Radical Covalent Catalysis: Recent Advances and Future Developments

Abstract

Dedicated to Professor Li-Xin Dai on the occasion of his 100th birthdayChemCatChem www.chemcatchem.org

Recent advances in hydrogen atom transfer induced C(sp3)–H functionalizations initiated by radical addition to alkynes

“…Radical covalent catalysis, which is initiated by radical addition and terminated by elimination of an identical radical in the catalytic cycle, has recently emerged as a very potent strategy to promote transformations that are otherwise challenging to achieve. 21 In this scenario, Zhang, Fu, Wang and co-workers disclosed a conceptually novel N-heterocyclic carbene (NHC)-boryl radical catalyzed asymmetric alkyne cycloisomerization reaction (Scheme 8). 22 Catalyzed by a chiral NHC-ligated boryl radical, generated in situ via HAT between B-1 and azobiscyclohexanecarbonitrile (ACCN) or B-2 and 2,2′-azobis(2,4-dimethyl)valeronitrile (ABVN), N -benzyl homopropargyl amines 13 and 1-benzyl-7-alkynyl-indoles 15 could be smoothly converted into pyrrolidines 14 and pyrrolo[3,2,1- ij ]quinolines 16 with excellent enantioselectivity.…”

Vinylcyclopropane-Cyclopentene (VCP-CP) Rearrangement Enabled by Pyridine-Assisted Boronyl Radical Catalysis

“…Until very recently, an array of pioneering contributions in this area were disclosed. Strategically, the generated radicals from ring opening of VCPs can be mainly utilized through three pathways: (1) quenching by other radicals or radical precursors to afford linear products (Scheme C, a); (2) undergoing ring expansion reactions with alkenes/alkynes to access cyclopentanes/cyclopentenes (Scheme C, b); (3) adjusting to attack the vinyl motifs of another VCP to induce radical ring opening polymerization processes (Scheme C, c) . For example, in 2021, the You group developed a dearomatization reaction of indole- or pyrrole-tethered VCPs, affording structurally diverse polycyclic molecules .…”

Boryl radical catalysis enables asymmetric radical cycloisomerization reactions

“…Free organic radicals are commonly involved as stoichiometric promoters or intermediates in a variety of reactions ( 21 ). By contrast, the use of free organic radicals as the catalysts remains limited ( 22 ) because the short lifetime and the high reactivity of these species pose a marked challenge in the realization of efficient catalytic cycles. However, their specific chemical properties can promote notable molecular transformations that are not easily accessible by other known catalytic methods.…”