Metalloradical Catalysis: General Approach for Controlling Reactivity and Selectivity of Homolytic Radical Reactions

Abstract

Since Friedrich Wohler’s groundbreaking synthesis of urea in 1828, organic synthesis over the past two centuries has predominantly relied on the exploration and utilization of chemical reactions rooted in two‐electron heterolytic ionic chemistry. While one‐electron homolytic radical chemistry is both rich in fundamental reactivities and attractive with practical advantages, the synthetic application of radical reactions has been long hampered by the formidable challenges associated with the control over reactivity and selectivity of high‐energy radical intermediates. To fully harness the untapped potential of radical chemistry for organic synthesis, there is a pressing need to formulate radically different concepts and broadly applicable strategies to address these outstanding issues. In pursuit of this objective, researchers have been actively developing metalloradical catalysis (MRC) as a comprehensive framework to guide the design of general approaches for controlling over reactivity and stereoselectivity of homolytic radical reactions. Essentially, MRC exploits the metal‐centered radicals present in open‐shell metal complexes as on‐electron catalysts for homolytic activation of substrates to generate metal‐entangled organic radicals as the key intermediates to govern the reaction pathway and stereochemical course of subsequent catalytic radical processes. Different from the conventional two‐electron catalysis by transition metal complexes, MRC operates through one‐electron chemistry utilizing stepwise radical mechanisms.

Asymmetric Three-Component Radical Alkene Carboazidation by Direct Activation of Aliphatic C–H Bonds

“…Next, the alkyl radical adds to the chiral Fe­(III)–N 3 -bonded α,β-enone to give a γ-Fe­(III)-alkyl radical E . Finally, an asymmetric intramolecular radical substitution − delivers the chiral alkylazides and regenerates the Fe­(II) catalyst. In addition, the aryl carboxyl radical D potentially abstracts the trimethylsilyl from TMSN 3 , leading to the generation of a free azido radical (N 3 • ).…”