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

Abstract

4,4‐Dimethyl‐4‐silathiane and its S‐oxides [n = 0 (1), 1 (2), 2 (3)] were studied experimentally by variable temperature dynamic NMR spectroscopy down to 103 K and the frozen ring inversion was revealed for all three compounds. The barriers for the degenerate ring inversion in 1 and 3 were measured to be 4.8 and 5.0 kcal/mol at the coalescence temperatures of 111 and 116 K, respectively, and practically coincide with the calculated barriers of 4.60 kcal/mol in 1 and 4.46 kcal/mol in 3. The frozen equilibrium mixture 2‐ax/2‐eq contains 37% of the 2‐ax and 63% of the 2‐eq conformer. The ring inversion barrier proved to be ca. 4.8 kcal/mol. Calculations at the B3LYP/6‐311+G(d,p) level of theory showed the 2‐ax conformer to be 0.90 kcal/mol more stable than the 2‐eq conformer in the gas phase whereas in solution the relative stability of the conformers calculated using the PCM model at the same level of theory is inverted to become 0.19 (in CHCl3) or 0.36 kcal/mol (in DMSO) in favor of the 2‐eq conformer. The chair–chair interconversion mechanism of sulfoxide 2 includes two intermediate energetically equivalent 1,4‐twist forms and the 2,5‐boat transition state: 2‐ax (chair) ⇆ 2 (1,4‐twist) ⇆ [2 (2,5‐boat)]≠ ⇆ 2 (1,4‐twist) ⇆ 2‐eq (chair). The calculated ring inversion barriers are 5.1 (2‐ax → 2‐eq) and 4.2 kcal/mol (2‐eq → 2‐ax) in the gas phase, and 4.03 and 4.22 kcal/mol, respectively, in chloroform. Copyright © 2011 John Wiley & Sons, Ltd.

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

“…[17,18] 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 ΔG ≠ values for 3-methyl-3-silathiane (5.6-6.0), [17] 3-fluoro-3-methyl-3-silathiane (4.6), [17] 3,3-dimethyl-3-silathiane (6.3), [19] 2,3,3-trimethyl-3silathiane (6.8), [19] or 4,4-dimethyl-4-silathiane and its S-oxides (4.5-4.8 kcal mol −1 ). [20] Unfortunately, no reliable data on the ring inversion barrier are available for the Si,O-containing or Si,Ncontaining heterocyclohexanes; only the upper limit of ca. 8.4 kcal mol −1 was estimated for 3,3-dimethyl-1,3-oxasilinane and 1,3, 3-trimethyl-1,3-azasilinane in the early NMR study.…”

Conformational flexibility of 4,4-dimethyl-3,4-dihydro-2H-1,4-thiasiline and its monoheterocyclic analogs

“…Using the same approach as we used for the earlier studied by us silaheterocyclohexanes [2][3][4][5] we tried to examine the conformational behavior of compound I by 1 Н NMR, but even at the lowest accessible temperature of 103 K we observed only broadening of the signals but not their decoalescence. This is indicative of a very low barrier to the ring inversion ΔG ≠ in molecule I (4-5 kcal/mol depending on the difference of the chemical shifts of the decoalescing signals Δν) which is in compliance with a higher flexibility of the cyclohexene ring (ΔG ≠ = 5.37 kcal/mol, [6]) as compared to the cyclohexane ring (ΔG ≠ = 10.3 kcal/mol, [7]).…”

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

“…The conformational equilibrium in solution is shifted to the equatorial conformer, the SOeq:SOax being 63:37, which is comparable with the 55:45 ratio found for thiane S-oxide [47]. Interestingly, theoretical DFT calculations showed the SOax conformer to be more stable by 0.93 kcal/mol [49], which seems to contradict the experiment. However, due to a larger dipole moment of SOeq (5.41 vs. 4.21 D in SOax) the use of the PCM in CHCl3 as the solvent led to the inversion of the relative stability and the SOeq conformer was found to be 0.19 kcal/mol more stable [49].…”

“…It should be mentioned, that in addition to the ax and eq conformers of the sulfimide motif, two rotamers, namely, with the "inward" or "outward" CF3 group may exist Interestingly, theoretical DFT calculations showed the SO ax conformer to be more stable by 0.93 kcal/mol [49], which seems to contradict the experiment. However, due to a larger dipole moment of SO eq (5.41 vs. 4.21 D in SO ax ) the use of the PCM in CHCl 3 as the solvent led to the inversion of the relative stability and the SO eq conformer was found to be 0.19 kcal/mol more stable [49]. This gives the ratio SO eq :SO ax of 70:30 in excellent agreement with the experiment.…”

“…For N-phenylsulfonyl-4-silathiane S-sulfimide ( Figure 7) the conformational equilibrium is almost degenerate (1:1), whereas in N-triflyl-4-silathiane S-sulfimide the equilibrium is shifted to the axial conformer (~55:45) due to electronegative CF3 group [52]. It should be mentioned, that in addition to the ax and eq conformers of the sulfimide motif, two rotamers, namely, with the "inward" or "outward" CF3 group may exist Interestingly, theoretical DFT calculations showed the SO ax conformer to be more stable by 0.93 kcal/mol [49], which seems to contradict the experiment. However, due to a larger dipole moment of SO eq (5.41 vs. 4.21 D in SO ax ) the use of the PCM in CHCl 3 as the solvent led to the inversion of the relative stability and the SO eq conformer was found to be 0.19 kcal/mol more stable [49].…”