N-Heterocyclic Carbene Catalyzed Research Articles

Updating the Benzoin Condensation of Benzaldehyde Using Microwave-Assisted Organic Synthesis and N-Heterocyclic Carbene Catalysis

The benzoin condensation of benzaldehyde is a staple of the undergraduate organic chemistry laboratory, introducing the current topic of nucleophilic carbene organocatalysis. Our procedure makes notable improvements to current practice, including the elimination of solvent waste and an 18-fold reduction of reaction time. We have demonstrated that microwave heating provides yields comparable to current practice in 5 min, using significantly less power (70 W) than traditional heating methods. The highest yields were obtained with N,N′-(2,4,6-trimethylphenyl)imidazolium chloride (IMes·HCl) as the catalyst and 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) as the base. Our procedure has been successfully executed by multiple cohorts of organic chemistry undergraduates, both with and without pandemic social distancing constraints.

N-Heterocyclic Carbene (NHC) Catalysis Permits Diastereoselective Radical Aminoacylation of Olefins

N-Heterocyclic Carbene (NHC) Catalysis Permits Diastereoselective Radical Aminoacylation of Olefins

Visible light-induced synthesis of 1,3-disubstituted bicyclo[1.1.1]pentane ketonesviacooperative photoredox and N-heterocyclic carbene catalysis

Visible light-induced highly efficient synthesis of 1,3-disubstituted bicyclo[1.1.1]pentane with mild conditions, high atom economy, and superior functional group tolerance.

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.

Construction of alkenyl-isoquinolinones through NHC-catalyzed remote C(sp3)–H acylation and cascade cyclization of benzamides and enals

Available online A general and efficient method for constructing alkenyl-substituted isoquinolinones through N-heterocyclic carbene catalysis was described. The functionalization of benzamides with various enals proceeds smoothly through an N-center radical-mediated remote C (sp3)–H acylation and a cascade intramolecular hemiacetalization process. The newly designed thiazolium NHCs, exhibiting superior catalytic efficacy to activate the challenging enal substrates, is key to the success of this chemistry. This catalytic protocol further expands the generality of N-heterocyclic carbene radical catalysis.

Merging N‐Heterocyclic Carbene Organocatalysis with Hydrogen Atom Transfer Strategy

AbstractRadical N‐heterocyclic carbene (NHC) organocatalysis has progressed significantly in recent years, enabling various incredible transformations. However, prefunctionalized substrates bearing a redox functionality were always required. As a result, the direct functionalization of aliphatic C−H bonds in simple substrates remains a challenge. This situation was changed once the hydrogen transfer atom (HAT) strategy was successfully applied in radical NHC catalysis very recently. This concept paper focused on the discussion of merging NHC catalysis with a classical intramolecular 1,5‐HAT process. Moreover, the unprecedented application of intermolecular HAT in radical NHC catalysis is highlighted as a novel catalysis model showing great synthetic promise.

Metal-Free Generation of γ-Cyanoalkyl Radicals by N-Heterocyclic Carbene Catalysis: Assembly of 6-Cyanoalkyl Phenanthridines

A number of γ-cyanoalkyl radicals were generated by sustainable N-heterocyclic carbene catalysis in tin-, transition-meal-, and light-free conditions, followed by insertion into biaryl isonitriles, thus leading to the rapid assembly of a variety of diversely functionalized 6-cyanoalkyl phenanthridines. A preliminary mechanism study revealed that a single-electron transfer radical process was possibly involved.

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

A Markovnikov-selective hydroacylation of alkenes has been achieved via the synergistic merger cobalt, photoredox and N-heterocyclic carbene catalysis. The closely incorporated catalytic cycles allow for Co(III) generation by photochemical oxidation instead of chemical oxidants or anodizing process. This mild, operationally simple protocol converts a wide variety of commercially available alkenes and aroyl fluorides into the corresponding ketones in high yield with branched selectivity.

Diastereoselective Radical Aminoacylation of Olefins through N-Heterocyclic Carbene Catalysis.

