Proximity effects in 29Si, 13C, and 1H NMR spectra of ortho substituted phenoxytrimethylsilanes

Abstract

NMR Spectra of eight ortho substituted phenoxytrimethylsilanes, 2-X-C6H4-OSi(CH3)3 (X = Cl, Br, OCH3, NH2, NO2, OSi(CH3)3, CH3, and H), are reported. In contrast to analogous ortho substituted methoxybenzenes the 13C chemical shifts of C-2 and C-6 aromatic carbons do not exhibit consistent trends indicating different conformational preferences in the trimethylsiloxybenzenes. Under the influence of the ortho substituents the nuclei of OSi(CH3)3 group (29Si, 13C, and 1H) are deshielded; compounds with X = CH3 (and H) appear anomalous in this respect. It is argued that this proximity effect is not due to an interaction involving terminal methyl groups but involves the oxygen atom of the OSi(CH3)3 group; it is most likely due to an interaction with unshared electrons of the ortho substituent.