COMPARISON OF BIRADICAL FORMATION BETWEEN ENEDIYNE AND ENYNE ALLENE - ABINITIO CASSCF AND MRSDCI STUDY

We theoretically compared energetics of the Bergman-type biradical formation between enediyne and enyne-allene. The structures of transition states as well as reactants and products for the reaction of (Z)-hexa-1,5-diyn-3-ene (1) and (Z)-hepta-1,2,4-trien-6-yne (3) are determined at the CASSCF level. Energy calculations at the CASSCF and the MRSDCI level show that the reaction of enyne-allene 3 is more exothermic and its activation energy is lower than that of enediyne 1, consistent with the experiments. The reaction of enyne-allene is more exothermic, since a pi-radical on the benzylic methylene group in the product of enyne-allene is more stable than the sigma-radical of the product of enediyne, partly because of the conjugation with the aromatic ring and partly because of the intrinsically stronger aryl-H bond than the alkyl-H bond. This conjugation, however, does not take place at the enyne-allene transition state where the methylene remains perpendicular. Thus, the lower activation energy for enyne-allene is not ascribed to the conjugation but to the smaller four-electron repulsion between the in-plane pi-bonds than that for enediyne.