A theoretical ab-initio MO investigation on the relative stability of exo/endo isomers of substituted cycloalkanes-cycloalkenes

A theoretical MO ab-initio investigation was carried out on methylenecycloa lkanes (exo-isomer) and methylcycloalkenes (endo-isomer) bearing electronwi thdrawing substituents (CN,COOR) at the exo carbon atom and a pentadiene pu sh-pull structure in the endo isomer. These isomers were found previously t o be generated in variable amounts as a function of ring dimension, of the substituents at the exo carbon, and of the push-pull character of the struc tures. The relative stability of the two isomers was obtained from the tota l molecular energies at MP2/6-31G*//HF/6-31G* theoretical level taking into account the complex conformational patterns due to several degrees of free dom for internal rotation in these molecules. The results from the theoreti cal study, carried out also on model compounds of simpler structure and als o employing the heats of formation calculated with CBS-4M model chemistry, allow the different effects acting on the relative stability of exolendo is omers of cyclic compounds to be rationalized. (C) 2001 Elsevier Science B.V . All rights reserved.