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.

Direct access to β-arylketoesters by NHC/photoredox catalyzed alkoxycarbonylation–carbonylation of styrenes

“…Steady-state fluorescence quenching experiments showed that the excited 4CzIPN* was quenched by potassium 2-ethoxy-2-oxoacetate 3 (section 3.2, ESI†). Based on the above experiments and previous reports, 11,13,15,17 we deduced the possible mechanism of the reaction (Scheme 5). The reaction involves a total of two catalytic cycles: the NHC catalytic and photocatalytic cycles, respectively.…”

“…10 However, NHC-mediated 1,2-acylation of olefins was also reported. 11 In 2022, Larionov's group described the NHC-photocatalyzed tricomponent regioselective 1,2-diacylation of alkenes. 12 Very recently, our group reported that olefin diacylation products could also be obtained using benzoic acid as a reducing agent under the promotion of visible light.…”

On the Redox Properties of the Dimers of Thiazol-2-ylidenes That Are Relevant for Radical Catalysis

“…In addition to ionic-type (2-electron) mechanisms, thiazol-2-ylidenes have also recently become privileged organocatalysts for radical transformations of aldehydes. − In these reactions, the Breslow-type enolates C – , which result from the condensation of aldehydes with NHCs in basic conditions, are able to undergo spontaneous single-electron transfer (SET) to challenging substrates ( E ox > −1.6 V vs SCE). − In the absence of aldehydes, free NHCs are milder reducing reactants. For instance, imidazol-2-ylidenes perform stoichiometric reduction of the ferrocenium cation ( E = +0.38 V vs SCE) and the trityl cation ( E = +0.27 V vs SCE) .…”

Organocatalytic radical relay trifunctionalization of unactivated alkenes by a combination of cyano migration and alkylacylation

“…However, to the best of our knowledge, the reported protocols were mainly focused on the difunctionalization of activated alkenes (Scheme 1B). 6 The radical functionalization of unactivated alkenes via NHC catalysis has not been reported yet and remains challenging.…”