they were significantly more aromatic than the one previously reported example of this heterocycle which contained a nitrogen substituent on boron. Martin and co-workers reported thermal and photochemical cyclization reactions of cis and trans diazenes (43) with pentaphenylborole (42) (Scheme 13) [17]. The cyclization products (44 and 45) of thermal reactions of trans diazenes depended on the diazene substituents. Trans-diphenyldiazene (43) gave a 2:1 mixture of two products (44 and 45) that differ in which nitrogen coordinates to boron. Both products were thought to arise from initial coordination of N to B followed by an intramolecular electrophilic addition of the borole diene to the carbon ortho to the coordinated nitrogen. The reaction of pentaphenylborole (42) with 2′,6′-dimethylazobenzene (46) was thought to start similarly but steric interactions between the two methyls and the phenyl groups in the analog to the major isomer (44) were proposed to cause a ring expansion/ring contraction sequence that led to the isolated product (47) (Scheme 14). Photochemical reactions with both trans diazenes led to the same type of 1,3,2-diazaborepins. This same ring system was formed via thermal reactions of pentaphenyl borole with cis diazenes so the authors proposed that photochemical trans-cis isomerization of the trans diazenes prior to cyclization accounted for the observed products. Formation of these 1,3,2-diazaborepins (51) (Scheme Martin and co-workers reported thermal and photochemical cyclization reactions of cis and trans diazenes (43) with pentaphenylborole (42) (Scheme 13) [17]. The cyclization products (44 and 45) of thermal reactions of trans diazenes depended on the diazene substituents. Trans-diphenyldiazene (43) gave a 2:1 mixture of two products (44 and 45) that differ in which nitrogen coordinates to boron. Both products were thought to arise from initial coordination of N to B followed by an intramolecular electrophilic addition of the borole diene to the carbon ortho to the coordinated nitrogen. Martin and co-workers reported thermal and photochemical cyclization reactions of cis and trans diazenes (43) with pentaphenylborole (42) (Scheme 13) [17]. The cyclization products (44 and 45) of thermal reactions of trans diazenes depended on the diazene substituents. Trans-diphenyldiazene (43) gave a 2:1 mixture of two products (44 and 45) that differ in which nitrogen coordinates to boron. Both products were thought to arise from initial coordination of N to B followed by an intramolecular electrophilic addition of the borole diene to the carbon ortho to the coordinated nitrogen. The reaction of pentaphenylborole (42) with 2′,6′-dimethylazobenzene (46) was thought to start similarly but steric interactions between the two methyls and the phenyl groups in the analog to the major isomer (44) were proposed to cause a ring expansion/ring contraction sequence that led to the isolated product (47) (Scheme 14). Photochemical reactions with both trans diazenes led to the same type of 1,3,2-diazaborepins. This same ring system was formed via thermal reactions of pentaphenyl borole with cis diazenes so the authors proposed that photochemical trans-cis isomerization of the trans diazenes prior to cyclization accounted for the observed products. Formation of these 1,3,2-diazaborepins (51) (Scheme The reaction of pentaphenylborole (42) with 2 ,6 -dimethylazobenzene (46) was thought to start similarly but steric interactions between the two methyls and the phenyl groups in the analog to the major isomer (44) were proposed to cause a ring expansion/ring contraction sequence that led to the isolated product (47) (Scheme 14). Martin and co-workers reported thermal and photochemical cyclization reactions of cis and trans diazenes (43) with pentaphenylborole (42) (Scheme 13) [17]. The cyclization products (44 and 45) of thermal reactions of trans diazenes depended on the diazene substituents. Trans-diphenyldiazene (43) gave a 2:1 mixture of two products (44 and 45) that differ in which nitrogen coordinates to boron. Both products were thought to arise from initial coordination of N to B followed by an intramolecular electrophilic addition of the borole diene to the carbon ortho to the coordinated nitrogen. The reaction of pentaphenylborole (42) with 2′,6′-dimethylazobenzene (46) was thought to start similarly but steric interactions between the two methyls and the phenyl groups in the analog to the major isomer (44) were proposed to cause a ring expansion/ring contraction sequence that led to the isolated product (47) (Scheme 14). Photochemical reactions with both trans diazenes led to the same type of 1,3,2-diazaborepins. This same ring system was formed via thermal reactions of pentaphenyl borole with cis diazenes so the authors proposed that photochemical trans-cis isomerization of the trans diazenes prior to cyclization accounted for the observed products. Formation of these 1,3,2-diazaborepins (51) (Scheme Photochemical reactions with both trans diazenes led to the same type of 1,3,2-diazaborepins. This same ring system was formed via thermal reactions of pentaphenyl borole with cis diazenes so the authors proposed that photochemical trans-cis isomerization of the trans diazenes prior to cyclization accounted for the observed products. Formation of these 1,3,2-diazaborepins (51) (Scheme 15) was thought to originate from an initial Diels-Alder reaction (52) followed by N-N bond heterolysis via a [1,3] sigmatropic shift. Coordination of the imine nitrogen thus liberated (53) to boron was proposed to set off a ring expansion that led to the observed product (54).
Molecules 2020, 25, x FOR PEER REVIEW 7 of 24 15) was thought to originate from an initial Diels-Alder reaction (52) followed by N-N bond heterolysis via a [1,3] sigmatropic shift. Coordination of the imine nitrogen thus liberated (53) to boron was proposed to set off a ring expansion that led to the observed product (54).
Molecules 2020, 25, x FOR PEER REVIEW 7 of 24 15) was thought to originate from an initial Diels-Alder reaction (52) followed by N-N bond heterolysis via a [1,3] sigmatropic shift. Coordination of the imine nitrogen thus liberated (53) to boron was proposed to set off a ring expansion that led to the observed product (54).

