Recent Advances in NHC-Catalyzed Chemoselective Activation of Carbonyl Compounds

N-Heterocyclic carbenes (NHCs) catalysts have been employed as effective tools in the development of various reactions, which have made notable contributions in developing diverse reaction modes and generating significant functionalized molecules. This review provides an overview of the recent advancements in the chemo- and regioselective activation of different aldehydes using NHCs, categorized into five parts based on the different activation modes. A brief conclusion and outlook is provided to stimulate the development of novel activation modes for accessing functional molecules.

Recent Advances in Enantioselective Organocatalytic Reactions Enabled by NHCs Containing Triazolium Motifs

N-Heterocyclic carbenes (NHCs) containing triazolium motifs have emerged as a powerful tool in organocatalysis. Recently, various NHC pre-catalyst mediated organic transformations have been developed successfully. This article aims to compile the current state of knowledge on NHC-triazolium catalysed enantioselective name reactions and introduce newly developed catalytic methods. Furthermore, this review article framework provides an excellent opportunity to highlight some of the unique applications of these catalytic procedures in the natural product synthesis of biologically active compounds, notably the wide range of preparation of substituted chiral alcohols, and their derivatives. This article provides an overview of chiral NHC triazolium-catalyst libraries synthesis and their catalytic application in enantioselective reactions.

Remote N–H activation of indole aldehydes: an investigation of the mechanism, origin of selectivities, and role of the catalyst

Density functional theory investigation on the N-heterocyclic carbene-catalysed synthesis of oxazinoindole derivatives via N–H activation of indole aldehydes.

Photocatalytic Redox-Neutral Reaction of γ-Indolyl α-Keto Esters

The direct γ-C(sp3)-H activation of saturated α-keto esters has long been an elusive transformation. We found that photo-redox catalysis IrIII in combination with DABCO as dual hydrogen-bonding donor and organic...

A Radical Addition/Cyclization and Se-Group Transfer Strategy for the Facile Synthesis of Se-Containing Cyclopentenes under Metal-Free and Peroxide-Free Conditions

A novel intermolecular radical addition/cyclization and Se-group transfer reaction of terminal alkynes and unsaturated alkyl selenide is presented which offers a straightforward and facile approach for the synthesis of valuable Se-containing cyclopentenes. Remarkable features of this strategy include easily accessible starting materials, metal-free and peroxide-free conditions, high atom economy, simple operation and broad substrate scope. More importantly, the reaction is easy to scale up and can be extended to the construction of six-membered carbon ring.

Mechanism and regio- and stereoselectivity in an NHC-catalyzed Mannich/lactamization domino reaction

Density functional theory (DFT) calculations (M06-2X) have been employed to disclose the mechanisms and regio- and stereo-selectivities of the N-heterocyclic carbene (NHC)-catalyzed reaction of 2-benzothiazolimines and α-chloroaldehydes. The preferred mechanism is initiated by the nucleophilic attack of NHC on α-chloroaldehyde (first step), followed by 1,2-proton transfer which was assisted by the Brønsted acid DABCO·H+ to generate the Breslow intermediate (second step). The cleavage of the C-Cl bond (third step) and deprotonation (fourth step) form the enolate intermediate. This further reacts with 2-benzothiazolimine which leads to the formation of a new C-C bond (fifth step). Subsequent cyclization takes place via the formation of a new C-N bond (sixth step). Catalyst regeneration completes the whole catalytic cycle and affords the final product (seventh step). The DFT results indicate that the fifth step determines the stereochemistry of the reaction and leads to benzothiazolopyrimidinone with the SS configuration, which agrees well with experimental observations. Intramolecular cyclization is found to be the regioselectivity-determining step, for which the [4+2] annulation pathway is more preferred than that via [2+2] annulation, which again agrees well with experimental observations. Based on the mechanism proposed, the origins of regio- and stereoselectivities have also been investigated by performing distortion/interaction, natural bond orbital (NBO) and non-covalent interaction (NCI) analyses. The mechanistic insights gained in this work should be helpful in the rational design of potential catalysts for analogous reactions.

