Conformational properties of 1-fluoro-1-methyl-silacyclohexane and 1-methyl-1-trifluoromethyl-1-silacyclohexane: Gas electron diffraction, low-temperature NMR, temperature-dependent Raman spectroscopy, and quantum chemical calculations

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.…”

Molecular structure and conformational analysis of 3-methyl-3-phenyl-3-silatetrahydropyran. Gas-phase electron diffraction, low temperature NMR and quantum chemical calculations

“…The calculated and experimental relative energies and Gibbs free energies of the conformers are summarized in Table 4 So far, the experimental preferences of the geminally substituted silaheterocyclohexanes were confined to silacyclohexanes containing two substituents with sharply different electronegativity, that is, 1-fluoro-1-methylsilacyclohexane, 28 1-methyl-1-trifluoromethylsilacyclohexane 28 and 1-chloro-1-methylsilacyclohexane. 29 For 1-fluoro-and 1-chloro-1-methylsilacyclohexanes the predominance of the F ax or Cl ax conformers proved by GED, 19 Although the difference in solution is small one should keep in mind that it was obtained from the ratio of the conformers, which is measured very precisely by NMR.…”

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

“…The analysis of their conformational preferences for the silylated species containing the methyl and an electronegative substituent at silicon however shows that they are clearly distinct from their carbon analogues. Thus, while 1-chloro-1-methylcyclohexane strongly prefers the Me eq Cl ax conformation (94:6), 30 the similar conformational preferences in its analogues, 1-chloro-1-methyl-1-silacyclohexane 33 and 1-fluoro-1-methyl-1-silacyclohexane, 34 are notably attenuated, being e.g. Me eq F ax :Me ax F eq = 75:25 for the latter.…”

“…On the other hand, strong axial preference was found even for the bulky CF 3 group in 1-methyl-1-trifluoromethyl-1-silacyclohexane (Me eq CF 3ax :Me ax CF 3eq = 84:16). 34 The only so far conformationally studied silaheterocyclohexane system containing the exocyclic Si-O bond is 1,3-dimethyl-3-isopropoxy-3-silapiperidine, 36 for which, in contrast to steric effects of the methyl and alkoxy groups, the ratio Me eq i-PrO ax :Me ax i-PrO eq = 1:2 was measured experimentally. The intriguing question is if and how the conformational equilibrium would change if the nitrogen atom in the ring is replaced by oxygen.…”