NMR, solvation and theoretical investigations of conformational isomerism in 2-X-cyclohexanones (X = NMe2, OMe, SMe and SeMe)

The conformational equilibria of 2-N,N-dimethylamino- (1), 2-methoxy- (2), 2-methylthio- (3) and 2-methylselenocyclohexanone (4) were determined in various solvents by measurement of the (3)J(H-2,H-3) couplings. The observed couplings were analyzed using theoretical and solvation calculations to give both the conformer energies in the solvents studied plus the vapor-phase energies and the coupling constants for the distinct conformers. These gave the conformer energies and couplings of 2-4. The intrinsic couplings for the 2-N,N-dimethylamino compound were determined by the molecular mechanics PCMODEL program. The axial conformation in 1 is the most polar and also more stable in DMSO solution (E-eq - E-ax = 0.05 kcal mol(-1)) and the pure liquid, while the equatorial conformer predominates in the remaining solvents studied (except in CCl4, where self-association is observed). In the methoxy ketone (2) the equatorial conformation is more stable in the vapor (E-eq - E-ax = -0.30 kcal mol(-1)) and in all solvents. The opposite behavior is shown by 3 and 4, where the axial conformation is the more stable one in the vapor phase (E-eq - E-ax = 1.60 and 2.95 kcal mol(-1) for 3 and 4, respectively) and is still the prevailing conformer in solution. The axial predominance for 3 and 4 is attributed to hyperconjugation between the electron lone pair of the heterosubstituent and the pi*(CO) orbital. This interaction is stronger for 3 and 4 than in the case of 1 and 2, where the 'gauche effect' in the equatorial conformation should be more effective in stabilizing this conformation. Copyright (C) 2003 John Wiley Sons, Ltd.