Boron Dienophiles
Several reports of boron-substituted dienophiles for use in Diels-Alder reactions have appeared in the last few years as well. In 2018, Houck, Morgan, and co-workers reported a quinine-promoted, enantioselective, boron-tethered Diels-Alder reaction of a boron-substituted dienophile (82) (Scheme

Boron Dienophiles
Several reports of boron-substituted dienophiles for use in Diels-Alder reactions have appeared in the last few years as well. In 2018, Houck, Morgan, and co-workers reported a quinine-promoted, enantioselective, boron-tethered Diels-Alder reaction of a boron-substituted dienophile (82) (Scheme 23) [21]. In this work, a variety of dienols were tethered to phenylethenyl boronic acid and quinine was used as a chiral promoter to coordinate to boron for the subsequent intramolecular Diels-Alder reaction. Highly substituted cyclohexenols (83) were recovered in good yields and enantioselectivities and quinine was recovered in good yield as well. DFT calculations were used to understand the origins of the enantioselection.

Boron Dienophiles
Several reports of boron-substituted dienophiles for use in Diels-Alder reactions have appeared in the last few years as well. In 2018, Houck, Morgan, and co-workers reported a quinine-promoted, enantioselective, boron-tethered Diels-Alder reaction of a boron-substituted dienophile (82) (Scheme 23) [21]. In this work, a variety of dienols were tethered to phenylethenyl boronic acid and quinine was used as a chiral promoter to coordinate to boron for the subsequent intramolecular Diels-Alder reaction. Highly substituted cyclohexenols (83) were recovered in good yields and enantioselectivities and quinine was recovered in good yield as well. DFT calculations were used to understand the origins of the enantioselection.

Scheme 23. Boron tethered Diels-Alder reactions.
Pellegrinet and co-workers reported preparation of some alkylhalovinylboranes and their Diels-Alder reactions (Scheme 24) [22]. They reasoned that these vinyl boranes would be less sterically hindered and more electron deficient than dialkylvinylboranes and hence should be more reactive in Diels-Alder chemistry. In this work, they calculated activation free energies and endo/exo selectivities for a number of vinyl borane reactions with cyclopentadiene (87). They then tested a few of those predictions experimentally. Their proof of concept reaction involved hydroboration of cyclohexene (84) with dichloroborane (85) and Et3SiH followed by transmetallation with vinyltributyltin (86) in the presence of cyclopentadiene (87). Oxidation of the boron-substituted cycloadduct (88) produced 5-norbornen-2-ol (89) in 48% yield and 4:1 endo:exo selectivity.

