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

Abstract

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 ⇆s 3eae). The normal Perlin effect (J _CHax < J _CHeq) 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 (J _CHax > J _CHeq). The experimental findings are interpreted in terms of the σ (C‒Hax) → σ* (S˭O) stereoelectronic effect for the C‒H bonds in the 2- and 6-positions, and the buttressing effect of the axial SO group on the C‒Hax bonds in the 3- and 5-positions and confirmed by GIAO-B3LYP/6-311G(d,p) theoretical calculations.

Keywords:

• Stereoelectronic effects
• Perlin effect
• Silathiane S-oxides

Acknowledgements

This work was financially supported by the Russian Foundation for Basic Research (Grant Number 04-03-32131). B.A.S. is grateful to Professor C.L. Perrin (University of California, San Diego), and to Professor E. Juaristi (Instituto Politécnico Nacional, Mexico) for helpful discussions and valuable comments.

Notes

^†Here we follow the terminology used in 19 where it was referred to as β- and γ-effect as reflecting the effect on atoms β and γ to oxygen. This is distinct from α- and β-effect used by the same authors 20 to describe the effect of the whole substituent on the carbons α and β to a sulfur-containing moiety.