Conformational analysis of 3-methyl-3-silathiane and 3-fluoro-3-methyl-3- silathiane

摘要

The conformational equilibria of 3-methyl-3-silathiane 5, 3-fluoro-3-methyl-3-silathiane 6 and 1-fluoro-1-methyl-1-silacyclohexane 7 have been studied using low temperature ~(13)C NMR spectroscopy and theoretical calculations. The conformer ratio at 103K was measured to be about 5_(ax):5_(eq)=15:85, 6_(ax):6_(eq)=50:50 and 7ax:7eq=25:75. The equatorial preference of the methyl group in 5 (0.35kcalmol-1) is much less than in 3-methylthiane 9 (1.40kcalmol~(-1)) but somewhat greater than in 1-methyl~(-1)- silacyclohexane 1 (0.23kcalmol~(-1)). Compounds 5-7 have low barriers to ring inversion: 5.65 (ax→eq) and 6.0 (eq→ax) kcal mol-1 (5), 4.6 kcalmol-1 (6), 5.1 kcalmol-1 (Me ax→Meeq), and 5.4 kcalmol-1 (Me eq→Meax) (7). Steric effects cannot explain the observed conformational preferences, like equal population of the two conformers of 6, or different conformer ratio for 5 and 7. Actually, by employing the NBO analysis, in particular, considering the second order perturbation energies, vicinal stereoelectronic interactions between the Si-X and adjacent C-H, C-S, and C-C bonds proved responsible.