Collect. Czech. Chem. Commun. 1990, 55, 2019-2026

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

Jan Schramla, Václav Chvalovskýa, Harald Janckeb, Peter Koehlerb, Mikhail F. Larinc, Vadim A. Pestunovichc and Mikhail G. Voronkovc

a Institute of Chemical Process Fundamentals, Czechoslovak Academy of Sciences, 165 02 Prague 6-Suchdol, Czechoslovakia
b Central Institute of Physical Chemistry, Academy of Sciences of G.D.R., 1199 Berlin, G.D.R.
c Irkutsk Institute of Organic Chemistry, Siberian Division of Academy of Sciences of the U.S.S.R., 664033 Irkutsk, U.S.S.R.


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.