Copper-Catalyzed Oxidative C-O Coupling by Direct C—H Bond Activation of Formamides: Synthesis of Enol Carbamates and 2-Carbonyl-Substituted Phenol Carbamates

摘要

Coupling chemistry is an important synthetic strategy, widely used in both industry and academia for the formation of carbon—carbon and carbon—heteroatom bonds. The traditional coupling procedures involve either the use of stoichiometric organometallic reagents, such as Grignard and organo-lithium reagents, or the transition-metal-catalyzed coupling of functionalized hydrocarbons. There has been substantial progress in these methods over the last few decades, and they are successfully applied in the synthesis of commercially important products. However, the use of prefunctionalized starting materials in these methods, thus adding steps towards the formation of desired chemical bond, is a major concern for the synthetic chemist from an atom-economical and environmental point of view. The best way to address this issue is to utilize unfunctionalized starting materials by the direct activation of C—H bonds. Several reports have appeared in the literature on transition-metal-catalyzed C-H bond activation and its further application to carbon-carbon and carbon—heteroatom bond formations. In recent years, more systematic and concerted efforts have been made in C—H bond activation and its application in coupling chemistry. As a result exceptionally useful methods for organic synthesis have been developed, such as, the transition-metal-catalyzed functional-group-directed C—H bond functionalization to achieve C-C and C-X bonds, and pioneering work by Li in the area of cross-dehydrogenative couplings (CDC), where the activation of two different C—H bonds under oxidative conditions have been achieved (Scheme 1).