Trialkylsilyl groups stereoselectivity

Secondly it has been found that a number of trialkylsilyl groups can be added stereoselectively to the /j-position of the C —C double bond in dioxanone 10 (derived from enantiomerically pure 3-hydroxybutanoic acid)23. In this case the presumed silylcuprate is generated from a chlorosilane and a higher-order organocuprate. In the example shown, with addition of phenyldimethylsilyl chloride, a diastereomeric ratio of 91 9 was obtained. Improved ratios were obtained with other groups (e g., 98 2 for trimethylsily 1), but unfortunately none of the latter were susceptible to oxidative cleavage. The first two steps of the procedure produce a mixture of 11 and its silyl enol ether, which is then desilylated to 11 by treatment with tetrabutylammonium fluoride. 

The stereoselectivity of this cleavage reaction can be modulated by the size of the trialkylsilyl group and by the reactivity of the nucleophile, as demonstrated by the use of tert-butyldimethyl-silyl iodide and trifluoroacetate, respectively. ... 

After the stereoselective and regioselective reduction of 44 with trialkylalurninum hydride had been shown to provide the ( )-vinylacetylene 45, it was rewarding to note that the hydroboration was also directed by the trialkylsilyl group. 

Trialkylsilyl vinyl ketenes (72) have been shown to react stereoselectively with a-benzotriazolyl organolithium species to give highly substituted cyclopentenones. The selectivity was found to be kinetic, not thermodynamic, in origin. Several possible mechanisms have been proposed (Scheme 10). It has been suggested that the observed stereoselectivity may result from torquoselectivity in a concerted reaction, or from stereospecific conrotatory cyclization of cation (73), formed stereoselectively because of the interaction shown between the electron-withdrawing group Z and the metal ion.75... 

---