Very low‐temperature dynamic 29Si NMR study of the conformational equilibrium of (1,1′‐phenyl‐1,1′‐silacyclohex‐1‐yl)disiloxane

Abstract

Low‐temperature solution NMR spectroscopy is the most enlightening method for the conformational analysis of silacyclohexanes. In view of much higher flexibility of silacyclohexanes (due to elongated C─Si bonds and expanded C─Si─C angles) as compared with cyclohexanes, the temperatures of decoalescence of appropriate NMR signals in the ring interconversion process are much lower and fall in the range from 140 to 90 K. Most commonly used nuclei for conformational analysis by NMR spectroscopy are H, C, and often F (in organofluorine compounds) nuclei. To the best of our knowledge, there are no examples of the use of Si NMR spectroscopy at low temperature for the conformational analysis of silacyclohexanes and, more generally, of any conformationally flexible organosilicon molecules. In continuation of our ongoing research on the conformational analysis of silacyclohexanes and sila(hetero) cyclohexanes, we synthesized (1,1′‐phenyl‐1,1′‐ silacyclohex‐1‐yl)disiloxane 1 and examined its low‐ temperature H and C NMR spectra; unfortunately, neither were informative as far as the conformational composition of compound 1 is concerned. Therefore, we estimated the relative energies of the three possible conformers (Figure 1) theoretically at the M062X/6‐ 311G(d,p) and MP2/6‐311G(d,p) levels of theory and showed the ax–ax conformer to be the most stable and the eq–eq conformer the least stable one (cf. Figure 1 and Table 1). In this paper, we present the results of the Si low‐ temperature NMR study of the siloxane 1, which allowed to freeze the conformational equilibrium, to determine the ratio of the three conformers and to assign them in accordance with the relative stability by applying the theoretically calculated Si chemical shifts. Si NMR chemical shifts of the conformers of compound 1 and of the reference compound hexamethyldisiloxane, which is most structurally close to compound 1, were calculated at the GIAO/B3LYP/6‐