Oxidative Radical NHC Catalysis: Divergent Difunctionalization of Olefins through Intermolecular Hydrogen Atom Transfer

Abstract

Oxidative N-heterocyclic carbene (NHC) organocatalysis, typically leading to the formation of acyl azolium reactive intermediates, constitutes one of the most important activation strategies for the NHCcatalyzed chemical transformations. Here, we report an unprecedented oxidative radical NHC catalysis by using peroxyester as the external single-electron oxidant to realize divergent difunctionalization of olefins. The key to success of this chemistry is the catalytic generation of oxygen radicals that could trigger an intermolecular hydrogen atom transfer to activate the inert CÀ H bonds, thereby enabling the productive radical relay process. With this protocol, commonly used general chemicals could serve as radical precursors to allow efficient synthesis of value-added products in a straightforward and cost-effective manner. Preliminary mechanistic investigations, including control experiments and DFT calculations, shed light on the NHC organocatalytic radical reaction mechanism.

Radical acylation: concepts, synthetic applications and directions

“…Recent efforts from Li's group revealed that peroxyesters could also act as oxidants and intermolecular hydron atom initiators in the NHC catalytic system. 56 The reaction of vinylarenes and aldehydes in the presence of peroxyester as oxidant and Cs 2 CO 3 (1.5 equiv.) as base in DMSO at 60 °C could produce a collection of 1,4-diketones (Scheme 30).…”

Visible light-mediated NHC and photoredox co-catalyzed 1,2-sulfonylacylation of allenesviaacyl and allyl radical cross-coupling

“…N-heterocyclic carbenes (NHCs) 9 could stabilize acyl radicals 10 and open a new avenue for single-electron-transfer (SET)-mediated radical acylation chemistry. 11 Since the pioneering works of Studer 12 and Ohmiya, 13 a elegant progress has been achieved in NHC-involved cross-couplings of ketyl radicals with O-centered radicals, 12 alkyl radicals, 13–18 and highly active allene radicals 19 by employing aldehydes or derivatives of carboxylic acid (Scheme 1b). The attachment of NHCs could modulate the stability of transient acyl radicals to persistent ones, which may offer the opportunity for radical acylation of allyl radicals.…”