NMR study of orgnasilicon compounds : X. The transmission of substituent effects in phenoxysilanes as studied by 29Si and 13C NMR

Abstract

29Si and 13C NMR chemical shifts for a series of meta and para substituted phenoxytrimethylsilanes are given and compared with those in phenyltrimethylsilanes using the formal single and dual substituent parameter analysis of substituent effects. The silicon chemical shift is found to be about twice as sensitive to substituent effects in phenoxytrimethylsilanes as in phenyltrimethylsilanes. The chemical shift sensitivity to substituent effects, ϱ, is considered to be a product of two factors, ϱ el and ϱ shield, which describe the sensitivity of the electron density to substituent effects and the sensitivity of the shielding to the electron density, respectively. Using 13C chemical shifts and CNDO/2 net atomic charges, it is shown that the substituent effects propagate within XC 6H 4 fragment of phenoxysilanes no better than in phenylsilanes. The 13C chemical shifts of the terminal methyl groups are affected by the substituents in the former series of compounds much less than in the latter. An increase in the relative basicity of oxygen is accompanied by an increase in silicon shielding in phenoxytrimethylsilanes. According to CNDO/2 calculations, the substituents cause larger changes in net atomic charges on the silicon atom if it is bonded directly to the benzen ring rather than via the oxygen bridge. In spite of the fact that the possibility of a dative O·→Si interaction, not reflected by the CNDO/2 calculations, cannot be completely excluded, the results that the increased silicon shift sensitivity to substituent effects in phenoxysilanes is due to higher sensitivity of silicon shielding (ϱ shield) to electron density in these compounds rather than to a bettr transmission of electronic effects (ϱ el). The existing theory of silicon shielding must be improved or refined if it has to accomodate the increased sensitivity in the phenoxysilanes.