Thermodynamics of exo-endo isomerization of carbon-carbon double bonds in five-membered ring compounds. A quantum chemical study

The energies and enthalpies of isomerization of methylenecyclopentane to 1- methylcyclopentene, 1,3-dimethylenecyclopentane to 1-methyl-4-methylenecycl opentene, and 3-methylenecyclopentanone to 3-methyloyclopent-3-en-1-one hav e been determined by DFT calculations at the B3LYP/6-31G* level of theory. Related studies were also carried out for some structurally related five-me mbered heterocyclic compounds containing one or two O atoms in the ring. Th e aim of the work was to study how the energetics of the exo --> endo isome rization of the C=C bond in methylenecyclopentane, 2-methylenetetrahydrofur an and 4-methylene-1,3-dioxolane is affected by replacement of an sp(3) hyb ridized ring C atom with a trigonal one. This effect turned out to be small -only a few kilojoules per mole in carbocycles, favoring the endo isomer; i n the heterocyclic compounds, especially those containing two ring O atoms, the effect is much greater.