Unusual conformational preferences of 1,3‐dimethyl‐3‐isopropoxy‐3‐silapiperidine

Abstract

The conformational analysis of the first representative of the Si-alkoxy substituted six-membered Si,N-heterocycles, 1,3-dimethyl-3-isopropoxy-3-silapiperidine, was performed by low-temperature 1 H and 13 C NMR spectroscopy and DFT theoretical calculations. In contrast to the expectations from the conformational energies of methyl and alkoxy substituents, the Me ax i-PrO eq conformer was found to predominate in the conformational equilibrium in the ratio Me ax i-PrO eq : Me eq i-PrO ax of ca. 2 : 1 as from the 1 H and 13 C NMR study. The thermodynamic parameters obtained by the complete line shape analysis showed that the main contribution to the barrier to ring inversion originates from the entropy term of the free energy of activation.

Conformational analysis of 4,4‐dimethyl‐1‐(trifluoromethylsulfonyl)‐1,4‐azasilinane and 2,2,6,6‐tetramethyl‐4‐(trifluoromethylsulfonyl)‐1,4,2,6‐oxazadisilinane

“…The barriers in Table are rather high for silacyclohexanes; for example, the values of Δ G ≠ in 1‐methylsilacyclohexane, 1,1‐dimethylsilacyclohexane, 1‐fluorosilacyclohexane, and 1‐fluoro‐1‐methylsilacyclohexane are equal to, respectively, 5.8 ± 0.1, 5.5, 5.0, and 5.1–6.2 kcal mol −1 . Introduction of the sulfur atom to the α‐position or β‐position to silicon does not affect much the ring inversion barrier as proved by the Δ G ≠ values for 3‐methyl‐3‐silathiane (5.6–6.0), 3‐fluoro‐3‐methyl‐3‐silathiane (4.6), 3,3‐dimethyl‐3‐silathiane (6.3), 2,3,3‐trimethyl‐3‐silathiane (6.8), or 4,4‐dimethyl‐4‐silathiane and its S‐oxides (4.5–4.8 kcal mol −1 ) . Unfortunately, no reliable data on the ring inversion barrier are available for the Si,O‐containing or Si,N‐containing heterocyclohexanes; only the upper limit of ca.…”

Conformational analysis of 4,4‐dimethyl‐4‐silathiane and its S‐oxides

“…An interesting question is why the barrier to ring inversion in 1,1‐dimethylsilacyclohexane (5.5 kcal/mol)24 is increased on going to 3,3‐dimethyl‐3‐silathiane (6.3 kcal/mol)21 but decreased on going to compound 1 (4.8 kcal/mol, Table 1). Although the changes are small, the trend is evident.…”

“…Structural studies of thiasilacyclohexanes (silathianes) and their S‐oxides are much more recent and can be easily surveyed. They are confined to computational,14–22 gas phase electron diffraction, IR and Raman spectroscopy,20 and NMR21–23 studies of 1‐thia‐3‐silacyclohexane and some its derivatives. The conformational analysis of 3,3‐dimethyl‐3‐silathiane was published by us recently21 and the barrier to ring inversion and the frozen proton NMR spectra of the parent compound, 1,1‐dimethylsilacyclohexane, have been published previously by Bushweller et al 24.…”

“…They are confined to computational,14–22 gas phase electron diffraction, IR and Raman spectroscopy,20 and NMR21–23 studies of 1‐thia‐3‐silacyclohexane and some its derivatives. The conformational analysis of 3,3‐dimethyl‐3‐silathiane was published by us recently21 and the barrier to ring inversion and the frozen proton NMR spectra of the parent compound, 1,1‐dimethylsilacyclohexane, have been published previously by Bushweller et al 24. The much lower barrier to ring inversion of 3,3‐dimethyl‐3‐silathiane (Δ G ≠ = 6.3 kcal/mol)21 compared with thiacyclohexane (Δ G ≠ = 9.4 kcal/mol)25 can still be attributed to the more flexible chair conformer due to the longer CSi bonds and flattening of the ‘Si‐part’ of the ring 21.…”

“…The conformational analysis of 3,3‐dimethyl‐3‐silathiane was published by us recently21 and the barrier to ring inversion and the frozen proton NMR spectra of the parent compound, 1,1‐dimethylsilacyclohexane, have been published previously by Bushweller et al 24. The much lower barrier to ring inversion of 3,3‐dimethyl‐3‐silathiane (Δ G ≠ = 6.3 kcal/mol)21 compared with thiacyclohexane (Δ G ≠ = 9.4 kcal/mol)25 can still be attributed to the more flexible chair conformer due to the longer CSi bonds and flattening of the ‘Si‐part’ of the ring 21. However, the barrier in 3,3‐dimethyl‐3‐silathiane (6.3 kcal/mol) is higher than in dimethylsilacyclohexane (5.5 kcal/mol),24 which apparently is due to the fact that puckering of the saturated 6‐membered ring from the sulfur moiety outweighs its higher flexibility due to the longer CS bonds.…”