Relative Stabilities of the Geometrical and Conformational Isomers of 2-Alkylideneoxacycloalkanes. A DFT Study

The relative stabilities of the geometrical and rotational isomers of 2-alkylideneoxacycloalkanes (-oxiranes, -oxetanes, -tetrahydrofurans, and -tetrahydropyrans; alkyl = Et, Pr, i-Bu, 2,2-dimethylpropyl) have been studied by DFT calculations. Independent of the size of the alkyl group, the E and Z isomers of alkylideneoxiranes have almost comparable stabilities (the Z form, however, being slightly favored), but, with increasing size of the heterocyclic ring, the relative stability of the E isomer decreases. This is particularly prominent for the tetrahydropyran derivatives with alkyl = 2,2-dimethylpropyl, in which marked repulsive interactions between the t-Bu group and the 3-CH₂ group of the tetrahydropyran ring make the E form, ca. 13 kJ mol^–1 less stable than the Z isomer. On the other hand, for alkyl = Et, Pr, and i-Bu, the relative stabilities of the geometrical isomers are almost independent of the alkyl group. Besides the relative stabilities of the geometrical isomers, energetics of the rotational isomerism of the alkyl group about the C(sp³)—C(sp²) bond is also surveyed.