Cooperative NHC/Photoredox Catalysis: Three Component Radical Coupling of Aroyl Fluorides, Styrenes and Alcohols

Abstract

Cooperative NHC/photoredox catalysis has emerged as an important research field in recent years. Herein, the use of tertiary alcohols as alkyl radical precursors to perform three component radical coupling with styrenes and aroyl fluorides for the preparation of α‐arylated alkyl aryl ketones is described. Reaction of an aroyl fluoride with the NHC catalyst provides an acyl azolium ion that can be reduced by the photoredox catalyst to the corresponding ketyl radical anion. Oxidatively generated C‐radicals derived from the alcohol add to the styrene derivative and subsequent cross coupling of the adduct radical with the ketyl radical delivers after NHC fragmentation the targeted ketone.

Radical NHC Catalysis

“…Very recently, the Studer group also developed a photoredox-catalyzed radical relay coupling of tertiary alcohols, styrenes, and aroyl fluorides to afford α-arylated alkyl aryl ketones (Scheme ). The reaction starts by the condensation of alcohols with benzoxazolium salts to provide ortho ester amides, which allow for the generation of tertiary alkyl radicals upon single-electron oxidation, deprotonation, and fragmentation. In analogy to previous examples, the alkyl radical then adds onto a styrene and the adduct radical formed is finally trapped by the in situ generated ketyl radical.…”

“…Very recently, the Studer group also developed a photoredox-catalyzed radical relay coupling of tertiary alcohols, styrenes, and aroyl fluorides to afford α-arylated alkyl aryl ketones (Scheme 38). 119 The reaction starts by the condensation of alcohols with benzoxazolium salts to provide ortho ester amides, which allow for the generation of tertiary alkyl radicals upon single- We hope that our perspective will attract even more attention to radical NHC catalysis and lay grounds for the further development of this relatively young branch of NHC chemistry.…”

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.…”

“…Despite the conceptual simplicity, these approaches are rarely employed in the difunctionalization of allenes, which might be obstructed by the challenges in compatibility and regio- and chemo-selective control. Motivated by recent pioneering work from Studer, 14 Scheidt, 15 Chi, 16 and Ohmiya 13 f , g employing the acylazolium intermediate in NHC-catalyzed radical transformations, we envisaged that an NHC and photocatalysis (PC) co-catalyzed system could afford transient radicals and persistent NHC-stabilized ketyl radicals under extremely mild reaction conditions, which might serve as a solution to this scenario via unprecedented allyl and acyl radical cross-couplings. In continuation of our efforts in NHC catalysis, 18 k ,19 c ,21 we now report the NHC and PC co-catalyzed radical 1,2-sulfonylacylation of allenes for the synthesis of important sulfonyl 22 substituted tetrasubstituted 23 olefins (Scheme 1c).…”

Branched-Selective Hydroacylation of Alkenes via Photoredox Cobalt and N-Heterocyclic Carbene Cooperative Triple Catalysis

“…We anticipated whether this branched selectivity could be realized through cobalt hydride chemistry. Considering that significant advances in radical reactions of NHC catalysis via single electron transfer (SET) from Breslow intermediates and acyl azolium intermediates have been described by Ohmiya, − Studer, − Scheidt, − and many other groups, − we envisioned that a Markovnikov hydroacylation of alkenes could be achieved by a delicate triple catalysis of cobalt hydride merged with NHC and photoredox catalysis. Herein, the transient carbon radical is generated by MHAT of Co­(III)-H with the photocatalyst.…”