The photochemical Formation of Fulvene from Benzene via Prefulvene-a Theoretical Study

Results of semiempirical MN‐DOC‐CI and ab initio CASSCF calculations reveal that fulvene is a primary product of the photolysis of benzene. The photochemical step is assumed to lead to prefulvene, and both a synchronous conversion and a two‐step process via bicyclo[3.1.0]hexa‐1,3‐diene (isofulvene) could be ruled out as possible pathways from prefulvene to benzene. The most probable mechanism for the photochemical isomerization of benzene to fulvene involves the intermediate structures prefulvene and 1,3‐cyclopentadienylcarbene and has to compete with the almost barrierless formation of benzvalene and rearomatization to benzene. The short‐lived intermediates are rather flexible structures with negligible barriers to inversion at the radical center in the threemembered ring of prefulvene and prebenzvalene and to rotation around the exocyclic single bond in 1,3‐cyclopentadienylcarbene. What happens when benzene is photoisomerized? A three‐step mechanism for the formation of the primary photoproduct fulvene (right) has been derived on the basis of potential energy hypersurfaces and state diagrams obtained from semiempirical MN‐DOC‐CI and ab initio CASSCF calculations.