Silacyclohexanes, Sila(hetero)cyclohexanes and Related Compounds: Structure and Conformational Analysis

Conformational analysis of Si-mono- and Si,Si-disubstituted silacyclohexanes as well as their analogues with a heteroatom(s) in the ring is reviewed with the focus on the recent results. Experimental measurements in the gas phase (gas electron diffraction, GED) and low temperature NMR spectroscopy (LT NMR) on ¹H, ¹³C and ²⁹Si nuclei are described along with theoretical calculations at the DFT and MP2 levels of theory. Structural and conformational specific features are shown to be principally different from those of the carbon predecessors—the corresponding cyclohexanes, oxanes, thianes and piperidines. The role of various effects (steric, hyperconjugation, stereoelectronic, electrostatic) is demonstrated.

Molecular Structure and Conformational Analysis of 1-Phenyl-1-X-1-Silacyclohexanes (X = F, Cl) by Electron Diffraction, Low-Temperature NMR, and Quantum Chemical Calculations

The molecular structure and conformational preferences of 1-phenyl-1-X-1-silacyclohexanes C₅H₁₀Si(Ph,X) (X = F (3), Cl (4)) were studied by gas-phase electron diffraction, low-temperature NMR spectroscopy, and high-level quantum chemical calculations. In the gas phase only three (3) and two (4) stable conformers differing in the axial or equatorial location of the phenyl group and the angle of rotation about the Si-C_Ph bond (axi and axo denote the Ph group lying in or out of the X-Si-C_Ph plane) contribute to the equilibrium. In 3 the ratio Ph_eq:Ph_axo:Ph_axi is 40(12):55(24):5 and 64:20:16 by experiment and theory, respectively. In 4 the ratio Ph_eq:Ph_axo is 79(15):21(15) and 71:29 by experiment and theory (M06-2X calculations), respectively. The gas-phase electron diffraction parameters are in good agreement with those obtained from theory at the M06-2X/aug-ccPVTZ and MP2/aug-cc-pVTZ levels. Unlike the case for M06-2X, MP2 calculations indicate that 3-Ph_eq conformer lies 0.5 kcal/mol higher than the 3-Ph_axo conformer. As follows from QTAIM analysis, the phenyl group is more stable when it is located in the axial position but produces destabilization of the silacyclohexane ring. By low-temperature NMR spectroscopy the six-membered-ring interconversion could be frozen at 103 K and the present conformational equilibria of 3 and 4 could be determined. The ratio of the conformers is 3-Ph_eq:3-Ph_ax = (75-77):(23-25) and 4-Ph_eq:4-Ph_ax = 82:18.