Theoretical study of the NHC-catalyzed C–S bond cleavage and reconstruction reaction: mechanism, stereoselectivity, and role of catalysts

The mechanism of the NHC-catalyzed C–S bond activation reaction has been theoretically studied and ELF analysis along with IRC results shows that the NHC promotes the cleavage of the C–S bond via an SN2 process.

Stereoselective synthesis and applications of spirocyclic oxindoles

This review summaries recent synthetic developments towards spirocyclic oxindoles and applications as valuable medicinal and synthetic targets.

Mechanistic study on the NHC-catalyzed [3+4] annulation of enals and thiazolones

Reaction mechanisms and origins of regio- and stereo-selectivities of NHC catalyzed [3+4] annulation of enals and thiazolones.

Mechanism and stereoselectivity in NHC-catalyzed β-functionalization of saturated carboxylic ester

To understand the mechanism and origin of the stereoselectivity of the [3 + 2] annulation reaction between a carboxylic ester and an isatin generating spirooxindole lactone catalyzed by N-heterocyclic carbene (NHC), density functional theory (DFT) calculations have been carried out. DFT results indicate that the catalytic cycle begins with the coupling of the catalyst with benzotriazole ester, followed by α-deprotonation to produce the enolate intermediate. The subsequent 1,4-proton transfer affords the homoenolate intermediate. The next crucial step is the stereoselective C-C bond formation. Then proton transfer takes place leading to the formation of the lactone intermediate. Finally, the elimination of the catalyst furnishes the final product. The presence of 1-hydroxybenzotriazole (HOBt) dramatically accelerates the proton transfer step. More importantly, HOBt has a non-negligible impact on stereoselective C-C bond formation, and the SR-configured product is the major stereoisomer of the annulation product, which is in good agreement with the experimental observations. The differential π⋯π stacking, C-H⋯π, lone pair (LP)⋯π and repulsion interactions are found to be responsible for the stereoselectivity. The obtained mechanistic insights should provide valuable information for understanding the important roles of the NHC catalyst and HOBt additive and be helpful for designing better NHC catalysts for this kind of reaction.

Understanding the mechanism and stereoselectivity of NHC-catalyzed [3 + 2] cycloaddition of 3-bromoenals and isatin N-Boc ketimines

The mechanism and origin of stereoselectivities in N-heterocyclic carbine (NHC)-catalyzed [3 + 2] cycloaddition of 3-bromoenals and isatin N-Boc ketimines have been studied using density functional theory (DFT) calculations. Various possible reaction pathways were examined and analyzed. Our calculation results reveal that the catalytic cycle can be characterized by six steps: nucleophilic attack of a catalyst on 3-bromoenal (step I); [1,2]-proton transfer (step II); addition to the other reactant isatin N-Boc ketimine (step III); debromination (step IV); intramolecular cycloaddition concerted with hydrogen-elimination (step V) and elimination of the NHC catalyst (step VI). For step II, besides the direct [1,2]-proton transfer, the mediator (base and water)-assisted proton transfer has been investigated, and the DFT results show that the 2H2O cluster-mediated proton transfer is most energy favorable. The carbon-carbon bond formation step (step III) is calculated to be the stereoselectivity-determining step and the S-configuration product is the predominant product, which are in good agreement with the experimental observations. The mechanistic insights gained in this work should be helpful for other NHC-catalyzed reactions.

A tandem dearomatization/rearomatization strategy: enantioselective N-heterocyclic carbene-catalyzed α-arylation

In this study, the first example of carbene-catalyzed tandem dearomatization/rearomatization reaction of azonaphthalenes with α-chloroaldehydes is described.

In this study, the first example of the carbene-catalyzed tandem dearomatization/rearomatization reaction of azonaphthalenes with α-chloroaldehydes is described. This protocol enables the efficient assembly of chiral dihydrocinnolinone derivatives in good yields with excellent enantioselectivities (up to 99% ee). Moreover, this strategy enables not only the highly enantioselective NHC-catalyzed nucleophilic aromatic substitution, but also a formal Csp²–Csp³ bond formation.

