Simple
catalysts that use atom-economical oxygen as the terminal
oxidant to accomplish selective ortho–ortho, ortho–para, or para–para homo-couplings
of phenols are described. In addition, chromium salen catalysts have
been discovered as uniquely effective in the cross-coupling of different
phenols with high chemo- and regioselectivity.
A highly chemoselective phenol cross-coupling reaction catalyzed by a Cr-salen catalyst was developed. Kinetic studies showed that the oxidation of Cr(III) to Cr(V) is the rate-determining step of the reaction. In addition, experimental stoichiometric analysis showed that a high valentCr(V) specie is the active catalyst for this process. The selectivity of the reaction was found to be determined by the cross-coupling carbon-carbon bond forming reaction, rather than any precoordination species. It appears that the lowest energy cross-coupling pathway requires a lesser degree of electronic reorganization in its transition state vs the lowest energy homo-coupling pathway. This result was supported by stoichiometric Cr(V) kinetics, 13 C kinetic isotope effects, and DFT calculations. The understanding of the full landscape of this reaction, allowed us to develop a general analysis to predict the regioselectivity of the cross-coupling reaction.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.