Thioacetals reaction with allylsilanes

The reactions of titanium-alkylidenes prepared from thioacetals with unsymmetrical olefins generally produce complex mixtures of olefins. This complexity arises, at least in part, from the concomitant formation of the two isomeric titanacyclobutane intermediates. However, the regiochemistry of the titanacyclobutane formation is controlled when an olefin bearing a specific substituent is employed. Reactions of titanocene-alkylidenes generated from thioacetals with trialkylallylsilanes 30 afford y-substituted allylsilanes 31, along with small amounts of homoallylsilanes 32 (Scheme 14.16) [28]. 

Allylstannanes are much better nucleophiles for dithioacetals than allylsilanes, and dimethyl(methyl-thio)sulfonium fluoroborate is a particularly good catalyst in this reaction. The reaction has been used tellingly in a macrocyclization (Scheme 56). ° Monothioacetals also react with allylstannanes, with cleavage of either the carbon-oxygen or the carbon-sulfur bond, depending upon the choice of Lewis acid (Scheme 57).Sugar thioacetals react in the same way (Scheme 58), but with the opposite anomeric stereoselectivity from that of the corresponding radical chain substitution. ... 

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