Prediction on the origin of selectivities of NHC-catalyzed asymmetric dearomatization (CADA) reactions

The mechanism and origin of selectivities of NHC-catalyzed dearomatization reaction have been theoretically studied.

Oxidative NHC catalysis: direct activation of β sp3 carbons of saturated acid chlorides

The first activation of saturated acid chlorides by an oxidative N-heterocyclic carbene catalyst has been utilized to synthesize enantio-enriched spirooxindole lactones and δ-lactones.

N-Heterocyclic carbene-catalyzed β-addition of enals to 3-alkylenyloxindoles: synthesis of oxindoles with all-carbon quaternary stereocenters

The N-heterocyclic carbene-catalyzed β-addition of enals to 3-alkylenyloxindoles was developed. All of the β-aryl, β-alkenyl and β-alkyl enals worked well for the reaction to give the corresponding 3,3-disubstituted oxindoles bearing contiguous all-carbon quaternary and tertiary stereocenters in good yields with moderate to high diastereoselectivities and excellent enantioselectivities.

N-Heterocyclic Carbene (NHC)-Organocatalyzed Kinetic Resolutions, Dynamic Kinetic Resolutions, and Desymmetrizations

The last couple of decades have witnessed tremendous development within N-heterocyclic carbene (NHC) organocatalysis. NHCs have been used as powerful organic catalysts in asymmetric synthesis. Although great achievements have been made in asymmetric NHC catalysis, their applications in kinetic resolution (KR), dynamic kinetic resolution (DKR), and desymmetrization processes have been relatively less developed. Moreover, limited activation modes have been involved in these processes. This review provides an overview of the NHC-organocatalyzed KR, DKR, and desymmetrization reactions in the preparation of chiral functional molecules. The aim is to highlight both the importance and elegance of these methods in the construction of challenging chiral compounds and to provide our own perspective on future development in this direction.

Insights into the N-Heterocyclic Carbene (NHC)-Catalyzed Oxidative γ-C(sp3)-H Deprotonation of Alkylenals and Cascade [4 + 2] Cycloaddition with Alkenylisoxazoles

The N-heterocyclic carbene (NHC)-catalyzed oxidative C-H deprotonations have attracted increasing attention; however, the general mechanism regarding this kind of oxidative organocatalysis remains unclear. In this paper, the competing mechanisms and origin of the stereoselectivity of the NHC-catalyzed oxidative γ-C(sp³)-H deprotonation of alkylenals and cascade [4 + 2] cycloaddition with alkenylisoxazoles were systematically investigated for the first time using density functional theory (DFT). The computed results indicate that the oxidation of the Breslow intermediate by 3,3',5,5'-tetra- tert-butyl diphenoquinone (DQ) via a hydride transfer to oxygen (HTO) pathway is the most favorable among the four competing pathways. In addition, the analyses demonstrate that oxidant DQ plays a double role, i.e., strengthening the acidity of the hydrogen of the γ-carbon of alkylenal and forming π···π interactions with conjugated C═C bonds to promote the γ-C(sp³)-H deprotonation. The NHC catalyst acts as a Lewis base, and the hydrogen-bond network between the NHC and the substrate formed in the key Michael addition step is responsible for the origin of the stereoselectivity. Further DFT calculations reveal that the nonpolar solvent can stabilize the nonpolar R isomer but destabilize the polar S isomer for the stereoselectivity-determining transition states, thus improving the stereoselectivity.

Metal-Free Oxidative Decarbonylative [3+2] Annulation of Terminal Alkynes with Tertiary Alkyl Aldehydes toward Cyclopentenes

A new metal-free oxidative decarbonylative [3+2] annulation of terminal alkynes with tertiary alkyl aldehydes is presented, which features broad substrate scope and excellent selectivity. The selectivity of this reaction toward cyclopentenes and indenes relies on the nature of the aldehyde substrates. While treatment of tertiary γ,δ-unsaturated aldehydes with common terminal alkynes assembles cyclopentenes, 2-methyl-2-arylpropanals succeed in accessing indenes.

