Stereodynamics of 4‐substituted cyclohexenes. Formyl, vinyl and methyl substituents. Dynamic NMR and molecular mechanics studies

Abstract

The 1H and 13C{1H} NMR spectra of 4-formylcyclohexene in 50% CF2Cl2–50% CHF2Cl decoalesce at very low temperatures and, at 108 K, have sharpened into a major (77%) and a minor (23%) subspectrum. Based on the NMR spectra and molecular mechanics calculations, the major subspectrum is assigned to a family of equatorial conformations and the minor subspectrum to a family of axial conformations. The free energy of activation for conversion of the equatorial conformations to the axial forms is 5.6 kcal mol−1 (1 kcal = 4.184 kJ) at 117 K. Within a family of conformations, interconversion occurs rapidly at 108 K via formyl group rotation. The conformational preference in 4-formylcyclohexene is solvent dependent. In 50% CF2Cl2–50% CHF2Cl, CHF2Cl and CF2Cl2, the respective ratios of equatorial to axial conformations are 77:23, 77:23, and 89:11 at 108 K. The 1H and 13C{1H} NMR spectra of 4-methylcyclohexene and 4-vinylcyclohexene in 50% CF2Cl2–50% CHF2Cl show no evidence of decoalescence at very low temperatures but do show differential broadening and subsequent sharpening of various resonances characteristic of exchange between strongly dominant equatorial conformations and axial conformations present at too low a concentration to be detectable by NMR. A lower limit on the free energy preference for the equatorial conformations is estimated to be 1.0 kcal mol−1. Molecular mechanics calculations also predict a stronger preference for equatorial conformations in 4-methylcyclohexene and 4-vinylcyclohexene than in 4-formylcyclohexene. © 1998 John Wiley & Sons, Ltd.