Thermal and Photochemical Rearrangement of Bicyclo[3.1.0]hex-3-en-2-one to the Ketonic Tautomer of Phenol. Computational Evidence for the Formation of a Diradical Rather than a Zwitterionic Intermediate
journal contributionposted on 26.11.2002, 00:00 by Isabel Gómez, Santiago Olivella, Mar Reguero, Antoni Riera, Albert Solé
The ground state (S0) and lowest-energy triplet state (T1) potential energy surfaces (PESs) concerning the thermal and photochemical rearrangement of bicyclo[3.1.0]hex-3-en-2-one (8) to the ketonic tautomer of phenol (11) have been extensively explored using ab initio CASSCF and CASPT2 calculations with several basis sets. State T1 is predicted to be a triplet ππ* lying 66.5 kcal/mol above the energy of the S0 state. On the S0 PES, the rearrangement of 8 to 11 is predicted to occur via a two-step mechanism where the internal cyclopropane C−C bond is broken first through a high energy transition structure (TS1-S0), leading to a singlet intermediate (10-S0) lying 25.0 kcal/mol above the ground state of 8. Subsequently, this intermediate undergoes a 1,2-hydrogen shift to yield 11 by surmounting an energy barrier of only 2.7 kcal/mol at 0 K. The rate-determining step of the global rearrangement is the opening of the three-membered ring in 8, which involves an energy barrier of 41.2 kcal/mol at 0 K. This high energy barrier is consistent with the fact that the thermal rearrangement of umbellulone to thymol is carried out by heating at 280 °C. Regarding the photochemical rearangement, our results suggest that the most efficient route from the T1 state of 8 to ground state 11 is the essentially barrierless cleavage of the internal cyclopropane C−C bond followed by radiationless decay to the S0 state PES via intersystem crossing (ISC) at a crossing point (S0/T1-1) located at almost the same geometry as TS1-S0, leading to the formation of 10-S0 and the subsequent low-barrier 1,2-hydrogen shift. The computed small spin−orbit coupling between the T1 and S0 PESs at S0/T1-1 (1.2 cm-1) suggests that the ISC between these PESs is the rate-determining step of the photochemical rearrangement 8 → 11. Finally, computational evidence indicates that singlet intermediate 10-S0 should not be drawn as a zwitterion, but rather as a diradical having a polarized CO bond.