Intrinsic conformational preferences of and an anomeric‐like effect in 1‐substituted silacyclohexanes

Abstract

A series of ab initio computations have been performed to estimate the CCSD(T) complete basis set limit of the axial/equatorial energy difference for the chair conformation of 1-fluoro, 1-chloro, 1-methyl, 1-hydroxy, and 1-methoxysilacyclohexanes. The equatorial conformation is more stable than the axial by 0.21 kcal mol Ϫ1 for 1-methylsilacyclohexane, while the axial position is more stable for 1-fluoro, 1-chloro, and 1-methoxysilacyclohexane (by 0.09, 0.40, and 0.15 kcal mol Ϫ1 , respectively). The axial and equatorial orientations of 1-hydroxysilacyclohexane are nearly electronically isoenergetic (equatorial favored by 0.03 kcal mol Ϫ1). Zero point vibrational energy corrections have very little effect on the relative energies (less than 0.08 kcal mol Ϫ1 ). These results suggest the presence of an interesting stereoelectronic effect, similar to the anomeric effect, in these silacyclohexanes even without an additional heteroatom, like O, in the ring.

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

“…Unexpectedly, the introduction of the silicon atom into the cyclohexene ring (compound II), in contrast to expectations based on a high conformational flexibility of silacyclohexanes [11][12][13][14][15][16], increases the barrier to inversion by 1-1.5 kcal/mol with respect to cyclohexene. The presence of two heteroatoms in the molecule (compound I) results in the values of the barrier to inversion intermediate between those of compounds II and III.…”

“…Even stronger is the decrease in the inversion barrier upon introduction of the silicon atom in the saturated ring. The lowering of the barrier in silacyclohexanes to 5-6 kcal/mol as compared to 10-11 kcal/mol in the cyclohexane derivatives was attributed to a longer Si-C bond (1.904 Å) in comparison with the C-C bond (1.534 Å) [11,12], although stereoelectronic effects were also allowed to play an important role [13][14][15][16]. There are no data in the literature on the conformational properties of silacyclohexenes, to say nothing of thiasilacyclohexenes.…”

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

“…Taking into account much lower conformational energies А for alkoxy groups as compared to the methyl group in cyclohexanes as well in silacyclohexanes, one might anticipate the predominance of the i ‐PrО ax Me eq conformer in the equilibrium. For cyclohexanes, such a predominance was proved experimentally, and for silacyclohexanes, this follows from the results of theoretical calculations also (partly) proved by the experiment . Therefore, in the presence of two conformers, their correct assignment is principal.…”

Carbohydrate force fields

“…However, the anomeric effect cannot be fully explained in all situations by VSEPR arguments alone, as was pointed out recently 106. In the article, a series of silacyclohexanes have been substituted at the silicon atom.…”