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

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

“…The answer to the question of conformational equilibrium of the studied siloxane was obtained from the 29 Si-LT NMR spectra, which contained the signals of all three conformers [87]. This allowed to determine the ratio of the conformers and to assign them in accordance with the relative stability using the theoretically calculated 29 Si chemical shifts at the GIAO/B3LYP/6-311++G(d,p)//B3LYP/6-311++G(d,p) level.…”

“…The calculated shifts are reliable because (i) they lie in the interval of 2.3 ppm, which is well consistent with the experiment (2.6 ppm); (ii) the highest signal in the spectrum in Figure 21 is the most upfield one; the calculated signal for the most stable and, hence, most abundant ax-ax conformer also lies the most high field; (iii) the signal of lowest intensity is the most downfield one ( Figure 21) and corresponds to the least stable eq-eq conformer; (iv) two signals of equal intensity appear between them and, judged from the relative energies of conformers in Figure 19, belong to the ax-eq conformer. It deserves to be mentioned that the spectrum in Figure 21 represents the first example of application of the low temperature 29 Si-NMR spectroscopy in conformational analysis [87] (cf. also Figure 6 and the text above).…”

“…So, we had to limit our study to theoretical computational analysis [91]. The ring inversion barrier ∆G # measured from the temperature of decoalescence T c and the value of ∆δ of the ax-ax and eq-eq conformers was found to be one of the lowest ever measured for six-membered rings, 4.8 kcal/mol [87].…”

“…It deserves to be mentioned that the spectrum in Figure 21 represents the first example of application of the low temperature 29 Si-NMR spectroscopy in conformational analysis [87] (cf. also Figure 6 and the text above).…”

“…The first sulfur-containing silacyclohexene, 4,4-dimethyl-3,4-dihydro-2H-1,4-thiasiline, was obtained as shown in Scheme 3, in 66% yield by the rearrangement of 4,4-dimethyl-1,4-thiasilinane 1-oxide, using trifluoroacetic anhydride (R = CF 3 ) [88]. It deserves to be mentioned that the spectrum in Figure 21 represents the first example of application of the low temperature 29 Si-NMR spectroscopy in conformational analysis [87] (cf. also Figure 6 and the text above).…”