Synthesis of Cyclohexanones by a Tandem Photocatalyzed Annulation

N-Heterocyclic Carbene/Organic Photoredox-Cocatalyzed Acyl Azolation of Alkenes with Acyl Azoles

We have developed the N-heterocyclic carbene/organic photoredox-cocatalyzed acyl azolation of styrenes using acyl azoles. The cooperative action of organic photoredox catalysis and NHC catalysis enabled C-N bond cleavage of the acyl azole and the insertion of the C-C double bond in styrene with complete regioselectivity. The radical cation species, generated by single electron oxidation of alkene, subsequently reacted with the nitrogen nucleophile and ketyl radical intermediate to accomplish insertion of alkene into the C-N bond.

Mechanism and Origins of Regio- and Stereoselectivities of NHC-Catalyzed Dearomative Annulation of Benzoazoles and Cinnamaldehydes from DFT

A theoretical study on the mechanism, regioselectivity, and enantioselectivity of NHC-catalyzed dearomatizing annulation of benzoxazoles with enals has been conducted using density functional theory calculations. Our calculated results indicate that the favored mechanism occurs through eight reaction steps: initial binding of the NHC to enals, followed by formation of the Breslow intermediate via proton transfer. Subsequent oxidation generates the α,β-unsaturated acylazolium intermediate, which can undergo Michael addition with benzoxazoles. Sequential protonation/deprotonation/cyclization produces the six-membered cyclic intermediate that undergoes catalyst elimination, leading to the final product. DABCO·H+ was found to play important roles in proton transfer and cyclization. Without DABCO·H+, the energy barrier up to 44.2 kcal/mol for step 2 is too high to be accessible. With DABCO·H+, the corresponding value is lowered to 18.6 kcal/mol. The energy barrier for cyclization can be lowered by 7.4 kcal/mol by using DABCO·H+. The Michael addition step determines both the enantioselectivity and the regioselectivity. According to NCI analysis, the enantioselectivity is controlled by the strong interactions (such as C-H···O, C-H···N, and π···π) between the α,β-unsaturated acylazolium intermediate and benzoxazoles. We also discuss the solvent and substituent effects on the enantioselectivity and the role of the NHC. The mechanistic insights obtained in the present study would help improving current reaction systems or designing new synthetic routes.

N-Heterocyclic carbene (NHC) organocatalysis: from fundamentals to frontiers

N-Heterocyclic carbenes (NHCs) have been used as organocatalysts for a multitude of C-C and C-heteroatom bond-forming reactions. They enable diverse modalities of activating a wide range of structurally distinct substrate classes and allow access to electronically distinct intermediates. The easy tunability of the NHC scaffold contributes to its versatility. Recent years have witnessed a surge of interest in various organocatalytic reactions of NHCs, leading to the forays of NHC catalysis into the relatively newer domains such as reactions involving radical intermediates, atroposelective synthesis, umpolung of electrophiles other than aldehydes, and the use of NHCs as non-covalent templates for enantioinduction. This tutorial review provides an overview of various important structural features and reactivity modes of NHCs and delves deep into some frontiers of NHC-organocatalysis.

Cooperative photoredox and N-heterocyclic carbene-catalyzed formal C-H acylation of cyclopropanes via a deconstruction-reconstruction strategy

Cyclopropanes are ubiquitous and key structural motifs in commercially available drugs and bioactive molecules. Herein, we present regio-selective acylation of aryl cyclopropanes with cooperative photoredox and N-heterocyclic carbene catalysis. This approach involves a deconstruction-reconstruction strategy via γ-chloro-ketones as intermediates and fulfills the formal C(sp3)-H functionalization of cyclopropanes.

Cooperative NHC and Photoredox Catalyzed Radical Aminoacylation of Alkenes to Tetrahydropyridazines

Tetrahydropyridazines constitute an important structural motif found in numerous natural products and pharmaceutical compounds. Herein, we report an aminoacylation reaction of alkenes that enables the synthesis of 1,4,5,6-tetrahydropyridazines through cooperative N-heterocyclic carbene (NHC) and photoredox catalysis. This approach involves the 6-endo-trig cyclization of N-centered hydrazonyl radicals, generated via single-electron oxidation of hydrazones, followed by a radical-radical coupling step. The mild process tolerates a wide range of common functional groups and affords a variety of tetrahydropyridazines in moderate to high yields. Preliminary investigations using chiral NHC catalysts demonstrate the potential of this protocol for asymmetric radical reactions.

