Enol ethers, silylene reactions

Disilanyl enol ethers (22) are the main product of the reaction of alkyl ketones with photochemically generated phenyltrimethylsilylsilylene (Scheme 30) (77JOM(135)C45). They could result from silylene insertion into the enol O—H bond, particularly as many have been isolated from dodecamethylcyclohexasilane and alkyl ketones or aldehydes on photolysis (Scheme 31) (78CL609). 

Products of the type (24) also result from enolizable ketones without the formation of silyl enol ethers if the reaction is carried out in the presence of tertiary phosphines. The proposed mechanism involves the betaine R3P—SiMe2 as the silylene transfer agent. In preventing a 1,3-hydrogen migration, the phosphine may well induce dimerization prior to oxasilacyclopropane formation. The dioxadisilacyclohexane (24) can be reduced with LiAIHU to give dimethylsilyl-substituted carbinols, so the reaction is of synthetic value (Scheme 34) (78JA7074). 

Steady-state kinetic analysis of a competition experiment led to the conclusion that the siloxolane is formed by reaction of a vinylsilirane intermediate with acetone, and that the vinylsilirane arises from addition of the free silylene to butadiene. Since silylenes are known to react more rapidly with acetone than with butadiene, the kinetic analysis further suggested that the carbonyl sila-ylide dissociates more rapidly than it rearranges to the silyl enol ether shown in equation 64140. 

Diketones often are protected as enol ethers or enamines and these selectively functionalized compounds may be subjected to complementary transformations (Scheme 94). Also silylenes can be prepared from diketones and -hydroxycarbonyl compounds by reaction with dimethyldicyanosilane. Naturally, these blocking groups are relatively sensitive to hydrolysis. On the other hand, partial solvolysis can open a route to monoprotected derivatives (e.g. 101), usually blocked at the sterically less demanding carbonyl function as 0-silyl cyanohydrins (see Scheme 95). Deprotection is finally achieved with silver fluoride in THF. 

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