Z-a-Silyl vinyl sulfides

Under strictly anhydrous conditions which minimize competing protiodesilylation, (Z)-a-silyl vinyl sulfides with an co-carbonyl moiety attached to the S atom undergo a fluoride-promoted intramolecular cyclization which yields 2-alkenyltetrahydropyran-3-ols. The substrates are derived from (Z)-a-silyl enethiols by alkylation with co-halocarbo-nyl compounds (Scheme 157) <2000EJ02391>. 

A large variety of Z-a-silyl vinyl sulfides 55 were prepared starting from Z-a-silyl enethiols 4a-d with two different procedures46,47 (Scheme 36). According to path a (Scheme 36), products 55 were stereoselectively obtained by reacting 4 with halides in acetone in the presence of dry K2C03 at room temperature. The... 

Z-a-Silyl vinyl sulfides are intriguing species since the silyl and the thioether functions exert an opposing polarization on the olefinic bond (Scheme 38). 

The presence of a substituent in the p-position of the a-silyl vinyl sulfides introduces the problem of geometry in the resulting olefins. As yet, relatively few synthetic methods for -substituted a-silyl vinyl sulfides are known and some of them afford the olefins as a mixture of E- and Z-isomers. For example, the Peterson olefination of bis(trimethylsilyl)alkylthiomethyllithium 47 with aldehydes55 and the treatment of the sulfoxides 48 with LDA and trimethylchlorosilane56 gave... 

Stereoselectivity of radical addition is not limited to sulfonyl radicals. The (rani-addition has also been observed for tin, bromine, chlorine, and silyl radicals. Varying degrees of selectivity has been observed for addition of carbon-centered radicals, depending on the substituents size and effect on the inversion barrier. Because the importance of negative hyperconjugation decreases for radicals in comparison to the anions, the barriers for inversion decrease in parallel. As a result, the selectivity can erode under conditions when trapping of the radical is slower than the equilibration, e.g. in the thiol-yne click reaction, which often provide a mixture of E and Z-vinyl sulfides. ... 

Me2AlCl catalyzes the ene reactions of a variety of aldehydes with (Z)-3P-acetoxy-5,17(20)-pregnadiene at —78°C. The stereoselectivity with aliphatic aldehydes is >10 1 in favor of the 22oi-isomer, while aromatic aldehydes produce predominantly the 22p-isomer (eq 7). Me2AlCl is also the Lewis acid of choice for ene reactions of a-halo aldehydes (eq 8). Ene reactions of vinyl sulfides to produce enol silyl ethers are also catalyzed by Me2 AlCl (eq9).i ... 

In the original study by Peterson, the alkenation procedure was found to be compatible with sulfur and phosphorus substitution. The alkenation reaction has been tqrplied successfully to a variety of substituted alkenes. Because of the aiuon-stabilizing nature of the thiophenyl, the p-hydroxysiliuie is not isolated and the elimination to the alkene takes place directly to form a 1 1 mixture of ( )- and (Z)-isomers. Ager studied the reaction of the lithio anions of phenyl (trimethylsilyl)methyl sulfides (318) with a variety of carbonyl compounds (equation 72). Yields of this process were good, and addition occurred even with enolizable substrates. This reaction was extended to vinyl sulfones. In contrast to the sulfide case, the substituted sulfone silyl anion behaves as a base, leading to undesired enolization. The best yields were observed for the case where R is a hydrogen or phenyl. 

---