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

Abstract

The molecular structure and conformational preferences of 1-phenyl-1-X-1-silacyclohexanes CHSi(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 bond (axi and axo denote the Ph group lying in or out of the X-Si-C plane) contribute to the equilibrium. In 3 the ratio Ph:Ph:Ph is 40(12):55(24):5 and 64:20:16 by experiment and theory, respectively. In 4 the ratio Ph:Ph 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 conformer lies 0.5 kcal/mol higher than the 3-Ph 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:3-Ph = (75-77):(23-25) and 4-Ph:4-Ph = 82:18.

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

“…The results are compiled in Table 4. For comparison, the recently published data for 1-cyano- [74] and 1-methoxy-1-silacyclohexane [75] as well as disubstituted silacyclohexanes with the phenyl group and Hlg = F, Cl [76] or other electronegative groups at silicon are included. Note, that 1-bromo-1-phenylsilacyclohexane has also been synthesized [77] but, because of low stability, its conformational analysis could not be performed.…”

“…Note, that 1-bromo-1-phenylsilacyclohexane has also been synthesized [77] but, because of low stability, its conformational analysis could not be performed. [76] * No decoalescence in NMR. Slight predominance of the ax conformer according to Raman spectroscopy.…”

“…In light of this, the predominance of Ph ax F ax , though small, is puzzling. To rationalize the observed conformational ratios, the energy partitioning analysis was employed [76].…”

“…In solution, the assignment of signals in the 13 C-LT NMR spectra is not unequivocal, either. By comparing with the spectra of 1-Ph and 1-Ph,1-X-silacyclohexanes (X = Me, OMe, F, Cl) after decoalescence [6,72,76], all experimental and theoretically calculated chemical shifts, except those for C-1,5 signal, argue for 1-Ph eq N ax to be the preferred conformer of 1-(dimethylamino)-1-phenylsilacyclohexane.…”

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

“…In continuation of our ongoing research on the conformational analysis of silacyclohexanes and sila­(hetero)­cyclohexanes summarized in the recent review, we synthesized 1-phenyl-1- tert- butyl-silacyclohexane ( 1 ) and examined its molecular structure and conformational preferences by gas-phase electron diffraction (GED), temperature variable IR spectroscopy and quantum chemistry . 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, ,− failed because a trial to freeze the equilibrium led to complete solute precipitation at already 124 K …”