Vinylsilanes substitution reactions

Some recent examples of vinylsilane substitutions demonstrating the synthetic utility of such reactions follow. 

Vinylsilanes react with a variety of electrophiles to give substitution or addition products. In substitution reactions, the silyl group is replaced by the electrophile because formation of the carbon-carbon rc-bond is faster than capture of the incipient P-carbocation by the nucleophile. 

There is an extensive review of the electrophilic substitution reactions of allylsilanes and vinylsilanes in Organic Reactions and the reactions of allylstannanes have been reviewed briefly. The major books on synthetic organosilicon and organotin chemistry also cover the types of reaction discussed here. 

Vinylsilanes undergo a large variety of substitution reactions, for example154, those in equations 48 and 49. Often, the stereochemical outcome is retention of configuration of... 

As mentioned in Section n.B.5, the te/t-butyldiphenylsilyl group attached to the olefinic carbon is a poor electrofugal group and hence does not leave easily in electrophilic substitution reactions. On the otha- hand, the epoxide 172, obtained from the corresponding vinylsilane having such a bulky sUyl substituent, gives the carbonyl compound upon... 

TMS-alkynes are oxidized at the terminal carbon to carboxylic acids by hydroboration/oxidation (dicyclohexylborane/NaOH, H2O2). This does not work with TIPS-alkynes. Instead, TIPS-alkynes are cleanly monohydroborated at the internal carbon by 9-borabicyclo[3.3.1]nonane dimer to give (Z)- -borylvinyl-silanes. These can be oxidized in high yields to a-silyl ketones, or cross coupled with a bromide R Br (R = aryl, benzyl, dimethyl-vinyl) in the presence of NaOH and tetrakis(triphenylphos-phine)palladium(0) to give /3,/3-disubstituted vinylsilanes (Suzuki reaction eq 14). The same nucleophilic substituted vinylsilane can be added to an aromatic aldehyde to provide access to ( )-3-silyl allyl alcohols. ... 

For the preparation of divinyl ketones, as required for the Nazarov reaction, various synthetic routes have been developed. A large variety of substituted divinyl ketones, including vinylsilane derivatives, can thus be prepared. The Nazarov cyclization, and especially the vinylsilane variant, has found application for the synthesis of complex cyclopentanoids. 

The Ireland-Claisen reaction of ( )-vinylsilanes has been applied to the stereoselective synthesis of syn- and c/nti-2-substituted 3-silyl alkcnoic acids. a R-2-Alkyl-3-silyl acids are prepared by rearrangement of ( )-silyl ketene acetals which are generated in situ from the kinetically formed (Z)-enolate of the corresponding propionate ester40. Chelation directs the stereochemistry of enolization of heteroelement-substituted acetates in such a way that the syn-diastereomers are invariably formed on rearrangement403. 

The electophilic substitution of allylsilanes and vinylsilanes I, Fleming, J. Dunogues and R. Smithers, Organic Reactions 37, Chapter 2 (1990). 

Alkyne hydrosilylation continues as a focus of current research. Despite the relative simplicity of the transformation, it is becoming increasingly clear that different catalysts often utilize unique mechanisms. In addition, the demands placed by the need to access vinylsilanes of differing substitution patterns, stereochemistries, and functional groups require a diverse, complementary set of methodologies. This discussion covers hydrosilylation reactions... 

The intramolecular coupling of enolethers with enolethers, styrenes, alkyl-substituted olefins, allylsilanes, and vinylsilanes was systematically studied by Moeller [69]. Many of these coupling reactions turned out to be compatible with the smooth formation of quaternary carbon atoms (Eq. 11) [70], which were formed diastere-oselectively and led to fused bicyclic ring skeletons having a ds-stereochemistry [71]. The cyclization is compatible with acid-sensitive functional groups as the allylic alkoxy group. Moeller has demonstrated in some cases that these reactions can be run without loss of selectivity and yield in a simple beaker with either a carbon rod or reticulated carbon as anode without potential control and a 6-V lantern battery as power supply [71]. 

Electroreductive coupling of ketones with silyl-substituted olefins promotes interesting reactions that are useful for organic synthesis. For example, coupKng of ketones with trimethylvinylsilanes affords /I-trimethylsilyl alcohols, which are easily transformed to the corresponding olefins (Scheme 40). This reaction is interesting from the synthetic point of view since vinylsilane behaves as the equivalent to a /I-trimethylsilyl group-substituted anion [77, 83]. 

Vinylsilanes (8, 491 492) attylic alcohols (9, 340). Details are available for conversion of a ketone to a vinylsilane in which the C—Si bond has replaced the C -O group (cncsilylation). The reaction affords the less substituted vinylsilane in the case of unsymmetrical ketones. The paper includes details for use of vinylsilanes for cyclopcntenone annelalion by l riedel-Crafts acylation with acryloyl chlorides and subsequent cyclization of pentadienyl cations (9, 498-499).1... 

However, the directing influence of silicon can be overcome if the vinylsilane contains another substituent that can stabilize a carbocation more strongly than silicon. For example, when the silyl group is attached to C-2 of a terminal alkene, reaction occurs to give the more substituted carbocation 82 (equation 44)107. Similarly, if the silicon is bound to the same carbon atom as a phenyl group, reaction occurs via the benzyl cation to give the product shown in equation 45108. 

Allenylsilanes may behave as allyl or vinylsilanes. In general, allylsilane behaviour dominates over vinylsilane behaviour. The reactions of allenylsilanes with electrophiles have been comprehensively reviewed by Fleming, Dunogues and Smithers118. Electrophilic substitution of allenylsilanes gives alkynes (equation 106)173. 

Vinylsilanes are proper compounds which have won great appreciation as facile intermediates for many sorts of application. They are prepared either by addition reactions of acetylenes (vide supra) or by elimination reactions of saturated organosilanes. The main reactions, carried out with vinylsilanes, are addition reactions to obtain saturated organosilanes or electrophilic substitutions of the silyl group under Friedel-Crafts conditions where a rapid cleavage of the silyl moiety occurs (Scheme 8). 

A novel application of dimethylsulfonium methylide for the general preparation of non-readily accessible and synthetically valuable 1-substituted vinylsilanes and styrene derivatives has been developed.55 It has been found that by varying conditions, the reactions can be tuned in either direction to give an alkene or cyclopropane exclusively. 

Electrophilic substitution of vinylsilanes may be combined with the fazarov cyclization to build five-membered rings, as depicted in Figures i5.11 and Si5.12 (LA = Lewis acid). Note that the reaction depicted in igure Si5.12 is anomalous because the vinylsilane is attacked by the lectrophile at the end of the double bond remote from silicon. This is ecause the sulphur lone pairs stabilize a carbocation more effectively than le silicon P effect. 

---