Chang Xue-Ping, Cui Ganglong, Fang Wei-Hai, Thiel Walter
Chemistry College, Beijing Normal University, Xin-Jie-Kou Outer St. 19#, 100875 Beijing (China).
Chemphyschem. 2015 Apr 7;16(5):933-7. doi: 10.1002/cphc.201402897. Epub 2015 Jan 28.
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 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.
最近，通过以2,3-二氯-5,6-二氰基对苯醌（DDQ）作为光催化剂，在温和条件下进行选择性光氧化，实现了苯向苯酚的高效催化一步转化。本文报道了在气相和乙腈溶液中进行的高水平电子结构计算，以探索其潜在机制。DDQ最初占据的(1)ππ态可以通过附近的暗态(1)nπ通道态有效地弛豫到DDQ的(3)ππ*态，该态被发现是参与苯向DDQ初始分子间电子转移的前体状态。计算得出，随后DDQ自由基阴离子、苯自由基阳离子和水之间的三重态反应很容易发生。形成的DDQH和苯-OH自由基可以经历T1→S0系间窜越以及伴随的质子耦合电子转移（PCET），以生成产物DDQH2和苯酚。所考虑的四条非绝热途径中的两条涉及取向依赖性三重态PCET过程，随后系间窜越到基态（S0）。另外两条首先经历非绝热的T1→S0跃迁以产生两性离子S0络合物，随后进行无势垒质子转移。目前的理论研究确定了新型的非绝热PCET过程，并为DDQ催化的光氧化提供了详细的机理见解。
