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Mechanism for the Nonadiabatic Photooxidation of Benzene to Phenol: Orientation‐Dependent Proton‐Coupled Electron Transfer
Author(s) -
Chang XuePing,
Cui Ganglong,
Fang WeiHai,
Thiel Walter
Publication year - 2015
Publication title -
chemphyschem
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.016
H-Index - 140
eISSN - 1439-7641
pISSN - 1439-4235
DOI - 10.1002/cphc.201402897
Subject(s) - intersystem crossing , chemistry , photochemistry , electron transfer , proton coupled electron transfer , benzene , proton , radical , triplet state , intermolecular force , excited state , molecule , atomic physics , organic chemistry , physics , quantum mechanics , singlet state
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 ππ* state of DDQ can relax efficiently through a nearby dark 1 nπ* doorway state to the 3 ππ* 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 T 1 →S 0 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 T 1 →S 0 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.