Catalyst Complexity in a Highly Active and Selective Wacker-Type Markovnikov Oxidation of Olefins with a Bioinspired Iron Complex
Résumé
Palladium-catalyzed Wacker-type reactions occupy a central place in organic synthesis with important implications in industry. Pursuing more benign protocols by replacing palladium by first-row transition metals allowed the identification of iron as a privileged one in the last years. Although the anti-Markovnikov selectivity for iron catalysts is well developed, the Markovnikov-selective reactions still afford significant quantities of alcohol side-products and identification of reaction intermediates remains elusive so far. Herein, we present an iron catalyst that affords Markovnikov ketone products from (hetero)aromatic and aliphatic olefins in up to 99% selectivity under ambient conditions with 190,000 turnover numbers and turnover frequencies of 74 h-1 at 50 o C. The catalyst design is based on the promiscuous activity encountered in the family of the cytochromes P-450 enzymes and it enables the formation of iron-hydride species under catalytically relevant reaction conditions. Substrate scope assessment and mechanistic investigations suggest that the Markovnikovselective catalytic cycle competes with unprecedented three additional catalytic cycles (alcohol formation, hydrogenation and reductive homo-coupling) depending on the nature of the olefin and the reaction conditions.
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