Redox-Paired Reductive Heck Reaction and Oxidative Esterification Catalyzed by Mesoionic Carbenes

Paring a reductive reaction and an oxidative reaction in one reaction could be immensely important in achieving atom economic and environmental advantages. Herein, we report a simple protocol that combines two such reductive Heck reactions and oxidative esterification by using mesoionic carbenes as catalysts to synthesize multiple valuable products under mild conditions.

Synthesis and Photocatalytic sp3 C-H Bond Functionalization of Salen-Ligand-Supported Uranyl(VI) Complexes

Recent years have seen increasing interest in uranyl(VI) photocatalysis. In this study, uranyl complexes were successfully synthesized from ligands L1-L6 and UO2(NO3)2·6H2O under reflux conditions, yielding products 1-6 with yields ranging from 30% to 50%. The complexes were thoroughly characterized using NMR spectroscopy, single-crystal X-ray diffraction, and elemental analysis. The results indicate that complexes 1-5 possess a pentagonal bipyramidal geometry, whereas complex 6 exhibits an octahedral structure. The photocatalytic properties of these novel complexes for sp3 C-H bond functionalization were explored. The results demonstrate that complex 4 functions as an efficient photocatalyst for converting C-H bonds to C-C bonds via hydrogen atom transfer under blue light irradiation.

N-Heterocyclic carbene catalytic 1,2-boron migrative acylation accelerated by photocatalysis

The transformation of organoboron compounds plays an important role in synthetic chemistry, and recent advancements in boron-migration reactions have garnered considerable attention. Here, we report an unprecedented 1,2-boron migrative acylation upon photocatalysis-facilitated N-heterocyclic carbene catalysis. The design of a redox-active boronic ester substrate, serving as an excellent β-boron radical precursor, is the linchpin to the success of this chemistry. With the established protocol, a wide spectrum of β-boryl ketones has been rapidly synthesized, which could further undergo various C─B bond transformations to give multifunctionalized products. The robustness of this catalytic strategy is underscored by its successful application in late-stage modification of drug-derived molecules and natural products. Preliminary mechanistic investigations, including several control experiments, photochemistry measurements, and computational studies, shed light on the catalytic radical reaction mechanism.

Mechanism, Chemoselectivity, and Stereoselectivity of an NHC-Catalyzed Reaction of Aldehydes and Hydrazones: A DFT Study

To elucidate the mechanism and origins of chemo- and enantioselectivities of the reaction between aliphatic aldehydes and hydrazones catalyzed by triazolium-derived NHC, density functional theory computations have been performed. According to our calculated results, the whole catalytic cycle for the formation of dihydropyridazinones proceeds via the initial nucleophilic addition of NHC to an aliphatic aldehyde, followed by the concerted intramolecular proton transfer and C-Cl bond cleavage. Subsequent deprotonation generates an enolate intermediate. The enolate intermediate then undergoes 1,4-addition to hydrazone to construct a new carbon-carbon bond. The following ring-closure would lead to a six-membered ring intermediate, which, upon the release of NHC, affords the final product dihydropyridazinone. The computation results reveal that intramolecular proton transfer is significantly promoted by the Brønsted acid DIPEA·H+. The carbon-carbon bond formation step could determine not only the chemoselectivity but also the stereoselectivity and lead to the S-isomer product. It was found that the stereoselectivity arises from a combination of weak interactions, including C-H···O, C-H···N, C-H···π, and LP···π. NHC could enhance the nucleophilicity of the aliphatic aldehyde and facilitate further reaction with hydrazone. This work could be beneficial for the development of new catalytic strategies in the future.

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.

Photoinduced cerium-catalyzed C-H acylation of unactivated alkanes

Ketones are ubiquitous motifs in the realm of pharmaceuticals and natural products. Traditional approaches to accessing these species involve the addition of metal reagents to carboxyl compounds under harsh conditions. Herein, we report a cerium-catalyzed acylation of unactivated C(sp3)-H bonds using bench-stable acyl azolium reagents under mild and operationally-friendly conditions. This reaction exhibits excellent generality, accommodating a wide range of feedstock chemicals such as cycloalkanes and acyclic compounds as well as facilitating the late-stage functionalization of pharmaceuticals. We demonstrate further applications of our strategy with a three-component radical relay reaction and an enantioselective N-heterocyclic carbene (NHC) and cerium dual-catalyzed reaction.

