Proton-coupled electron transfer of macrocyclic ring hydrogenation: The chlorinphlorin

Abstract

Redox noninnocence of pyrrole macrocycles allows for their hydrogenation by proton-coupled electron transfer (PCET). The initial reduction of porphyrins by PCET occurs at a bridging methine carbon, yielding a phlorin. In macrocyclic cores that are reduced beyond porphyrins, the phlorin is unusual. The chlorinphlorin is an especially rare compound, and consequently, its formation and chemistry have eluded characterization. We now report the chemical preparation of a chlorinphlorin, including its X-ray crystal structure, which is distinguished by a tetrahedral geometry about the methine carbon of the macrocycle, resulting in the formal reduction of an olefin bond. We establish that the reductive electrochemistry of a series of hangman chlorins furnishes the chlorinphlorin by electrochemical hydrogenation according to a PCET mechanism. The rate constant for proton transfer from the pendant acid in the secondary coordination sphere to the chlorin anion radical is 103 s21. This initial proton transfer forms the N-H bond of the chlorinphlorin; upon a second reduction, the C-H bond at the tetrahedral methine bridge is formed. In the presence of exogenous acid, the hangman group prepositions the acid to trigger a concerted PCET reaction, thereby bypassing the chlorin anion radical intermediate. When the chlorin macrocycle is metalated, the formation of the initial N-H bond is prevented, and the hydrogenation of the chlorin macrocycle by PCET leads to the isobacteriochlorin.