Vinylsilanes addition reactions

Spectroscopic data of a-silylketones and the chemical behaviour of vinylsilanes (addition reactions opposite to Markownikoff s rule) are in agreement with this (p- -d) . model. 

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... 

This reaction was successfully applied to various hydrazones derived from acyclic or cyclic ketones and extended to a variety of unsubstituted vinylsilanes as acceptors (trialkyl, triphenyl and alkoxy vinylsilanes). In all cases, the regioselectivity of the addition reaction was consistent with the formation of a silicon-stabilized organozinc reagent. 

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). 

Carbolithiation reactions of ketone a,-dianions, generated by the above amine-free method with several alkenes, such as styrenyl derivatives, vinyl sulfides and vinylsilanes, can lead to the generation of ketone a,5-dianions (Scheme 15)14. For example, when one equivalents of triphenylvinylsilane was treated with a ketone a,-dianion, in THF, at 0 C for 1 h and the resulting reaction mixture was quenched by 2.2 mol equivalents of trimethylchlorosilane, the corresponding bis-silylated enol silyl ether was obtained. Substituted styrenyl derivatives, such as 1,1-diphenylethylene and cinnamyl alcohol, also underwent a smooth carbolithiation to give the corresponding ketone a,5-dianions. Similar addition reactions of ketone a,f)-dianions to vinyl phenyl sulfide took place smoothly to give a,5-dianions with a sulfur attached in the 5-position. 

Addition reactions are not the sole processes of the transformation of the vinylsilane group to another functional group. A silylative coupling reaction recently discovered by Marciniec et al. [12], may be used for the introduction of functional groups to polysiloxane, too. 

The intermolecular addition of vinylsilanes to C—X ir-systems is a well-studied reaction j however, not until 1981 had the inuamolecular variant been demonsuated. Thus, coverage of vinylsilane additions in this chapter will focus on the regio- and stereo-selectivity in inuamolecular (cyclization) reactions. Two modes of cyclization are possible, as determined by the position of the alkene with respect to the ring system cyclization can occur in an endocyclic or an exocyclic mode with respect to the vinylsilane group (Figure 10). 

In addition reactions to carbonyl compounds C-3 attack generally predominates,as illustrated in Scheme 72 with the formation of vinylsilane (163). C-1 addition also predominates in alkylation and acylation reactions involving a-silylallyl anions when alkyl halides and acyl halides are used as electrophiles. 

The stability of the -C -Si< bond has been known for a long time But on the other hand they are reactive compounds which undergo either - as precursors to vinylsilanes - various types of addition reactions or - as only silyl-protected acetylenes - an electrophilic substitution under Friedel-Crafts conditions in presence of catalytic amounts of Lewis acids The — SiR3 moiety has a highly useful protecting and/or activating function. 

The main synthetic pathways to vinylsilanes are (a) direct processes in the gas phase, (b) Grignard reactions, (c) metallation reactions (e.g. with lithiated reagents), (d) hydrosilylation of alkynes and related addition reactions, (e) hydrogenation of alky-nylsilanes, and (f) elimination reactions of alkylsilanes. Methods (a)-(e) are mostly used method (f), however, has become less common nowadays. 

Vinylsilanes 132 have been reacted with bromomalononitrile to yield the intermediate 133, which was used for the synthesis of cyclopropane derivatives Addition reactions of dichloromalononitrile with substituted alkenes and alkadienes can also be used for the preparation of intermediates in carbo- and heterocyclic synthesis. 2-Arylmalononitriles 135 have been produced by coupling malononitriles with aryllead(IV) triacetates like 134163. 

The hydrosilylation of alkenes (Equation 16.12) and alkynes (Equation 16.13), alternatively termed hydrosilation, is the addition of a silicon-hydrogen bond across the C-C TT-bond to form a new alkylsilane or vinylsilane. This reaction has been catalyzed by complexes containing many different metals, but is most commonly conducted with complexes of platinum, rhodium, and palladium. The hydrosilylation of alkenes t3q>ically forms terminal alkylsilanes as the major regioisomer, and the hydrosilylation of vinylarenes often generates the chiral branched alkylsilane. The hydrosilylation of alkynes has also been developed. As shown generally in Equation 16.13, these reactions can occur by either cis or trans addition, depending on the catalyst. In some cases, the reactions of silanes with olefins form vinylsilanes (called dehydrogenative silylation. Equation 16.14). The addition of an Si-Si bond of a disilane across an olefin has also been reported (Equation 16.15), and this reaction is called disilation of olefins. 

The lower-order silyl cuprates are apparently less reactive than the higher-order cuprates in conjugate addition reactions. Reagent (1) has been added in an addition-elimination sequence to an alkylidenemalonate system as a route to vinylsilanes (eq 4). ... 

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