Photoredox cooperative N-heterocyclic carbene/palladium-catalysed alkylacylation of alkenes

Abstract

Three-component carboacylation of simple alkenes with readily available reagents is challenging. Transition metal-catalysed intermolecular carboacylation works for alkenes with strained ring or directing groups. Herein, we develop a photoredox cooperative N-heterocyclic carbene/Pd-catalysed alkylacylation of simple alkenes with aldehydes and unactivated alkyl halides to provide ketones in good yields. This multicomponent coupling reaction features a wide scope of alkenes, broad functional group compatibility and free of exogenous photosensitizer or external reductant. In addition, a series of chlorinated cyclopropanes with one or two vicinal quaternary carbons is obtained when chloroform or carbon tetrachloride is used as the alkyl halide. The reaction involves the alkyl radicals from halides and the ketyl radicals from aldehydes under photoredox cooperative N-heterocyclic carbene/Pd catalysis.

Iminoacylation of Alkenes via Photoredox N-Heterocyclic Carbene Catalysis

“…During the preparation of this manuscript, Houk and Zhao et al reported the NHC-catalyzed aminoacylation of alkenes tethered with O -aryl oxime . Inspired by the efficient generation of iminyl radicals from readily available α-imino-oxy acids and our ongoing interests in alkene 1,2-difunctionalizations and NHC catalysis via radicals, , we envisioned that the reaction of alkene-tethered α-imino-oxy acids, with acyl imidazoles under photoredox NHC catalysis, might be feasible to deliver the corresponding N-heterocyclic molecules via the N-radical and ketyl radical (Scheme c). The alkene-tethered α-imino-oxy acids could be easily prepared via condensation of the corresponding enones with α-amino-oxy acid.…”

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) .…”

Construction of C(CO)–C(CO) Bond via NHC-Catalyzed Radical Cross-Coupling Reaction

“…Recently N-heterocyclic carbene (NHC)-catalyzed radical cross-coupling reactions provide an novel method to give diverse ketone compounds . Using a myriad of C­(sp 3 )-centered radical precursors such as N -hydroxyphthalimide (NHPI) ester, Hantzsch ester, Katritzky pyridinium salt, and others, a C­(sp 2 )–C­(sp 3 ) bond can be efficiently constructed (Scheme b). Despite much progress, the direct cross-coupling between NHC-derived ketyl radicals and C­(sp 2 )-centered radicals to form C­(sp 2 )–C­(sp 2 ) bonds remains elusive.…”