There have been significant advancements in radical-mediated reactions through covalent-based organocatalysis. Here, we present the generation of iminyl and amidyl radicals via N-heterocyclic carbene (NHC) catalysis, enabling diastereoselective aminoacylation of trisubstituted alkenes. Different from photoredox catalysis, single electron transfer from the deprotonated Breslow intermediate to O-aryl hydroxylamine generates an NHC-bound ketyl radical, which undergoes diastereocontrolled cross-coupling with the prochiral C-centered radical. This operationally simple method provides a straightforward access to a variety of pyrroline and oxazolidinone heterocycles with vicinal stereocenters (77 examples, up to >19:1 d.r.). Electrochemical studies of the acyl thiazolium salts support our reaction design and highlight the reducing ability of Breslow-type derivatives. A detailed computational analysis of this organocatalytic system suggests that radical-radical coupling is the rate-determining step, in which π-π stacking interaction between the radical intermediates subtly controls the diastereoselectivity.

Oxidative N-Heterocyclic Carbene Catalysis.

N-Heterocyclic carbene (NHC) catalysis is a by now consolidated organocatalytic platform for a number of synthetic (asymmetric) transformations via diverse reaction modes/intermediates. In addition to the typical umpolung processes involving acyl anion/homoenolate equivalent species, implementation of protocols under oxidative conditions greatly expands the possibilities of this methodology. Oxidative NHC-catalysis allows for oxidative and oxygenative transformations through specific manipulations of Breslow-type species depending upon the oxidant used (external oxidant or O2 /air), the derived NHC-bound intermediates paving the way to non-umpolung processes through activation of carbon atoms and heteroatoms. This review is intended to update the state of the art in oxidative NHC-catalyzed reactions that appeared in the literature from 2014 to present, with a strong focus to crucial intermediates and their mechanistic implications.

Exploring Molecular Complexity by N-Heterocyclic Carbene Organocatalysis: New Activation and Reaction Diversity.

The development of catalytic synthetic approaches towards molecular complexity from simple materials continues to be an ultimate goal in synthetic chemistry. Over the past decades, N-heterocyclic carbene (NHC) organocatalysis has been extensively investigated to provide opportunities for a vast number of novel chemical transformations. Various activation modes and reactive intermediates enabled by NHC small-molecule catalysts, such as Breslow intermediates, (homo)enolates, acyl azoliums and their derived unsaturated azoliums exhibit great potential in the construction of complicated skeletons. This personal account will summarize our group's recent work in the exploration of new activation modes of NHC catalysis towards molecular complexity with a focus on the development and applications of NHC to achieve diversity and enantioselectivity in the preparation of functional molecules.

ε-Benzylation via Cooperative Photoredox and N-Heterocyclic Carbene Catalysis.

The ε-benzylation of γ-alkenyl-γ-oxidized enals via dual photoredox and N-heterocyclic carbene catalysis has been developed, affording the corresponding ε-benzyl-α,β-γ,δ-bisunsaturated esters in moderate to good yields with exclusive regioselectivities. The reaction is proposed via the generation of benzyl radical under photocatalysis, followed by its addition to an NHC-bound trienolate intermediate.

N-Heterocyclic Carbene Catalyzed Three-Component Reaction for the Synthesis of Multi-substituted Benzenes.

An N-heterocyclic carbene-catalyzed three-component reaction of ketone, acetylenedicarboxylate, and enal for the synthesis of multisubstituted benzenes has been developed. The key features of this reaction are mild reaction conditions and high efficiency and chemoselectivity. The corresponding pentasubstituted benzenes are obtained in good to high yields.

Divergent Construction of Heterocycles by SOMOphilic Isocyanide Insertion under N-Heterocyclic Carbene Catalysis.

A variety of phenanthridines are rapidly constructed by an N-heterocyclic carbene (NHC)-catalyzed SOMOphilic isocyanide insertion-initiated homolytic aromatic substitution-type radical cyclization in the absence of any light, transition metals, and external oxidants. The aldehyde-free, scalable, and operationally simple protocol tolerates diverse functionalized biaryl isonitriles and activated α-halides. Moreover, it can be further applied to the divergent construction of other N-heterocycles. Preliminary mechanistic studies disclose that an NHC-derived radical cation intermediate is possibly involved.

