Vinylsilanes metathesis reactions

In addition to transition metals, recent work has demonstrated that strong Lewis acids will catalyze the addition of silanes to alkynes in both an intra- and an intermolecular fashion.14,14a-14c The formation of vinylsilanes from alkynes is possible by other means as well, such as the synthetically important and useful silylcupration15,15a of alkynes followed by cuprate protonation to afford vinylsilanes. These reactions provide products which can be complementary in nature to direct hydrometallation. Alternatively, modern metathesis catalysts have made possible direct vinylsilane synthesis from terminal olefins.16,16a... 

In contrast, ruthenium catalysts gave the best results for the cross-metathesis reactions of vinylsilanes with a range of unfunctionalised alkenes [8] (a typical example is shown in Eq. 4). 

The chiral Mo-alkylidene complex derived from AROM of a cyclic olefin may also participate in an intermolecular cross metathesis reaction. As depicted in Scheme 16, treatment of meso-72a with a solution of 5 mol % 4a and 2 equivalents of styrene leads to the formation of optically pure 73 in 57% isolated yield and >98% trans olefin selectivity [26]. The Mo-catalyzed AROM/CM reaction can be carried out in the presence of vinylsilanes the derived optically pure 74 (Scheme 16) may subsequently be subjected to Pd-catalyzed cross-coupling reactions, allowing access to a wider range of optically pure cyclopentanes. 

Many other (cross-) metathesis reactions of functionalized olefins have been shown to be possible in the presence of rhenium-based catalysts, such as self-metathesis (or cross-metathesis with normal olefins) of allyl- and vinylsilanes, unsaturated nitriles, chlorides, bromides etc. The products of these reactions are not yet of use in fine chemistry, but this might be remedied by future developments in this area. 

Silyl group transfer. Allylsilanes and vinylsilanes serve as silyl group donors to alkenes such as styrenes and vinyl ethers. It is apparently a metathesis reaction. 

Alkenylsilanes, mainly vinyl silanes and allyl silanes or related compounds, being widely used intermediates for organic synthesis can be efficiently prepared by several reactions catalyzed by transition-metal complexes, such as dehy-drogenative silylation of alkenes, hydrosilylation of alkynes, alkene metathesis, silylative coupling of alkenes with vinylsilanes, and coupling of alkynes with vinylsilanes [1-7]. Ruthenium complexes have been used for chemoselective, regioselective and stereoselective syntheses of unsaturated products. 

Evidence for the migratory insertion of ethylene [46] and vinylsilane [47] into the Ru-Si bond yielding vinylsilane and two bis(silyl)ethene regioisomers [E-l,2-bis(silyl)ethene and l,l-bis(silyl)ethene],respectively,has proved that in the reaction referred to as the metathesis of vinylsilanes and their cometathesis with olefins, instead of the C=C bond cleavage formally characterizing alkene metathesis (Eq. 24a), a new type of olefin conversion that is a silylative coupling of olefins with vinylsilanes occurs (Eq. 24b). 

Vinylamides such as V-vinylpyrrolidone, V-vinylphtalimide and V-vinylformamide undergo, in the presence of (I), stereoselective silylative coupling with various vinylsilanes up to ElZ = 99/1 [17] as well as with octavinylsilsesquioxane [15] (E Z= 95/5). Extended heating of the reaction mixture allowed exclusive isolation of the -isomer because of Z isomerization of the products. In contrast to the cross-metathesis with vinyl ethers, no reaction of iV-vinylpyrrolidinone... 

It has been assumed that the reaction of vinylsilanes with vinyl alkyl sulfides would also occur in the same manner as the reaction with vinyl ether [9] or vinyl pyrrolidinone [17], exhibiting active silylative coupling and inactive cross-metathesis. Nevertheless, we have observed the reverse. 

The reaction of vinyl-substituted silanes and octavinylsilsesquioxane with vinyl-substituted amides, amines (carbazole) as welt as boronates catalyzed by I proceeds effectively to yield under optimum conditions stereo- and/or regio-selectively l-silyl-2-/V- and 1,1-silylboryl-substituted ethenes. 1-silylvinyl carbazole can also be obtained via cross-metathesis of vinylsilane with vinylcarbazole, but only in the presence of the 2nd generation Grubbs catalyst (IV). 

