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

摘要

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 (-°G~0 - 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 δE 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 (-δG~0 - 1.8 kcal/mol) and the predominance of the 3-SiMe _(3 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.