Scheme 23. Boron tethered Diels-Alder reactions.
Pellegrinet and co-workers reported preparation of some alkylhalovinylboranes and their Diels-Alder reactions (Scheme 24) [22]. They reasoned that these vinyl boranes would be less sterically hindered and more electron deficient than dialkylvinylboranes and hence should be more reactive in Diels-Alder chemistry. In this work, they calculated activation free energies and endo/exo selectivities for a number of vinyl borane reactions with cyclopentadiene (87). They then tested a few of those predictions experimentally. Their proof of concept reaction involved hydroboration of cyclohexene (84) with dichloroborane (85) and Et 3 SiH followed by transmetallation with vinyltributyltin (86) in the presence of cyclopentadiene (87). Oxidation of the boron-substituted cycloadduct (88) produced 5-norbornen-2-ol (89) in 48% yield and 4:1 endo:exo selectivity. Calculations were then performed on reactions of chiral alkylchlorovinylboranes that could be obtained by hydroboration of chiral alkenes with HBCl2. Diels-Alder reactions between cyclohexadiene, isoprene, and trans,trans-1,4-diphenyl-1,3-butadiene with these chiral boranes made by hydroboration of α-pinene and 2-and 3-carene were reported. Unfortunately, yields, enantioselectivities, and endo:exo selectivities of these Diels-Alder reactions were generally disappointing.

Silicon Dienes other than Siloles
In 2010, our group published an enyne metathesis route to 2-silicon-substituted 1,3-dienes (95) (Scheme 26) [24]. We then studied their Diels-Alder reactions and the Hiyama-Denmark cross coupling reactions (96) of the silicon-substituted cyclohexene products of those Diels-Alder reactions. The cross coupling allows the intermediate silicon dienes (95) to serve as synthons for a host of 1,3dienes.

Silicon Dienes Other Than Siloles
In 2010, our group published an enyne metathesis route to 2-silicon-substituted 1,3-dienes (95) (Scheme 26) [24]. We then studied their Diels-Alder reactions and the Hiyama-Denmark cross coupling reactions (96) of the silicon-substituted cyclohexene products of those Diels-Alder reactions. The cross coupling allows the intermediate silicon dienes (95) to serve as synthons for a host of 1,3-dienes. Martin and co-workers communicated a total synthesis of isokidamycin that relied on a key intramolecular Diels-Alder reaction of a silicon-substituted furan (97) and a naphthyne (Scheme 27) [25]. This communication was followed by a full paper [26]. The silicon-substituted diene (102) used in this total synthesis was prepared by furan metalation followed by treatment with chlorodimethylvinylsilane (100) followed by hydroboration/oxidation to yield 102. Martin and co-workers communicated a total synthesis of isokidamycin that relied on a key intramolecular Diels-Alder reaction of a silicon-substituted furan (97) and a naphthyne (Scheme 27) [25]. This communication was followed by a full paper [26]. The silicon-substituted diene (102) used in this total synthesis was prepared by furan metalation followed by treatment with chlorodimethylvinylsilane (100) followed by hydroboration/oxidation to yield 102.
Martin and co-workers communicated a total synthesis of isokidamycin that relied on a key intramolecular Diels-Alder reaction of a silicon-substituted furan (97) and a naphthyne (Scheme 27) [25]. This communication was followed by a full paper [26]. The silicon-substituted diene (102) used in this total synthesis was prepared by furan metalation followed by treatment with chlorodimethylvinylsilane (100) followed by hydroboration/oxidation to yield 102. Scheme 27. Diels-Alder reaction of a silicon-substituted furan.
Carboni and co-workers reported the preparation of a silicon and a boron-substituted heterodendralene (103) followed by six hetero-Diels-Alder reactions of the silicon dendralene and three inverse electron demand hetero-Diels-Alder reactions of the boron dendralene (Scheme 28) [27]. The silicon and boron groups in the resulting cycloadducts (104) were then used in halogenation, oxidation or coupling reactions to produce more highly substituted tricyclic products (105).