Vinyl and allyl sulfides undergo an unusual cross-metathesis transformation with vinylsilane and vinylsilsesquioxane in the presence of the 2nd generation Grubbs catalyst (IV) to yield 1,2-silylvinyl sulfide or 1,3-silylpropenyl sulfide, respectively. The SC reactions of these substrates performed in the presence of Ru-H (I) and Ru-Si (II) give no products, which is explained by formation of the ruthenium sulfide species - inactive in the SC reactions since there is no insertion of olefin into the Ru-S bond. 

Ito et al. have reported that a sequence of intramolecular hydrosilylation or cya-nosilylation and the Pd-catalyzed coupling reaction is useful for regio- and stereo-defined synthesis of tri- or tetrasubstituted homoallyl alcohols from homopropargyl alcohols (Scheme 10.212) [551, 552]. More recently Denmark et al. have used ring-closing metathesis for the alkene synthesis via vinylsilanes [553]. 

Cross-metathesis. Functionalization of terminal alkenes by the metathetic method using catalyst 1 has been well established. The reaction between styrene and vinylsilanes gives (o-silylstyrenes, between allylarenes and acrylonitrile leads to 4-aryl-2-butenonitriles. Alternatively, homo-metathesis of two allylarene molecules to give 1,4-diary 1-2-butene is first carried out and the cross-metathesis follows. Also of interest is the homo-metathesis of monosubstituted allenes to symmetrical allenes. ... 

Our recent study on the activity for metathesis revealed a high reactivity of 1,9-decadiene and cyclooctene in their reactions with triethoxy- and trisiloxy-substituted vinylsilanes [20]. When the mixture containing vinylsilane and 1,9-decadiene was heated in the presence of Grubbs catalyst, the formation of mono- and bis(silyl)dienes, accompanied by a polymeric product, was detected. The replacement of 1,9-diene with cyclooctene in the reaction mixture has resulted in the same products. The reactions can be illustrated by Eqs. 6 and 7. l,10-bis(silyl)-substituted dienes were isolated with 50-70 % yields and characterized spectroscopically. In the case of disubstituted products separated by distillation, no double bond migration was observed. The only process observed in that case was E/Z isomerization, so only Z,Z-disubstituted products could be isolated. 

High efficiency of the cross-metathesis of 1,9-decadiene and ROM/CM of cyclooctene with vinylsilanes points to a possibility of effective runs of the ADMET copolymerization of 1,9-decadiene [40] and tandem ROM/CM polymerization of cyclooctadiene [20], in both cases with divinylsilicon compounds. The reactions have proceeded according to Eq. 11, yielding polymeric material isolated and analyzed by GPC and NMR methods. 

Finally this type of reaction can be run as a sequence of ring opening and cross metathesis. For example, the norbomene derivative in Equation 21.15 imdergoes reaction with the vinylsilane to generate the optically active cyclopentane in high enantiomeric excess. ... 

The catalytic inactivity of metallacarbene species e.g. Schrock catalyst [32] and Grubbs complex RuCl2(PPh3)(CHPh) in metathesis of vinyl-trisubstituted silanes and siloxanes also supports such a mechanism. This reaction is also called homo(hetero)coupling or trans-silylation of olefins with vinylsilanes. 

Highly selective cross-metathesis of vinylsilanes and vinylsiloxanes with styrene occurs under very mild conditions even at room temperature. This provided evidence for a metallacarbene mechanism catalyzed by RuCl2(PCy3)2Ru=CHPh. The reaction proceeds as in equation 18. 

It is unusual that substitution of Me for OR gives rise to an increase in the conversion of the vinylsilanes in the stoichiometric reactions (from 55 to 95%) but it also drastically reduces the selectivity to the metathesis product from 100% to 15%. 

The effect of substituents at silicon on the reaction is outlined. We discuss advantages and drawbacks of cross-metathesis of vinylsilanes with olefins as a general method for the synthesis of vinyl-trisubstituted silanes and finally present a general mechanistic scheme that explains the processes taking place in the studied systems. Moreover, recent results on the cross-metathesis of a variety of (di)vinyl-substituted silanes, disiloxanes, and monovinyl-substituted siloxanes with olefins are reported [5]. 

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