Oxidative Radical NHC Catalysis: Divergent Difunctionalization of Olefins through Intermolecular Hydrogen Atom Transfer

AbstractOxidative N‐heterocyclic carbene (NHC) organocatalysis, typically leading to the formation of acyl azolium reactive intermediates, constitutes one of the most important activation strategies for the NHC‐catalyzed chemical transformations. Here, we report an unprecedented oxidative radical NHC catalysis by using peroxyester as the external single‐electron oxidant to realize divergent difunctionalization of olefins. The key to success of this chemistry is the catalytic generation of oxygen radicals that could trigger an intermolecular hydrogen atom transfer to activate the inert C−H bonds, thereby enabling the productive radical relay process. With this protocol, commonly used general chemicals could serve as radical precursors to allow efficient synthesis of value‐added products in a straightforward and cost‐effective manner. Preliminary mechanistic investigations, including control experiments and DFT calculations, shed light on the NHC organocatalytic radical reaction mechanism.

Oxidative Radical NHC Catalysis: Divergent Difunctionalization of Olefins through Intermolecular Hydrogen Atom Transfer.

Oxidative N-heterocyclic carbene (NHC) organocatalysis, typically leading to the formation of acyl azolium reactive intermediates, constitutes one of the most important activation strategies for the NHC-catalyzed chemical transformations. Here, we report an unprecedented oxidative radical NHC catalysis by using peroxyester as the external single-electron oxidant to realize divergent difunctionalization of olefins. The key to success of this chemistry is the catalytic generation of oxygen radicals that could trigger an intermolecular hydrogen atom transfer to activate the inert C-H bonds, thereby enabling the productive radical relay process. With this protocol, commonly used general chemicals could serve as radical precursors to allow efficient synthesis of value-added products in a straightforward and cost-effective manner. Preliminary mechanistic investigations, including control experiments and DFT calculations, shed light on the NHC organocatalytic radical reaction mechanism.

CF2Br2 as a Source for Difluoroolefination of 1,3-Enynes via N-Heterocyclic Carbene Catalysis.

Commercially available CF2Br2 has been used as a convenient source for the rapid and reliable incorporation of the gem-difluorovinyl motif into an allene framework via an N-heterocyclic carbene catalyzed difluoroolefination of 1,3-enynes. The reaction proceeds through a cascade three-component radical relay/elimination process. This protocol is distinguished by its mild conditions, readily accessible starting materials, wide substrate scope, and ease of late-stage functionalization, thus unlocking an untraditional strategy to construct a new class of functionalized gem-difluorovinyl allenes.

Recent Trends in Photocatalytic Enantioselective Reactions.

Enantioselective synthesis through photocatalysis is one of the highly preferred approaches towards preparation of optically active compounds. This review elaborates and critically analyzes the different strategies of photocatalytic enantioselective reactions through H-bonding, transition metal catalysis, phase-transfer catalysis (PTC), chiral Lewis acid catalysis, N-heterocyclic carbene catalysis, and amine catalysis, and also explores ion pairs. In addition, it explains the different catalysis modes with multifunctional approaches for enantioselective photocatalytic reactions.

N-Heterocyclic Carbene Catalyzed Cross Dehydrogenative Coupling of Aldehydes with Methanol: Combined Use of Eosin Y and Hexachloroethane

AbstractCross dehydrogenative coupling of aldehydes with methanol was investigated under organocatalytic conditions based on the cooperation­ between N-heterocyclic carbene and eosin Y·Na as an organophotocatalys­t. The combined use of eosin Y·Na and hexachloro­ethane (C2Cl6) was the effective method for the oxidative esterification of various aldehydes, because the oxidation steps are promoted by two pathways associated with the activated photocatalyst and C2Cl6. In contrast, the combined use of eosin Y·Na and bromotrichloromethane (BrCCl3) was effective only for the oxidative esterification of simple cinnamaldehyde derivatives, in which BrCCl3 promotes the oxidation as a brominating reagent toward radical intermediates.

Dark and Light Reactions of Carbenes─Merging Carbene Transfer Reactions with N-Heterocyclic Carbene Catalysis for the Synthesis of Hydroxamic Acid Esters

Herein, we report visible light and N-heterocyclic carbene (NHC) jointly promoted multicomponent carbene transfer reactions. Under the optimized reaction conditions, two kinds of important hydroxamic acid esters were obtained in good yields depending on the reaction media used. The key to this success was driven by the blue light-promoted generation of free carbene species and the fast in situ formation of hydroxamic acid under NHC-catalyzed conditions. The mild reaction conditions, excellent functional group tolerance, useful synthetic transformations, and successful modification of natural products and drug molecules proved the utility and practicality of this method.