Effects of methyl substitution in 4-silathiane S-oxides on the stereochemistry and 1JCH coupling constants: Buttressing effect of axial sulfinyl group as the origin of the reverse Perlin effect

摘要

An NMR study of the products of lithiation/methylation of 4,4-dimethyl-4-silathiane S-oxide 1, diastereomers of 2,4,4-trimethyl-4-silathiane S-oxide 2 and 2,4,4,6-tetramethyl-4-silathiane S-oxide 3, as well as 4,4-dimethyl-4-silathiane S,S-dioxide 4 and 2,4,4-trimethyl-4-silathiane S,S-dioxide 5 is reported. The 2-Me group in 2,4,4-trimethyl-4-silathiane S-oxides is always equatorial while the SO group may occupy either the equatorial (major isomer, 2ee) or axial (minor isomer, 2ae) position. 2,4,4,6-Tetramethyl-4-silathiane S-oxide exists in the form of the two isomers, the one with 2-Me, 6-Me and sulfinyl groups all equatorial (3eee), and the other one as an equilibrium mixture of the axial and equatorial sulfoxides with 2-Me axial and 6-Me equatorial (3aae ?s3eae). The normal Perlin effect (JCHax?JCHeq) is found for the CH2 groups in all studied compounds except for the 3- and 5-CH2 groups in 2ae and 5, which show a small reverse Perlin effect (JCHax?>?JCHeq). The experimental findings are interpreted in terms of the s (Csbnd;Hax)???s* (Sdbnd;O) stereoelectronic effect for the Csbnd;H bonds in the 2- and 6-positions, and the buttressing effect of the axial SO group on the Csbnd;Hax bonds in the 3- and 5-positions and confirmed by GIAO-B3LYP/6-311G(d,p) theoretical calculations.