Conformational behavior of 3,3-dimethyl-3-silathiane 1-oxide and its diastereomeric 2-methyl derivatives

Abstract

The conformational behavior of 3,3-dimethyl-3-silathiane 1-oxide 3 and its trans-4 and cis-4 2-methyl derivatives has been studied by 1 H and 13 C NMR. The equatorial conformer 3-eq for 3 and the diequatorial isomer trans-4-eq for trans-4 predominate (>95%) in CDCl 3 solution. In contrast, the cis-4-ax conformer is the more preferred form of cis-4. The conformational free energy ( G o ) in the equilibrium cis-4-ax ↔ cis-4-eq was determined to be about +0.01 and +0.52 kcal mol −1 at 26 and −120 • C, respectively. The Gibbs free energy of activation ( G ‡ ) for the inversion cis-4-ax → cis-4-eq was estimated to be 8.1 kcal mol −1 at −100 • C.

Crystal structures and theoretical calculations of trans-2,4,4-trimethyl-4-silathiane 1-oxide and 4,4-dimethyl-4-silathiane 1,1-dioxide

“…Their first representatives, 3,3-dimethyl-3-silathiane 1-oxide and 2,3,3-trimethyl-3-silathiane 1-oxide, have been synthesized in our group more than 10 years ago [1,2]. Later on, 4,4-dimethyl-4-silathiane 1-oxide [3], 2,2,3,3-tetramethyl-3-silathiane 1-oxide, 2,4,4-trimethyl-4-silathiane 1-oxide, and 2,4,4,6-tetramethyl-4-silathiane 1-oxide were obtained [4], some of their diastereomers separated and isolated as individuals, and their chemistry [5] and stereochemistry studied [6,7]. A series of works on theoretical conformational analysis of various Si-substituted 4-silathiane 1-oxides has also been published [8][9][10][11].…”

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

“…The trans-isomer can adopt only the eq-eq form, but for the cis-isomer the MeeqSOax and MeaxSOeq are in equilibrium (Figure 8) with the ratio MeeqSOax:MeaxSOeq equal to 1:1 at room temperature or 5:1 at −120 o C. This is consistent with the large AMe value (1.76 kcal/mol) and a small negative value of ASO (−0.18 kcal/mol). Although the presence of silicon in the ring lowers the AMe value and makes the ASO value positive, the MeeqSOax conformer is still preferable at low temperatures [48]. Smaller conformational energies A are inherent not only to substituents at silicon, but, because of longer C−Si and C−S bonds, also to the groups attached to the endocyclic carbon atoms.…”

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

“…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.…”