MCSCF/MP2 Study of the Cheletropic Addition of Singlet and Triplet CF(2) and C(OH)(2) to the Ethene Double Bond

CAS-MCSCF calculations describe the addition of singlet CF(2) and C(OH)(2) to the ethene double bond as a two-step reaction. The energy barriers that separate, in the first rate-determining step, loosely bound pi-complexes from stable CH(2)CH(2)CX(2) diradical intermediates show the expected ordering, smaller for CF(2) than for C(OH)(2). Back-dissociation of the diradicals into reactants requires the overcoming of non-negligible energy barriers. In both diradicals, the CAS-MCSCF activation energy for ring closure is smaller than that required for rotation of their terminal methylenic groups, which models, in these simple systems, an isomerization process. However, when the activation free energies are computed, in the case of the difluoro diradical the isomerization process appears to be less disfavored (and possibly competitive to some extent at higher temperatures); in contrast, in the case of the dioxy diradical, isomerization is never competitive with ring closure. The small energy barriers for ring closure of the diradicals disappear altogether when multireference MP2 energy calculations are carried out on the CAS-MCSCF critical points, casting doubts on the very existence of these intermediates. However, in contrast with the ethene reaction, the addition of singlet CF(2) to isobutene involves the formation of a diradical intermediate whose barrier for ring closure persists also at the MP2 level. These results suggest that cyclopropanation is likely to be a two-step process (with formation of a diradical intermediate) only with bulky substituted alkenes, while the attack to an unsubstituted double bond could be an asynchronous but concerted process. The analogous triplet reactions go through transition and stable structures of lower symmetry than the singlet and see the intervention of diradical intermediates. Their formation is easier than that in the singlet case and their stability with respect to back-dissociation higher. Also the isomerization processes (taking place again through rotation of the terminal methylenic group) are easier than those examined on the singlet surfaces.