Synthesis, conformational preferences in gas and solution, and molecular gear rotation in 1-(dimethylamino)-1-phenyl-1-silacyclohexane by gas phase electron diffraction (GED), LT NMR and theoretical calculations

Abstract

1-(Dimethylamino)-1-phenyl-1-silacyclohexane 1, was synthesized, and its molecular structure and conformational properties studied by gas-phase electron diffraction (GED), low temperature 13C NMR spectroscopy and quantum-chemical calculations. The predominance of the 1-Phax conformer (1-Pheq:1-Phax ratio of 20:80%, ΔG°(317 K) = −0.87 kcal/mol) in the gas phase is close to the theoretically estimated conformational equilibrium. In solution, low temperature NMR spectroscopy showed analyzable decoalescence of Cipso and C(1,5) carbon signals in 13C NMR spectra at 103 K. Opposite to the gas state in the freon solution employed (CD2Cl2/CHFCl2/CHFCl2 = 1:1:3), which is still liquid at 100 K, the 1-Pheq conformer was found to be the preferred one [(1-Pheq: 1-Phax = 77%: 23%, K = 77/23 = 2.8; −ΔG° = −RT ln K (at 103 K) = 0.44 ± 0.1 kcal/mol]. When comparing 1 with 1-phenyl-1-(X)silacylohexanes (X = H, Me, OMe, F, Cl), studied so far, the trend of predominance of the Phax conformer in the gas phase and of the Pheq conformer in solution is confirmed.