Conformational rivalry of geminal substituents in silacyclohexane derivatives: 1-Phenyl vs. 1-OR, R=H or Me

Add time:07/15/2019    Source:sciencedirect.com

Molecular structure and conformational equilibria of 1-methoxy- 1 and 1-hydroxy-1-phenylsilacyclohexanes 2 were studied by quantum chemical (QC) calculations and combined gas electron diffraction/mass spectrometry (GED/MS). Both molecules may exist in 5 or 6 forms, differing from each other by the substituents' position: (i) axial or equatorial and (ii) rotational orientation relative to the six-membered ring frame. The contribution of axial forms of both compounds varies from 35 to 60% depending on the theoretical method applied. From the GED data, the summarized molar fractions of the conformers were found to be Phax:Pheq = 70(15):30(20) and 50(20):50(20)% which corresponds to ΔG = Gax–Geq = −0.55(46) and 0.00(56) kcal/mol, for compound 1 and 2, respectively. The concentration of the Phax forms of 1-phenyl-1-(X)-silacyclohexanes (X = H, HO, Me, MeO and Me2N) increases with the size of the second substituent at the silicon atom: 38(10)<50(20)<58(15)<70(15) <80(15)%.

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