Light-Driven Enantioselective Carbene-Catalyzed Radical-Radical Coupling

An enantioselective carbene-catalyzed radical-radical coupling of acyl imidazoles and racemic Hantzsch esters is disclosed. This method involves the coupling of an N-heterocyclic carbene-derived ketyl radical and a secondary sp3 -carbon radical and allows access to chiral α-aryl aliphatic ketones in moderate-to-good yields and enantioselectivities without any competitive epimerization. The utility of this protocol is highlighted by the late-stage functionalization of various pharmaceutical compounds and is further demonstrated by the transformation of the enantioenriched products to biologically relevant molecules. Computational investigations reveal the N-heterocyclic carbene controls the double-facial selectivity of the ketyl radical and the alkyl radicals, respectively.

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.

Unified Approach to Diverse Heterocyclic Synthesis: Organo-Photocatalyzed Carboacylation of Alkenes and Alkynes from Feedstock Aldehydes and Alcohols

We report an organo-photocatalyzed carboacylation reaction that offers a springboard to create chemical complexity in a diversity-driven approach. The modular one-pot method uses feedstock aldehydes and alcohols as acyl surrogates and commercially available Eosin Y as the photoredox catalyst, making it simple and affordable to introduce structural diversity. Several biologically relevant skeletons have been easily synthesized under mild conditions in the presence of visible light irradiation by fostering a radical acylation/cyclization cascade. The proposed reaction mechanism was further illuminated by a number of spectroscopic studies. Furthermore, we applied this protocol for the late-stage functionalization of pharmaceuticals and blockbuster drugs.

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.

Exploring the Divergent Reactivity of Vinyl Radicals Emanating from Alkynes and Thiols via Photoredox Catalysis

Organic chemistry has seen a surge in visible-light-driven transformations, which offer unique reaction pathways and access to new synthetic possibilities. We aim to provide a comprehensive understanding of state-of-the-art photo-mediated alkyne functionalization, with a focus on the reactive behavior of vinyl radicals. This review outlines our contributions to the field, including developing new methods for forming carbon-carbon and carbon-heteroatom bonds.

Mechanisms and Stereoselectivities in the NHC-Catalyzed [4 + 2] Annulation of 2-Bromoenal and 6-Methyluracil-5-carbaldehyde

To disclose the reaction mechanism and selectivity in the NHC-catalyzed reaction of 2-bromoenal and 6-methyluracil-5-carbaldehyde, a systematic computational study has been performed. According to DFT computations, the catalytic cycle is divided into eight elementary steps: nucleophilic attack of the NHC on 2-bromoenal, 1,2-proton transfer, C-Br bond dissociation, 1,3-proton transfer, addition to 6-methyluracil-5-carbaldehyde, [2 + 2] cycloaddition, NHC dissociation, and decarboxylation. The Bronsted acid DABCO·H+ plays a crucial role in proton transfer and decarboxylation steps. The addition to 6-methyluracil-5-carbaldehyde determines both chemoselectivity and stereoselectivity, leading to R-configured carbocycle-fused uracil, in agreement with experimental results. NCI analysis indicates that the CH···N, CH···π, and LP···π interactions should be the key factor for determining the stereoselectivity. ELF analysis shows the main role of the NHC in promoting C-Br bond dissociation. The mechanistic insights obtained in the present work may guide the rational design of potential NHC catalysts.

Metal-Free Synthesis of 2H-Indazole Skeletons by Photochemistry or Thermochemistry

A simple and tuned synthesis of a 2H-indazole skeleton under metal-free conditions was developed. Under visible-light irradiation at room temperature, 2-((aryl/alkyl/H)ethynyl))aryltriazenes reacted with arylsulfinic acids to afford 3-functionalized 2H-indazoles without extra photocatalyst via an electron donor-acceptor complex. In the presence of arylsulfinic acid, 2-(ethynyl)aryltriazenes underwent an intramolecular oxidation/cyclization to provide 2H-indazole-3-carbaldehydes at 50 °C in air.

