Radical Hydrofunctionalization Using Light Driven, Reductive Cobalt Hydride Catalysis

Conspectus First row transition metals such as iron, cobalt and manganese with weak-field ligands are able to form metal hydride species in situ which have a weak M–H bond. In these cases, the formal addition of a hydrogen atom to an olefin is thermodynamically favored. The high regio and chemoselectivity of this process, termed metal hydride hydrogen-atom-transfer (MHAT) catalysis, together with the broad availability of olefins and the divergent reaction pathways from the carbon centered radical that is formed, has resulted in a wide range of radical hydrofunctionalization strategies that have been used extensively in complex synthetic settings. Until around 5 years ago, a typical catalytic cycle for forming the metal hydride relied on oxidation as a first step, followed by reaction with a hydride equivalent. The requirement to find compatible oxidant/reductant pairs defined and limited the scope of this chemistry. However, several groups including our own recognized that it would instead be possible to first reduce the metal and then protonate the low-valent form to yield the same metal hydride species. This net reductive approach would unlock new avenues for onward reactivity, circumventing the compatibility issues associated with the reagents used previously. A number of different approaches to this reductive MHAT paradigm have been reported, using chemical reductants, electrochemistry or photochemistry. Our group has focused on the merger of cobalt catalysis and photocatalysis to enable new hydrofunctionalization reactivity. Under reductive reaction conditions, it is possible to simultaneously form a persistent radical via single electron reduction and couple this with the radical formed from the alkene. We have used this concept to couple alkenes with cyanoarenes, pyridyl phosphonium salts, ketones and imines. Radical rearrangements, initiated by MHAT, can take place that were previously inefficient or not possible under oxidative MHAT conditions. We have used this to promote aryl migration reactions, building new C–C bonds with sterically demanding aromatic groups. And most recently, we have reported the combination of MHAT catalysis with reductive radical polar crossover, a new type of reactivity that enables the use of an electrophile as a coupling partner, contrasting with the commonly reported oxidative radical polar crossover pathway where hydrofunctionalization takes place with nucleophiles. In summary, the reductive MHAT concept has reinvigorated interest in radical hydrofunctionalization reactions. Here, we present our journey in uncovering new reactivities accessible only through this catalytic paradigm, enabled by photochemistry. We are confident that both the synthetic utility and the broader appeal of this modern approach to classical MHAT reactivity will continue to increase in the coming years, driven by the growing demand for catalytic systems that employ earth-abundant transition metals and which enable direct functionalization of native olefin functionality.