Conformational analysis of 3,3‐dimethyl‐3‐silathiane, 2,3,3‐trimethyl‐3‐silathiane and 2‐trimethylsilyl‐3,3‐dimethyl‐3‐silathiane—preferred conformers, barriers to ring inversion and substituent effects

The first conformational analysis of 3‐silathiane and its C‐substituted derivatives, namely, 3,3‐dimethyl‐3‐silathiane 1 , 2,3,3‐trimethyl‐3‐silathiane 2 , and 2‐trimethylsilyl‐3,3‐dimethyl‐3‐silathiane 3 was performed by using dynamic NMR spectroscopy and B3LYP/6‐311G(d,p) quantum chemical calculations. From coalescence temperatures, ring inversion barriers Δ G ≠ for 1 and 2 were estimated to be 6.3 and 6.8 kcal/mol, respectively. These values are considerably lower than that of thiacyclohexane (9.4 kcal/mol) but slightly higher than the one of 1,1‐dimethylsilacyclohexane (5.5 kcal/mol). The conformational free energy for the methyl group in 2 (−Δ G ° = 0.35 kcal/mol) derived from low‐temperature 13 C NMR data is fairly consistent with the calculated value. For compound 2 , theoretical calculations give Δ E value close to zero for the equilibrium between the 2 ‐Me ax and 2 ‐Me eq conformers. The calculated equatorial preference of the trimethylsilyl group in 3 is much more pronounced (−Δ G ° = 1.8 kcal/mol) and the predominance of the 3 ‐SiMe 3 eq conformer at room temperature was confirmed by the simulated 1 H NMR and 2D NOESY spectra. The effect of the 2‐substituent on the structural parameters of 2 and 3 is discussed. Copyright © 2010 John Wiley & Sons, Ltd.