NHCs and Visible Light-mediated Photoredox Co-catalyzed Radical 1,2-Dicarbonylation of Alkenes for 1,4-Diketones

Alkenes are ubiquitous, and radical difunctionalization of alkenes represents one of the most practical approaches to constructing value-added compounds. Dicarbonylation of alkenes provides direct access to value-added 1,4-dicarbonyl compounds. However, selectivity control for unsymmetric 1,2-dicarbonylation is an unclosed challenge. We herein describe NHCs and photocatalysis co-catalyzed three competent radical 1,2-dicarbonylation of alkenes by distinguishing two carbonyl groups, providing structurally diversified 1,4-diketones. Mechanistic studies indicated that NHCs-stabilized ketyl-type radicals originate from aroyl fluorides via oxidative quenching process of excited photocatalysis, and acyl radicals are generated from single-electron-oxidation of α-keto acids. Distinct properties of acyl radical and NHCs-stabilized ketyl radical contributed to selectivity control. Transient acyl radicals are rapidly added to alkenes delivering alkyl radicals, which undergo subsequent radical-radical cross-coupling with ketyl-type radicals, affording 1,2-dicarbonylation products. This transformation features mild reaction conditions, broad substruct scope, and excellent selectivity, providing a general and practical approach for the dicarbonylation of olefins.

DABCO-promoted photocatalytic C–H functionalization of aldehydes

Herein we present a direct application of DABCO, an inexpensive and broadly accessible organic base, as a hydrogen atom transfer (HAT) abstractor in a photocatalytic strategy for aldehyde C–H activation. The acyl radicals generated in this step were arylated with aryl bromides through a well stablished nickel cross-coupling methodology, leading to a variety of interesting aryl ketones in good yields. We also performed computational calculations to shine light in the HAT step energetics and determined an optimized geometry for the transition state, showing that the hydrogen atom transfer between aldehydes and DABCO is a mildly endergonic, yet sufficiently fast step. The same calculations were performed with quinuclidine, for comparison of both catalysts and the differences are discussed.

Divergent functionalization of aldehydes photocatalyzed by neutral eosin Y with sulfone reagents

While aldehydes represent a classic class of electrophilic synthons, the corresponding acyl radicals are inherently nucleophilic, which exhibits umpolung reactivity. Generation of acyl radicals typically requires noble metal catalysts or excess oxidants to be added. Herein, we report a convenient and green approach to access acyl radicals, capitalizing on neutral eosin Y-enabled hydrogen atom transfer (HAT) photocatalysis with aldehydes. The generated acyl radicals underwent SOMOphilic substitutions with various functionalized sulfones (X–SO2R’) to deliver value-added acyl products. The merger of eosin Y photocatalysis and sulfone-based SOMOphiles provides a versatile platform for a wide array of aldehydic C–H functionalizations, including fluoromethylthiolation, arylthiolation, alkynylation, alkenylation and azidation. The present protocol features green characteristics, such as being free of metals, harmful oxidants and additives; step-economic; redox-neutral; and amenable to scale-up assisted by continuous-flow technology.

Recent Advances on Acyl Radical Enabled Reactions between Aldehydes and Alkenes

Radical-mediated functionalization of alkenes has been emerging as an elegant and straightforward protocol to increase molecule complexity. Moreover, the abstraction of a hydrogen atom from aldehydes to afford acyl radicals...

Catalyst-Free Generation of Acyl Radicals Induced by Visible Light in Water to Construct C-N Bonds

We describe herein a catalyst-free and redox-neutral photochemical strategy for the direct generation of acyl radical from α-diketones, and its selective conversion of nitrosoarenes to hydroxyamide or amides with AcOH...

Synthesis of γ-Oxo-α-amino Acids via Radical Acylation with Carboxylic Acids

Herein we present a highly efficient, light-mediated, deoxygenative protocol to access g-oxo-a-amino acid derivatives.This radical methodology employs photoredox catalysis, in combination with triphenylphosphine, to generate acyl radicals from readily available (hetero)aromatic and vinylic carboxylic acids. This approach allows for the straightforward synthesis of g-oxo-aamino acids bearing a wide range of functional groups (e.g. Cl, CN, furan, thiophene, Bpin) in synthetically useful yields (~ 60% average yield). To further highlight the utility of the methodology, several deprotection and derivatization reactions were carried out.

Synthesis of γ-Oxo-α-amino Acids via Radical Acylation with Carboxylic Acids

Herein we present a highly efficient, light-mediated, deoxygenative protocol to access g-oxo-a-amino acid derivatives.This radical methodology employs photoredox catalysis, in combination with triphenylphosphine, to generate acyl radicals from readily available (hetero)aromatic and vinylic carboxylic acids. This approach allows for the straightforward synthesis of g-oxo-aamino acids bearing a wide range of functional groups (e.g. Cl, CN, furan, thiophene, Bpin) in synthetically useful yields (~ 60% average yield). To further highlight the utility of the methodology, several deprotection and derivatization reactions were carried out.