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-alk ylideneoxacycloalkanes (-oxiranes, -oxetanes, -tetrahydrofurans, and -tetra hydropyrans; 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 (Che Z for m, however, being slightly favored), but, with increasing size of the heter ocyclic ring, the relative stability of the E isomer decreases. This is par ticularly prominent for the tetrahydropyran derivatives with alkyl = 2,2-di methylpropyl, 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 almos t independent of the alkyl group. Besides the relative stabilities of the g eometrical isomers, energetics of the rotational isomerism of the alkyl gro up about the C(sp(3))-C(sp(2)) bond is also surveyed.