Scheme 27. Diels-Alder reaction of a silicon-substituted furan.
Carboni and co-workers reported the preparation of a silicon and a boron-substituted heterodendralene (103) followed by six hetero-Diels-Alder reactions of the silicon dendralene and three inverse electron demand hetero-Diels-Alder reactions of the boron dendralene (Scheme 28) [27]. The silicon and boron groups in the resulting cycloadducts (104) were then used in halogenation, oxidation or coupling reactions to produce more highly substituted tricyclic products (105). In 2012, we extended our earlier enyne metathesis work and showed that if the Hoveyda-Grubbs catalyst was used for the metathesis then enyne metathesis and Diels-Alder reactions could be run as one-pot multicomponent reactions (Scheme 29) [28]. In one case, we showed that an enyne metathesis/Diels-Alder/cross coupling sequence could be completed with only one reaction workup but addition of alumina (to chelate Ru) and its removal by filtration was required prior to cross coupling to yield 112.

Scheme 28. Reactions of boron and silicon-substituted dendralenes.
In 2012, we extended our earlier enyne metathesis work and showed that if the Hoveyda-Grubbs catalyst was used for the metathesis then enyne metathesis and Diels-Alder reactions could be run as one-pot multicomponent reactions (Scheme 29) [28]. In one case, we showed that an enyne metathesis/ Diels-Alder/cross coupling sequence could be completed with only one reaction workup but addition of alumina (to chelate Ru) and its removal by filtration was required prior to cross coupling to yield 112.
In 2012, we extended our earlier enyne metathesis work and showed that if the Hoveyda-Grubbs catalyst was used for the metathesis then enyne metathesis and Diels-Alder reactions could be run as one-pot multicomponent reactions (Scheme 29) [28]. In one case, we showed that an enyne metathesis/Diels-Alder/cross coupling sequence could be completed with only one reaction workup but addition of alumina (to chelate Ru) and its removal by filtration was required prior to cross coupling to yield 112. Scheme 29. One-pot enyne metathesis/Diels-Alder reactions.
Turks and co-workers reported an acid-catalyzed 1,2-silyl shift in propargyl silanes (142) as a route to silyl-substituted dienes (143) (Scheme 38) [35]. A variety of metal triflates were first investigated as catalysts for this reaction but the authors ultimately determined that small amounts of triflic acid sonicated in CH 2 Cl 2 caused rapid isomerizations in high yields for a variety of silanes. In one case, a t-butyldimethylpropargyl silane was treated with 1% HOTf in the presence of maleic anhydride for 3 h at 25 • C and the cycloadduct (144) resulting from a Diels-Alder reaction via an exo transition state was isolated in 66% yield. Johnson, Hendrix and Jennings reported syntheses of a number of silyl ketene acetals (141) from deprotonation and TMSCl trapping of α-silyl-α,β-unsaturated esters (150) (Scheme 37) [34]. No Diels-Alder reactions of these dienes (141) were reported but isolated yields of diene products ranged from 85% to 99% and E:Z ratios across both double binds in all products were >20:1. Turks and co-workers reported an acid-catalyzed 1,2-silyl shift in propargyl silanes (142) as a route to silyl-substituted dienes (143) (Scheme 38) [35]. A variety of metal triflates were first investigated as catalysts for this reaction but the authors ultimately determined that small amounts of triflic acid sonicated in CH2Cl2 caused rapid isomerizations in high yields for a variety of silanes. In one case, a t-butyldimethylpropargyl silane was treated with 1% HOTf in the presence of maleic anhydride for 3 h at 25 °C and the cycloadduct (144) resulting from a Diels-Alder reaction via an exo transition state was isolated in 66% yield. Lastly, while not technically involving a silyl-substituted diene or dienophile reaction (unless we consider the silyl benzyne as silyl diene + alkyne), Suzuki and co-workers reported using bromosilylphenyl tosylates (145) as precursors for intramolecular benzyne-diene Diels-Alder reactions utilizing a silicon tether (146) (Scheme 39) [36]. A variety of alkyl lithium and Grignard reagents were investigated as metalating agents but the magnesite (Ph3MgLi) provided the highest yields of products (147) in a variety of ether solvents. Acyclic E dienes and aromatic ring systems also reacted via [4+2] cycloaddition with these benzynes whereas acyclic Z dienes reacted via [2+2] cycloaddition. Scheme 39. Intramolecular benzyne-diene Diels-Alder reactions.

