Interconversions of Phenylcarbene, Cycloheptatetraene, Fulvenallene, and Benzocyclopropene. A Theoretical Study of the C7H6 Energy Surface

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Abstract
Ab initio calculations at the G2(MP2,SVP) and B-LYP/6-311+G(3df,2p)+ZPVE levels have been used to examine the potential energy surface of C7H6. Fulvenallene (6) is the most stable C7H6 isomer considered in this study. 1-Ethynylcyclopentadiene (9A), benzocyclopropene (10), and 1,2,4,6-cycloheptatetraene (4) lie 12, 29, and 49 kJ mol-1, respectively, above 6. Phenylcarbene (1) is calculated is to have a triplet (3A‘‘) ground state, 16 kJ mol-1 more stable than the singlet state (1A‘). Interconversion of 1 and 4 is predicted to have a moderate activation barrier, with the involvement of a stable bicyclic intermediate (bicyclo[4.1.0]hepta-2,4,6-triene, 2). However, 2 is found to lie in a shallow potential energy well with a small barrier (8 kJ mol-1) to rearrangement to 4. At the G2(RMP2,SVP)//QCI level, the 3A2 and 3B1 triplet states of cycloheptatrienylidene (33) are predicted to lie very close in energy. The singlet “aromatic” cycloheptatrienylidene (13, 1A1) is found to be a transition structure interconverting two chiral cyclic allenes (4) and it lies ∼25 kJ mol-1 below the triplet states. Bicyclo[3.2.0]hepta-1,3,6-triene (5) is predicted to be a stable equilibrium structure, lying in a significant energy well. Rearrangement of 4 to 5 constitutes the rate-determining step for the rearrangement of phenylcarbene to fulvenallene (6), the ethynylcyclopentadienes (9), and spiro[2.4]heptatriene (7). Rearrangement of 9A to 6, via a 1,4-H shift, requires a large barrier of 325 kJ mol-1. Rearrangement of benzocyclopropene (10) to 6 involves a methylenecyclohexadienylidene intermediate (27) and is associated with an energy barrier of 223 kJ mol-1. The calculated mechanisms and energetics for the interconversions of various C7H6 isomers are in good accord with experimental results to date.

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