On the mechanism of the reductive metallation of asymmetrically substituted silyl chlorides

An investigation of the stereochemical course of the reductive metallation of silyl chlorides with silicon-centred chirality has revealed two major events which are detrimental to stereoselection during silyl anion formation: (1) chloride-induced racemisation of silyl chlorides and (2) nonstereoselective formal dimerisation during metallation providing the corresponding disilane. In control experiments, the stereochemical course of these processes has been independently verified for the reductive metallation of the enantioenriched cyclic silyl chloride (SiS)-7a (R = H, er greater than or equal to 88:12). A screening of several related derivatives of (SiS)-7a led to the sterically encumbered silyl chloride (SiR)-7c (R = iPr, er greater than or equal to 94:6) which displays some unique features. This structural modification prevents racemisation by lithium chloride (T < -40 degreesdegreesC) as well as dimerisation (T < -100 degreesC) thus allowing for the first generation of an asymmetrically substituted silyl anion (SiS)-8c (er = 74:26) by reductive metallation of a silyl chloride with silicon-centred chirality. Moreover, the enantiospecificity of the preparation of (SiR)-7c by chlorination [(SiS)-9c --> (SiR)-7c] and its reduction with aluminium hydrides [(SiR)-7c --> (SiR)-9c] have been unambiguously determined by X-ray crystallography as retention (greater than or equal to99%) and inversion (greater than or equal to99%), respectively. (C) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005.