Mechanism for the Nonadiabatic Photooxidation of Benzene to Phenol: Orientation‐Dependent Proton‐Coupled Electron Transfer |
| |
Authors: | Dr Xue‐Ping Chang Prof Dr Ganglong Cui Prof Dr Wei‐Hai Fang Prof Dr Walter Thiel |
| |
Institution: | 1. Chemistry College, Beijing Normal University, Xin‐Jie‐Kou Outer St. 19#, 100875 Beijing (China);2. Max‐Planck‐Institut für Kohlenforschung, 45470 Mülheim an der Ruhr (Germany) |
| |
Abstract: | 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 1ππ* state of DDQ can relax efficiently through a nearby dark 1nπ* doorway state to the 3ππ* 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 T1→S0 intersystem crossing and concomitant proton‐coupled electron transfer (PCET) to generate the products DDQH2 and phenol. Two of the four considered nonadiabatic pathways involve an orientation‐dependent triplet PCET process, followed by intersystem crossing to the ground state (S0). The other two first undergo a nonadiabatic T1→S0 transition to produce a zwitterionic S0 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. |
| |
Keywords: | ab initio calculations electron transfer electronic structure photooxidation reaction mechanisms |
|
|