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

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 103 K was measured to be about 5 ax : 5 eq  = 15:85, 6 ax : 6 eq  = 50:50 and 7 ax : 7 eq  = 25:75. The equatorial preference of the methyl group in 5 (0.35 kcal mol −1 ) is much less than in 3‐methylthiane 9 (1.40 kcal mol −1 ) but somewhat greater than in 1‐methyl‐1‐silacyclohexane 1 (0.23 kcal mol −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 ( 6 ), 5.1 (Me ax  → Me eq ) and 5.4 (Me eq  → Me ax ) kcal mol −1 ( 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. Copyright © 2010 John Wiley & Sons, Ltd.