The beta effect in alkylsilanes

Abstract

It was suggested to us that compounds of the type XCH2SiR2CH2CH2Y might show interesting chemical and biological activity due to them possessing both an alpha group and a beta group. The aim of this research was to discover whether or not the alpha and beta effects interact with each other, and if so whether interaction is via steric or electronic effects. A series of compounds were made with a constant chloromethyl alpha function and varying beta functions (hydroxy, methoxy and chloro groups); plus a second series of trimethylsilyl substituted silanes with the same variety of beta functions were synthesised. The stereochemistry of the products was investigated by analysis of NMR spectra and of dipole moment data. It was found that the β-chloro-substituted compounds possessed restricted rotation. The methoxy- and hydroxy-substituted compounds which displayed more or less simple triplets, appear to possess free rotation; the smaller sized hydroxy and methoxy groups seemingly no great barrier to rotation. Similarly, compounds possessing larger alpha alkyl groups appeared also to possess restricted rotation, it was concluded that for the compounds possessing large alpha or a large beta function steric effects dominate. The kinetics of the solvolysis reaction were studied. β-functional alkylsilanes commonly undergo solvolysis by unimolecular elimination at remarkably enhanced rates. The β-hydroxy- and β-methoxy-substituted chloroethyl derivatives reacted substantially slower that their trimethylsilyl analogues, due to the electronegative chlorine pulling electrons into the Si-C bond. For compounds possessing an electronegative substituent alpha to silicon it seems it is the electronic effects that act to inhibit the beta effect. 2-Chloroethylchloromethyldimethylsilane initially appeared not to react solvolytically, however NMR analysis of the solvolysis products indicated that a reaction did occur but by an as yet unknown mechanism. For compounds with an a α-electronegative substituent in conjunction with a large β-function it was concluded both steric and electronic effects are important.