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 ΔG≠ 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 (−ΔG° = 0.35 kcal/mol) derived from low‐temperature 13C NMR data is fairly consistent with the calculated value. For compound 2, theoretical calculations give ΔE value close to zero for the equilibrium between the 2‐Meax and 2‐Meeq conformers. The calculated equatorial preference of the trimethylsilyl group in 3 is much more pronounced (−ΔG° = 1.8 kcal/mol) and the predominance of the 3‐SiMe3 eq conformer at room temperature was confirmed by the simulated 1H 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. The conformational analysis of 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 low temperature NMR spectroscopy and DFT calculations. The ring inversion barriers of 1 and 2 are lower than in thiacyclohexane but higher than in 1,1‐dimethylsilacyclohexane. Compound 2 shows very slight predominance of the conformer with 2‐Meeq group whereas compound 3 strongly prefers the 2‐equatorial conformation.