Novel Approaches to Cross-Coupling Enabled by Copper and Iron Metallaphotoredox Catalysis

Abstract

Transition metal-catalyzed cross-coupling is among the most robust and versatile methods for carbon–carbon bond formation; thus, it has become an indispensable tool for organic molecule construction as well as structural diversification. Cross-coupling mechanisms generally consist of a set number of polar, two-electron elementary steps that are thermally accessible to catalytic metal complexes in their ground electronic states. This paradigm, though modular and enabling, is unable to facilitate certain valuable transformations due to the limitations of these elementary steps.Over the past ten years, the convenient access to reactive organic radicals provided by light-absorbing catalysts, in combination with the robust single-electron chemistry of first-row transition metals, has provided a distinct approach to mechanistically avoid challenging steps within cross-coupling. This field, termed metallaphotoredox catalysis, has revolutionized the development of carbon–carbon or –heteroatom bond-forming reactions. Herein are described three studies in which novel reaction pathways circumvent long-standing mechanistic challenges that have historically limited the synthetic utility of transition metal-catalyzed cross-coupling. Chapter 2 describes the arylation of N-heterocyclic nucleophiles accomplished by a halogen abstraction-radical capture sequence that removes the need for oxidative addition by copper, enabling a room temperature Ullmann-Goldberg coupling. Chapter 3 discloses a biomimetic C(sp3)–C(sp3) cross-coupling of redox-active esters and alkyl bromides to form quaternary carbon centers via iron porphyrin catalysis. Detailed studies support the proposal of C–C bond formation via a key bimolecular homolytic displacement step, which circumvents a challenging tertiary alkyl reductive elimination. Chapter 4 details a mechanistically novel approach to the cross-electrophile coupling of tertiary and primary alkyl bromides, employing iron porphyrin-photoredox dual catalysis to form quaternary carbon centers through successive polar and radical displacement mechanisms.