Conformations of Silicon-Containing Rings. 5.^, Conformational Properties of 1-Methyl-1-silacyclohexane:  Gas Electron Diffraction, Low-Temperature NMR, and Quantum Chemical Calculations

Abstract

The molecular structure of 1-methyl-1-silacyclohexane 3 has been determined by gas electron diffraction (GED). The conformational preference of the methyl group was studied experimentally in the gas phase (GED) and in solution (low-temperature (13)C NMR) and by quantum chemical calculations (HF, MP2, and B3LYP with 6-31G basis sets and mPW1PW91/6-311G(2df,p)). Both experimental methods result in a preference of the equatorial position of the methyl group, 68(7)% in the gas phase at 298 K and 74(1)% in solution at 110 K. The calculations predict 68-73% equatorial conformer at room temperature. From coalescence temperatures, Gibbs free energies of activation for ring inversion DeltaG++ (eq --> ax) = 5.81(18) and DeltaG++ (ax --> eq) = 5.56(18) kcal mol(-1) were derived. The calculated values for DeltaG++ (eq --> ax) are 5.92 (B3LYP) and 5.84 kcal mol(-1) (mPW1PW91).

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

“…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.…”

Stereochemistry of 3-isopropoxy-3-methyl-1,3-oxasilinane–the first 3-silatetrahydropyran with an exo-cyclic RO–Si bond

“…A C C E P T E D ACCEPTED MANUSCRIPT [6] Along with this assignment conformer 1-RO eq proves to be slightly preferred (50.25 %); the second conformer 1-RO ax (49.75 %) is slightly at a disadvantage. The free energy difference proves with ∆G° = −0.02 kcal/mol to be adeqately low in preference of the 1-RO eq conformer.…”

Conformational properties of 1-tert-butyl-1-silacyclohexane, C5H10SiH(t-Bu): gas-phase electron diffraction, temperature-dependent Raman spectroscopy, and quantum chemical calculations

“…In recent years, A-values for some monosubstituted derivatives of 3 have been reported. Methyl [13][14][15] and phenyl [16] substituents were found to have positive A-values, albeit much lower in magnitude than the corresponding cyclohexane analogs [17][18][19]. Other substituents like CF 3 [20,21] and SiH 3 [22] were found to prefer the axial position contrary to their cyclohexane analogs.…”

“…A comparison with the smaller methyl group may be expected to deliver useful information. Included in Table 1 are the GED parameters for the methylated ring 5 [13]. It can be seen that the Si-C7 bond is considerably longer in the case of t-butyl than the methyl group (difference 0.047 Å ).…”