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

Abstract

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.

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 …”

“…Low temperature solution NMR spectroscopy is the most explicit and enlightening method for the conformational analysis of carbo- and heterocycles and especially for silacyclohexanes. , In view of much higher flexibility of silacyclohexanes (due to elongated C–Si bonds and flattened “Si-part” of the ring as compared to cyclohexanes), the temperatures of decoalescence T c /K of appropriate NMR signals in the ring interconversion process are much lower and usually fall in the range of 110–95 K (the latter being the lowest temperature reached so far in solution NMR spectroscopy). The nuclei most commonly used in conformational analysis by NMR spectroscopy are 1 H, 13 C, or 19 F (in organofluorine compounds). , 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 and 1-Ph-1-SMe-silacyclohexane …”

“…Note that in 1-Ph-1-Me-cyclohexane, the Ph ax Me eq conformer predominates, which may seem to indicate the reversal of the A values of the Me and Ph groups. The reasons were analyzed in a later work, and the different conformational behavior of cyclohexanes and silacyclohexanes was discussed in detail. ,, …”

“…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.…”

Structural and Conformational Aspects in the Chemistry of Heterocycles

“…The difference between the structure and conformational properties of heterocycles and those of their carbon predecessors is determined, first of all, by different bond distances and bond angles, the different electronegativity of carbon and heteroatoms and the presence of lone pairs. Thus, in the review of the Editor of this Special Issue on silacyclohexanes and sila(hetero)cyclohexanes as heterocycles with electropositive heteroatom [ 1 ], principally different structural and conformational features were demonstrated both in gas phase and in solution by GED, LT NMR, FT-IR spectroscopy, and theoretical calculations in comparison with corresponding cyclohexanes, oxanes, thianes and piperidines. The inversion of the role of steric, hyperconjugation, stereoelectronic, and electrostatic effects, as well as of conformational preferences, was clearly shown.…”