Bioinspired Oxidized Dicobalt Porphyrin Dimer-Catalyzed Cycloaddition Reactions: Cooperative Catalysis and Spectroscopic and Mechanistic Investigation

Diheme enzymes display redox cooperativity possibly via a tryptophan moiety placed in between two heme centers, thereby behaving as a single diheme unit rather than as two independent heme centers. Herein, we present an unprecedented, rationally designed dicobalt porphyrin dimer catalyzed hetero-Diels–Alder [4 + 2] reaction of unactivated aldehydes/imines with simple dienes under mild reaction conditions with extremely low catalyst loading. The high catalytic efficiency of the dimeric catalyst over the monomer and heterobimetallic Cu–Co catalyst highlights the importance of underlying cooperativity between two porphyrin units during catalysis. The redox cooperativity among metal, ligand, and bridge is further strengthened by the coexistence of both Co^II(por^•+) and Co^III(por) in this dimeric porphyrin framework. This dicobalt catalyst competently catalyzed both oxa- (ODA) and aza-Diels–Alder (ADA) reactions, whereas the diiron analogue did not show any catalytic activity for the ADA reaction. In situ monitoring of the progress of the reaction in solution enabled direct spectroscopic identification of the reactive intermediates for elucidation of the molecular mechanism of the ODA reaction. A dicobalt catalyst is most efficient compared to the mono-Co, Cu–Co, di-Fe, di-Cu, and di-Zn analogs. These extensive findings also shed light on the design principle for the development of sustainable, efficient cooperative catalysts.