1-t-Butyl-1-phenyl-1-silacyclohexane: Synthesis, conformational analysis in gas and solution by GED, FT-IR and theoretical calculations

At the Experimental Limit of the NMR Conformational Analysis: 29Si and 13C NMR Study of the Conformational Equilibrium of 1-Phenyl-1-tert-butylsilacyclohexane

“…There are two intriguing questions remaining unanswered in our previous work . The first is why the conformational energy ( A -value) of the phenyl group drops sharply when attached to silicon, becoming almost equal to that of the methyl group (0.25 kcal/mol), while the t -Bu group at silicon is still the anchoring group ( A = 1.30 kcal/mol), strongly preferring the equatorial position.…”

“…The second question is, why even with such a large difference of Δ A = 1.05 kcal/mol, does the 1 -Ph eq , t -Bu ax conformer amount to 4%, whereas its content should be <0.5%, that is, undetectable by NMR spectroscopy? The reason is that the presence of the tert -butyl group at silicon prevents free rotation about the Si–C ipso bond, as proved by the substantial energy difference between the “parallel” and “perpendicular” rotamers of the phenyl group . This increases the effective value of A Ph , thus decreasing the energy difference Δ A and allowing both conformers to be detected in the frozen equilibrium.…”

“…Compound 1 was synthesized by the reaction of 1-chloro-1-phenyl-1-silacyclohexane with the pentane solution of t- BuLi in an argon atmosphere at 10 °C . The crude product was purified by column chromatography on silica.…”

“…3,4 For silacyclohexanes, the low temperature 29 Si NMR spectroscopy can also be used, but so far only two papers have been published recently on its successful application for the conformational analysis of (1,1′phenyl-1,1′-silacyclohex-1-yl)disiloxane 5 and 1-Ph-1-SMe-silacyclohexane. 6 In continuation of our ongoing research on the conformational analysis of silacyclohexanes and sila(hetero)cyclohexanes summarized in the recent review, 2 we synthesized 1-phenyl-1tert-butyl-silacyclohexane (1) 7 and examined its molecular structure and conformational preferences by gas-phase electron diffraction (GED), temperature variable IR spectroscopy and quantum chemistry. 7 Low temperature 1 H and 13 C NMR spectroscopy, usually the most informative method for the analysis of the conformational equilibrium in solution and comparison of the results to those obtained by GED, as we did in many previous works, 6,8−12 failed because a trial to freeze the equilibrium led to complete solute precipitation at already 124 K. 7 In the present work, we dissolved 4−6 mg of the isolated and purified liquid [1-phenyl-1-tert-butylsilacyclohexane (1)] in the freon mixture CD 2 Cl 2 /CHFCl 2 /CHF 2 Cl in the ratio 1:1:3 and decreased the temperature.…”

“…6 In continuation of our ongoing research on the conformational analysis of silacyclohexanes and sila(hetero)cyclohexanes summarized in the recent review, 2 we synthesized 1-phenyl-1tert-butyl-silacyclohexane (1) 7 and examined its molecular structure and conformational preferences by gas-phase electron diffraction (GED), temperature variable IR spectroscopy and quantum chemistry. 7 Low temperature 1 H and 13 C NMR spectroscopy, usually the most informative method for the analysis of the conformational equilibrium in solution and comparison of the results to those obtained by GED, as we did in many previous works, 6,8−12 failed because a trial to freeze the equilibrium led to complete solute precipitation at already 124 K. 7 In the present work, we dissolved 4−6 mg of the isolated and purified liquid [1-phenyl-1-tert-butylsilacyclohexane (1)] in the freon mixture CD 2 Cl 2 /CHFCl 2 /CHF 2 Cl in the ratio 1:1:3 and decreased the temperature. While still at 100 K only broadened signals in both 29 Si (cf.…”