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 13 C NMR spectroscopy and quantum-chemical calculations. The predominance of the 1 -Ph ax conformer ( 1 -Ph eq : 1 -Ph ax ratio of 20:80%, Δ G °(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 C ipso and C(1,5) carbon signals in 13 C NMR spectra at 103 K. Opposite to the gas state in the freon solution employed (CD 2 Cl 2 /CHFCl 2 /CHFCl 2  = 1:1:3), which is still liquid at 100 K, the 1 -Ph eq conformer was found to be the preferred one [( 1 -Ph eq : 1 -Ph ax  = 77%: 23%, K  = 77/23 = 2.8; −Δ G ° = −R T 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 Ph ax conformer in the gas phase and of the Ph eq conformer in solution is confirmed.