Direct Benzylic C—H Activation for C-O Bond Formation by Photoredox Catalysis

摘要

A pressing demand of modern organic chemistry is to find a step-economic approach to build complex molecular structures through selective functionalization of a rather unreactive C—H bond.The most fundamental difficulty in such endeavors lies with the selective C—H activation in a molecule which contains a diversity of C—H groups. Intense research activities by several research groups in this area have resulted in the development of some very useful protocols for regioselective C_(sp~2)—H activation through transition-metal catalysis, however, the corresponding C_(sp~3)—H activation is still in its infancy.The benzyl group is an important motif in organic syntheses and has relatively active C_(sp~3)—H bonds, because of the proximal aromatic ring, and have therefore been functionalized using either heteroatom-chelated transition-metal catalysts' or nondirected oxidative activations.Nondirected oxidative activations have either required tert-butyl hydrogen peroxide (TBHP) in the presence of a metal catalyst, or a chemical oxidant such as 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) in the cases where the benzyl group is flanked by an a-nitrogen/oxygen atom. More recently, benzylic C—H activation of xanthenes as well as acridanes using molecular oxygen in the presence of a Bronsted acid was also reported. In spite of impressive progress made in this area, the reported protocols are far from being environmentally benign as they either require moisture-sensitive and expensive metal catalysts or special structural requirements along with excessive use of chemical oxidants. Therefore, further research in this area is still warranted.