Some asymmetric organosilicon compounds.
thesisposted on 19.11.2015, 08:45 by Peter William. Jones
A survey is made of the literature concerning the resolution of asymmetric organosilicon compounds and the application of stereochemical studies to the elucidation of the mechanism of nucleophilic substitution at silicon. The preparation and resolution of p-[ethylmethyl(p-methoxy- phenyl)silyl]benzoic acid (I) and its conversion into optically- active p-[ethylmethyl(p-methoxyphenyl)silyl]benzyl alcohol (II), p-[ethylmethyl(p-methoxyphenyl)silyl]benzyl methyl ether (III), and methyl p-[ethylmethyl(p-methoxyphenyl)silyl]benzoate (IV), is described. In all, eleven previously unreported compounds were isolated. Unsuccessful attempts were made to prepare optically-active silicon-functional arylethylmethylsilyl- compounds by selective bromine cleavage of the p-methoxyphenyl - silicon bond of each of the compounds I, III, and IV, and subsequent treatment of the reaction mixture with isopropyl alcohol or lithium aluminium hydride. Explanations for the apparent lack of stereospecificity in these reactions are offered. Optically-active methylphenyl(a-naphthyl)methoxysilane (V) was prepared, labelled with tritium in the methoxy group. The methanolysis of V in neutral, basic, and acidic, methanol was investigated: the pseudo first-order rate constant was determined for each reaction mixture, both polarimetrically and [by Dr. R. Baker and Mr. R. Spillett] radiochemically. For each reaction mixture, these two values of the rate constant were identical within experimental uncertainty (<5 %). The significance of this result is discussed, and an analysis is presented of the stereo-chemical requirements of those mechanisms of nucleophilic sub-stitution at silicon which are currently postulated. An attempt is made to rationalise, in terms of reaction mechanism, those stereochemical results of nucleophilic substitution at silicon as are available in the literature.