Direct allylic acylation via cross-coupling involving cooperative N‑heterocyclic carbene, hydrogen atom transfer, and photoredox catalysis

Visible Light-Mediated Monofluoromethylation/Acylation of Olefins by Dual Organo-Catalysis

Monofluoromethyl (CH2F) motifs exhibit unique bioactivities and are considered privileged units in drug discovery. The radical monofluoromethylative difunctionalization of alkenes stands out as an appealing approach to access CH2F-containing compounds. However, this strategy remains largely underdeveloped, particularly under metal-free conditions. In this study, we report on visible light-mediated three-component monofluoromethylation/acylation of styrene derivatives employing NHC and organic photocatalyst dual catalysis. A diverse array of α-aryl-β-monofluoromethyl ketones was successfully synthesized with excellent functional group tolerance and selectivity. The mild and metal-free CH2F radical generation strategy from NaSO2CFH2 holds potential for further applications in fluoroalkyl radical chemistry.

Chromium-Catalyzed Cross-Coupling of Methyl Ketones with Cyclic Ketones toward the Selective Synthesis of β-Branched β,γ-Unsaturated Ketones

Chromium-catalyzed cross-coupling of methyl ketones with cyclic ketones to β-branched β,γ-unsaturated ketones are reported. Interestingly, single-crossed aldol condensation products are formed, even in reactions in which a mixture of products is possible. The reaction is highly chemoselective and regioselective. This catalytic route gives a unique opportunity to integrate the chemistry of the synthetic challenge cross-coupling reaction of ketones and the alkene migration reaction into a reaction pot.

Chemo- and Regioselective Catalytic Cross-Coupling Reaction of Ketones for the Synthesis of β, γ-Disubstituted β, γ-Unsaturated Ketones

C-C bond forming reaction of ketone with aldehyde is well-studied for the synthesis of α, β-unsaturated ketones, however, the reaction with two different ketones to unsaturated carbonyl compound has not yet been systematically studied. Probably due to the relatively low reactivity of ketones as electrophiles (aldol acceptors), its propensity for retro-aldol reaction. The reactions often suffer from unsatisfactory chemoselectivity (self- vs. crossed aldol products) and regioselectivity (thermodynamic vs. kinetic enolate). In this quest, we report here for the first time selective cross-coupling reaction of ketones to β-branched β, γ-unsaturated ketones by using ruthenium catalysis. Interestingly, single crossed aldol condensation products are formed even in reactions where a mixture of products is possible. Reaction is highly chemoselective, regioselective and produces H2 O as the only byproducts making the protocol environmentally benign. Method is compatible with a wide variety of sensitive functional group and applicable for even problematic aliphatic ketones as substrates. Notably, acetone was found as a three-carbon feedstock for the syntheses of simple β, γ-unsaturated ketone compounds. The process can further be extended to the gram-scale reaction and late-stage functionalization of natural products. With the help of DFT calculations, several control experiments, and deuterium-labeling experiments, the mechanistic finding demonstrated that initial aldol-condensation of ketones to a β, β-disubstituted α, β-unsaturated ketone, which further isomerizes to a β, γ- unsaturated ketone via η3 -allyl ruthenium complex.

Recent advances in combining photo- and N-heterocyclic carbene catalysis

N-Heterocyclic carbenes (NHCs) are unique Lewis basic catalysts that mediate various organic transformations by means of polarity reversal. Although the scope of research on two-electron reactions mediated by NHC catalysts has been expanding, the types of these reactions are limited by the inability of NHCs to engage sp3-electrophiles. However, the revival of photocatalysis has accelerated the development of free-radical chemistry, and combining photoredox catalysis and NHC catalysis to achieve NHC-mediated radical reactions under mild conditions could overcome the above-mentioned limitation. This review summarizes recent advances in combining photoredox and NHC catalysis, focusing on elucidation and exploration of mechanisms, with the aim of identifying challenges and opportunities to develop more types of catalytic models.

Cooperative Carbene Photocatalysis for β-Amino Ester Synthesis

β-Amino acids are useful building blocks of bioactive molecules, including peptidomimetics and pharmaceutical compounds. The current limited accessibility to β2,2-type amino acids which bear an α-quaternary center has limited their use in chemical synthesis and biological investigations. Disclosed herein is the development of a new N-heterocyclic carbene/photocatalyzed aminocarboxylation of olefins, affording β2,2-amino esters with high regioselectivity. The generation of nitrogen-centered radicals derived from simple imides via a sequence of deprotonation and single-electron oxidation allows for the subsequent addition to geminal-disubstituted olefins regioselectively. The intermediate tertiary radicals then cross-couple with a stabilized azolium-based radical generated in situ to efficiently construct the quaternary centers. Mechanistic studies, including Stern-Volmer fluorescence quenching experiments, support the proposed catalytic cycle.

Fluoroalkylation of Activated Allylic Acetates through Radical-Radical Coupling: Organophotoredox/DABCO Catalytic System

A novel organophotoredox/DABCO catalytic system for the fluoroalkylation of activated allylic acetates via radical-radical coupling is described. The method offers mild reaction conditions, high selectivity, and broad substrate compatibility and enabled diverse bioactive molecules, FDA-approved drugs, and amino acid derivatives to be incorporated into transformation. This study expands the synthetic toolbox for the construction of fluorine-containing molecules.