A continuing challenge: N-heterocyclic carbene-catalyzed syntheses of γ-butyrolactones

Catalytic, stereoselective N-heterocyclic carbene-catalyzed reactions facilitate efficient construction of many different heterocyclic compounds, such as the enantioenriched 5-membered (γ) lactones highlighted in this tutorial review. Herein, various strategies to enable formal [3+2] type annulations between electrophilic carbonyl equivalents and homoenolate nucleophiles for the synthesis of γ-lactones are summarized.

Direct Activation of β-sp3-Carbons of Saturated Carboxylic Esters as Electrophilic Carbons via Oxidative Carbene Catalysis

An N-heterocyclic carbene-catalyzed oxidative LUMO activation of the β-cabons of saturated carboxylic esters is disclosed. This approach allows for efficient asymmetric access to lactams and lactones by directly installing functional groups to the typically inert β-sp³ carbons of saturated esters. The use of HOBt as an additive was found to significantly improve both yields and enantioselectivities of the reactions.

Visible-Light-Activated Asymmetric β-C-H Functionalization of Acceptor-Substituted Ketones with 1,2-Dicarbonyl Compounds

We report a visible-light-activated asymmetric β-C(sp3)-H functionalization of 2-acyl imidazoles and 2-acylpyridines with 1,2-dicarbonyl compounds (typically α-ketoesters) catalyzed by a tailored stereogenic-at-rhodium Lewis acid catalyst. The C-C bond formation products are obtained in high yields (up to 99%) and with excellent stereoselectivities (up to >20:1 dr and up to >99% ee). Experimental and computational studies support a mechanism in which a photoactivated Rh-enolate transfers a single electron to the 1,2-dicarbonyl compound followed by proton transfer and a subsequent stereocontrolled radical-radical recombination.

Diastereoselective [3 + 2] cycloaddition of 3-ylideneoxindoles with in situ generated CF2HCHN2: syntheses of CF2H-containing spirooxindoles

Structurally diverse CF₂H-containing spirooxindoles were constructed through a [3 + 2] cycloaddition of isatin-derived 3-ylideneoxindoles with in situ generated CF₂HCHN₂.

1-Hydroxybenzotriazole-Assisted, N-Heterocyclic Carbene Catalyzed β-Functionalization of Saturated Carboxylic Esters: Access to Spirooxindole Lactones

A 1-hydroxybenzotriazole-assisted, N-heterocyclic carbene catalyzed direct β-functionalization of saturated carboxylic esters is disclosed. This formal [3 + 2] annulation reaction of carboxylic esters with isatins affords optically pure spirooxindole lactones (on gram scale) bearing two vicinal stereogenic centers. A dual role of HOBt is proposed based on controlled experiments to rationalize the enhancement of diastereoselectivity and enantioselectivity.

Direct Regioselective [3 + 2]-Cyclization Reactions of Ambivalent Electrophilic/Nucleophilic β-Chlorovinyl Dithianes: Access to Cyclopentene Derivatives

The highly regioselective and operationally straightforward [3 + 2] cyclizations of β-chlorovinyl dithianes with α,β-unsaturated carbonyl compounds have been developed. This protocol provides direct access to highly functionalized cyclopentenes with perfect chemo- and regioselectivities under extremely mild reaction conditions. In particular, the unprecedented cyclization allows for the selective preparation of hydroxylated cyclopentenes.

Influence of the N-Substituents on the Nucleophilicity and Lewis Basicity of N-Heterocyclic Carbenes

The ability to modulate the nucleophilicity and Lewis basicity of N-heterocyclic carbenes is pivotal to their application as organocatalysts. Herein we examine the impact of the N-substituent on the nucleophilicity and Lewis basicity. Four N-substituents popular in NHC organocatalysis, namely, the N-2,6-(CH3O)2C6H3, N-Mes, N-4-CH3OC6H4, and N-tert-butyl groups, have been examined and found to strongly affect the nucleophilicity. Thus, the N-2,6-(CH3O)2C6H3 group provides the most nucleophilic imidazolylidene NHC reported and the N-tert-butyl group one of the least. This difference in nucleophilicity is reflected in the catalyst efficiency, as observed with a recently reported trienyl ester rearrangement.