Single-Electron-Elementary Steps in Homogeneous Organometallic Catalysis

Nature solves some of its most difficult and most interesting problems using one-electron (viz. radical) processes. Metalloenzymes catalyze radical reactions with high chemo-, regio-, diastereo-, and enantioselectivity by making use of several important tools, which are in particular (i) catalytic generation of radicals, (ii) shielding of the active site, (iii) substrate preorganization through interactions with specific amino acid residues, and (iv) interaction of the substrate with the metal d-orbitals. Whereas natural metalloenzymes contain exclusively earth abundant, inexpensive first-row transition metals, it remains a challenge to steer and control radical reactions with synthetic organometallic complexes based on these metals. In pursuit of performing chemical transformations at the minimum cost of energy and feedstocks, metalloenzymes thus serve as an important source of inspiration for the development of new base metal-catalyzed radical-type reactions. This chapter is intended as a tutorial overview of one-electron reactivity in contemporary fundamental research on homogeneous catalysis. Recent examples, possibilities, and limitations are discussed, aiming to bring together different threads and stimulate new work on the frontiers of metalloradical catalysis.