Siloles
Like boroles, there are many publications on siloles and their uses in photochemical and electrochemical applications. The number of reports on their synthesis and use in cyclization or cycloaddition reactions is much smaller. Lastly, while not technically involving a silyl-substituted diene or dienophile reaction (unless we consider the silyl benzyne as silyl diene + alkyne), Suzuki and co-workers reported using bromosilylphenyl tosylates (145) as precursors for intramolecular benzyne-diene Diels-Alder reactions utilizing a silicon tether (146) (Scheme 39) [36]. A variety of alkyl lithium and Grignard reagents were investigated as metalating agents but the magnesite (Ph 3 MgLi) provided the highest yields of products (147) in a variety of ether solvents. Acyclic E dienes and aromatic ring systems also reacted via [4+2] cycloaddition with these benzynes whereas acyclic Z dienes reacted via [2+2] cycloaddition. Lastly, while not technically involving a silyl-substituted diene or dienophile reaction (unless we consider the silyl benzyne as silyl diene + alkyne), Suzuki and co-workers reported using bromosilylphenyl tosylates (145) as precursors for intramolecular benzyne-diene Diels-Alder reactions utilizing a silicon tether (146) (Scheme 39) [36]. A variety of alkyl lithium and Grignard reagents were investigated as metalating agents but the magnesite (Ph3MgLi) provided the highest yields of products (147) in a variety of ether solvents. Acyclic E dienes and aromatic ring systems also reacted via [4+2] cycloaddition with these benzynes whereas acyclic Z dienes reacted via [2+2] cycloaddition. Scheme 39. Intramolecular benzyne-diene Diels-Alder reactions.

Siloles
Like boroles, there are many publications on siloles and their uses in photochemical and electrochemical applications. The number of reports on their synthesis and use in cyclization or cycloaddition reactions is much smaller.

Siloles
Like boroles, there are many publications on siloles and their uses in photochemical and electrochemical applications. The number of reports on their synthesis and use in cyclization or cycloaddition reactions is much smaller.
Matsuda and co-workers reported a synthesis of siloles (151) using a rhodium-catalyzed inter and intramolecular cyclization of alkynes and diynes with hexamethyldisilane (149) (Scheme 40) [37]. This reaction proceeded in 30-70% yields for a number of symmetrical alkynes (148) but yielded intractable mixtures or dienes rather than siloles with unsymmetrical alkynes. The reaction was proposed to start with transmetallation to produce a Rh-SiMe 3 species which does two alkyne insertions to produce intermediate (150). Oxidative addition of a SiMe bond followed by reductive elimination would produce the silole (151) plus a Rh-Me species that could continue the cycle. Molecules 2020, 25, x FOR PEER REVIEW 22 of 24 Scheme 45. Preparation of silicon-substituted naphthyne precursors.

Conclusion
Boron and silicon substituted dienes and dienophiles continue to be heavily studied. Their ease of synthesis and handling coupled with their ability to serve as surrogates for a variety of other functional groups via subsequent cross coupling, oxidation, or halogenation chemistry is responsible for their ongoing study and use by synthetic chemists. As new, milder and more functional group tolerant reaction conditions continue to be explored for cross coupling reactions, one would predict that utilization of main group element substituted dienes and dienophiles will become even more widespread in the future.
Funding: This review received no external funding.
Boron and silicon substituted dienes and dienophiles continue to be heavily studied. Their ease of synthesis and handling coupled with their ability to serve as surrogates for a variety of other functional groups via subsequent cross coupling, oxidation, or halogenation chemistry is responsible for their ongoing study and use by synthetic chemists. As new, milder and more functional group tolerant reaction conditions continue to be explored for cross coupling reactions, one would predict that utilization of main group element substituted dienes and dienophiles will become even more widespread in the future.
Funding: This review received no external funding.