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.
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 Citation19 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 Citation20 to describe the effect of the whole substituent on the carbons α and β to a sulfur-containing moiety.