Tuning Excited-State Reactivity by Proton-Coupled Electron Transfer

• Published in: Angewandte Chemie (International ed.) Vol. 55; no. 42; pp. 13132 - 13136
• Main Authors: Li, Han, Zhang, Ming-Tian
• Format: Journal Article
• Language: English
• Published: Germany Blackwell Publishing Ltd 10.10.2016; Wiley Subscription Services, Inc
• Edition: International ed. in English
• Subjects: Acids; Benzene; Benzophenone; Electron transfer; Hydrocarbons; hydrogen-atom transfer; kinetic isotope effect; Oxidation; photochemistry; Quenching; Reactivity; Substrates; Triplet state; triplet states; triplet states; kinetic isotope effect; electron transfer; photochemistry; hydrogen-atom transfer
• ISSN: 1433-7851; 1521-3773
• DOI: 10.1002/anie.201607176

The reactivity, and even reaction pathway, of excited states can be tuned by proton‐coupled electron transfer (PCET). The triplet state of benzophenone functionalized with a Brønsted acid (3*BP‐COOH) showed a more powerful oxidation capability over the simple triplet state of benzophenone (3*BP). 3*BP‐COOH could remove an electron from benzene at the rate of 8.0×105 m−1 s−1, in contrast to the reactivity of 3*BP which was inactive towards benzene oxidation. The origin of this great enhancement on the ability of the excited states to remove electrons from substrates is attributed to the intramolecular Brønsted acid, which enables the reductive quenching of 3*BP by concerted electron–proton transfer. Happily coupled: Both the reactivity and reaction pathway of excited states can be tuned by proton‐coupled electron transfer (PECT). Addition of a Brønsted acid unit to benzophenone enables its triplet state to oxidize benzene, which is not possible for nonfunctionalized benzophenone. The presence of the acid enables a switch of mechanism from hydrogen‐atom transfer (HAT) to a concerted electron–proton transfer process.