Mechanism for the Nonadiabatic Photooxidation of Benzene to Phenol: Orientation-Dependent Proton-Coupled Electron Transfer

An efficient catalytic one-step conversion of benzene to phenol was achieved recently by selective photooxidation under mild conditions with 2,3-dichloro-5,6-dicyano-p-benzoquinone (DDQ) as the photocatalyst. Herein, high-level electronic structure calculations in the gas phase and in acetonitrile solution are reported to explore the underlying mechanism. The initially populated (1)* state of DDQ can relax efficiently through a nearby dark (1)n* doorway state to the (3)* state of DDQ, which is found to be the precursor state involved in the initial intermolecular electron transfer from benzene to DDQ. The subsequent triplet-state reaction between DDQ radical anions, benzene radical cations, and water is computed to be facile. The formed DDQH and benzene-OH radicals can undergo T1S0 intersystem crossing and concomitant proton-coupled electron transfer (PCET) to generate the products DDQH(2) and phenol. Two of the four considered nonadiabatic pathways involve an orientation-dependent triplet PCET process, followed by intersystem crossing to the ground state (S-0). The other two first undergo a nonadiabatic T1S0 transition to produce a zwitterionic S-0 complex, followed by a barrierless proton transfer. The present theoretical study identifies novel types of nonadiabatic PCET processes and provides detailed mechanistic insight into DDQ-catalyzed photooxidation.