Metathesis, vinyl silanes

Cross-metathesis of enynes having various functional groups on the alkyne and an alkene gives dienes having useful functional groups such as vinyl silane or enol ether as the sole product ... 

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. 

Complex vinyl silanes may also be formed from simpler vinyl silanes by metathesis reactions. A ring closing metathesis (Section 8.3.3) to form a silicon heterocycle 2351, with a necessarily cis alkene was employed in a synthesis of brasilenyne 2.354 (Scheme 2.110). The RCM was followed by an intramolecular Hiyama... 

Some alkenes with heteroatoms can be used in cross-coupling reactions. Vinyl boranes are an important group, as the carbon-boron bond can subsequently be converted into so many other functional groups, such as halides (Scheme 8.97). ° An example of cross-metathesis of a vinyl borane, followed by Suzuki coupling, can be found in Scheme 11.40. An example of a vinyl silane metathesis can be found in Scheme 2.110. 

Whereas the vinyl groups of Da are accessible for functionalization by hydroboration or hydrosilylation, they are inert to functionalization by cross-metathesis. Alternatively, formal metathesis products can be obtained by the ruthenium-catalyzed silylative coupling reaction. This method involves the combination of a vinyl silane and an olefin in the presence of a ruthenium catalyst, to provide an alkenylsilane (see eq 7). The application of this reaction to Da provides substitution at each of the four vinyl groups, resulting in a cyclic tetraalkenyltetramethylcyclote-trasiloxane. The silylative coupling reaction of both Da and has been demonstrated with styrenes and enol ethers. ... 

The ability to provide highly functionalized reagents, such as unsaturated silanes and vinyl boronates, starting from terminal olefins is one of the most attractive attributes of CM, particularly when traditional methods for the preparation of such compounds are not synthetically straightforward. Therefore, significant research has been undertaken to determine the broad-spectrum chemoselectivity of olefin metathesis catalysts. In many cases, the use of a CM protocol in reagent synthesis is completely orthogonal to alternative methods of preparation. 

Summary Two catalytic reactions, i.e. silylative coupling (mms-silylation) (SC) catalyzed by complexes containing or generating Ru-H and/or Ru-Si bonds (I, II, V, VI) and cross-metathesis (CM) catalyzed by mthenium-carbene (i.e. 1st and 2nd generation mthenium Grubbs catalyst (ID, IV)) of vinyl and allyl-substituted hetero(N,S,B)organic compounds with conunercially available vinyltrisubstituted silanes, siloxanes, and silsesquioxane have been overviewed. They provide a universal route toward the synthesis of well-defined molecular compounds with vinylsilicon functionality. 

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). 

On the other hand, although well-defined or in-situ initiated metallacarbenes are inactive for selfmetathesis of vinyl-substituted silanes and siloxanes, we revealed recently a high catalytic activity of Grubbs catalyst in cross-metathesis of vinyltrialkoxysilanes and vinyltrisiloxanes with styrene, 1-alkenes and selected allyl ethers and other derivatives [10-12]. 

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. 

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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