Photocatalyzed Direct α-Alkylation of Esters Using Styrenes

A mild photocatalyzed approach to achieve the α-alkylation of esters via formation of an α -radical is disclosed here. Cesium enolates of esters were generated in situ using Cs2CO3 as a base. A subsequent photocatalyzed oxidation at the α-carbon of these enolates produced an α-radical that was added into activated alkenes. This is the first example accessing the α-carbon radical of esters in photoredox catalyed transformations.

Visible-light-induced coupling of carboxylic acids with alcohols/amines via a phosphorous linchpin strategy

The combination of activated carboxylic acids and alcohols/amines to access esters and amides, respectively, is a cornerstone of organic chemistry and has been well developed over the past century. These dehydrations are extensively used in medicinal chemistry and natural product synthesis due to the prevalence of these functional groups in bioactive molecules. Here, we report a divergent process from the expected ester/amide outcomes through a light-induced coupling of activated carboxylic acids and alcohols/amines to efficiently prepare α-hydroxy/amino ketones or β-ketophosphonates via single-electron chemistry. A phosphorus linchpin strategy allows for the combination of these simple reagents through an intramolecular triplet state radical process, thereby enabling new carbon-carbon bond formation.

Acyl Azolium-Photoredox-Enabled Synthesis of β-Keto Sulfides

α-Heteroatom functionalization is a key strategy for C-C bond formation in organic synthesis, as exemplified by the addition of a nucleophile to electrophilic functional groups, such as iminium ions; oxocarbenium ions; and their sulfur analogues, sulfenium ions. We envisioned a photoredox-enabled radical Pummerer-type reaction realized through the single-electron oxidation of a sulfide. Following this oxidative event, α-deprotonation would afford α-thio radicals that participate in radical-radical coupling reactions with azolium-bound ketyl radicals, thereby accessing a commonly proposed mechanistic intermediate of the radical-radical coupling en route to functionalized additive Pummerer products. This system provides a complementary synthetic approach to highly functionalized sulfurous products, including modification of methionine residues in peptides, and beckons further exploration in C-C bond formations previously limited in the standard two-electron process.

Photoinduced Acylations Via Azolium-Promoted Intermolecular Hydrogen Atom Transfer

Photoinduced hydrogen atom transfer (HAT) has been developed as a powerful tool to generate synthetically valuable radical species. The direct photoexcitation of ketones has been known to promote HAT or to generate acyl radicals through Norrish-type pathways, but these modalities remain severely limited by radical side reactions. We report herein a catalyst- and transition metal-free method for the acylation of C-H bonds that leverages the unique properties of stable, isolable acyl azolium species. Specifically, acyl azolium salts are shown to undergo an intermolecular and regioselective HAT upon LED irradiation with a range of substrates bearing active C-H bonds followed by C-C bond formation to afford ketones. Experimental and computational studies support photoexcitation of the acyl azolium followed by facile intersystem crossing to access triplet diradical species that promote selective HAT and radical-radical cross-coupling.

Visible light-mediated NHC and photoredox co-catalyzed 1,2-sulfonylacylation of allenes via acyl and allyl radical cross-coupling

Visible light-mediated NHC and photoredox co-catalyzed radical 1,2-sulfonylacylation of allenes via cross-coupling between an allyl radical and an NHC-stabilized acyl radical.

Research Progress on N-Heterocyclic Carbene Catalyzed Reactions for Synthesizing Ketones through Radical Mechanism

NHC-catalyzed radical cross-coupling reactions have been recently developed; they provide an efficient method to access ketones from aldehydes or carboxylic acid derivatives with sp3-hybridized carbon radical precursors. This reaction has indirectly solved the limitations in the scope of coupling partners in NHC umpolung catalyzed reactions of aldehydes. In this short review, we present some recent advances in NHC-catalyzed radical reactions, with a focus on the construction of the C–C(CO) bond.

1 Introduction

2 Oxidative Generation of NHC-Derived Ketyl Radical

2.1 NHPI Redox-Active Esters

2.2 Katritzky Pyridinium Salts

2.3 Alkyl Halides

2.4 Aryl Halides

2.5 Compounds Containing N–O Bond

2.6 Diazo Esters

2.7 Others

3 Reductive Generation of NHC-Derived Ketyl Radical

3.1 Hantzsch Esters

3.2 Sulfinates

3.3 Electron-Rich Arenes

3.4 Amines

3.5 Organoborane Reagents

4 Conclusion