NHC Organocatalytic Formal LUMO Activation of α,β-Unsaturated Esters for Reaction with Enamides

Stereodivergent Synthesis of All Stereoisomers of 2,3-Disubstituted δ-Lactam Derivatives via Organocatalytic Cascade Reactions and Base-Induced Epimerization

A protocol was developed to achieve stereodivergent synthesis of stereoisomers of δ-lactam bearing vicinal chiral centers. Organocatalytic cascade reactions were employed to produce the target products as the kinetic products, which exhibited remarkable enantioselectivities. In the presence of DBU, the kinetic product underwent epimerization to form a thermodynamically more stable diastereomer without loss in enantioselectivity. By simply switching the chiral organocatalyst and its enantiomer, we can efficiently obtain four stereoisomers with high enantioselectivities.

Asymmetric synthesis of chiral pyrazolo[3,4-b]pyridin-6-ones under carbene catalysis

A structurally diverse set of chiral pyrazolo[3,4-b]pyridin-6-ones was efficiently prepared in excellent yields with excellent enantioselectivities via N-heterocyclic carbene-catalyzed oxidative [3 + 3] annulation of enals with pyrazol-5-amines. The reaction features mild reaction conditions, a broad substrate scope, and easy scale-up.

Carbene-catalyzed chemoselective reaction of unsymmetric enedials for access to Furo[2,3-b]pyrroles

A carbene-catalyzed chemoselective reaction of unsymmetric enedials is disclosed. The reaction provides a concise access to bicyclic furo[2,3-b]pyrroles derivatives in excellent selectivity. A main challenge in this reaction is chemoselective reaction of the two aldehyde moieties in the enedial substrates. Mechanistic studies via experiments suggest that our chemoselectivity controls are mostly achieved on the reducing properties of different sited Breslow intermediates. Several side reactions processes and the corresponding side adducts are also studied by high resolution mass spectroscopy analysis. Our method allows for efficient assembly of the furo[2,3-b]pyrrole structural moieties and their analogues widely found in natural products and pharmaceuticals.

Alkynyl Acylazolium: a Versatile 1,3-Bielectrophilic 3C-Synthon in NHC-Organocatalysis

N-Heterocyclic carbene (NHC) organocatalysis has emerged as a powerful tool in the field of modern organic synthesis especially in the asymmetric construction of various cyclic skeletons. As an emerging NHC-bound 1,3-bielectrophilic intermediate, alkynyl acylazolium has drawn substantial attention in recent years, and has been used as a versatile 3C-synthon in synthesizing valuable organic molecules since its discovery. In this review, focused on the different pathways for the formation of alkynyl acylazoliums from different precursors like alkynoic esters, alkynoic acids and ynals, the recent advances in the transformations and applications of alkynyl acylazoliums pioneered or developed over the last decade under NHC-catalysis were summarized comprehensively. At the same time, the outlook for further investigation and exploration of novel reaction modes for alkynyl acylazoliums in the future was also discussed.

Enantioselective N-Heterocyclic Carbene Catalyzed α-Oxidative Coupling of Enals with Carboxylic Acids Using an Iodine(III) Reagent

The enantioselective α-oxidative coupling of enals with carboxylic acids was developed via the umpolung of an NHC-bound enolate with an iodine(III) reagent. The corresponding α-acyloxyl-β,γ-unsaturated esters were afforded in good yields, with high regio- and enantioselectivities. The key step of the reaction involves the formation of enol iodine(III) intermediate from the enolate with iodosobenzene, which changes the polarity of α-carbon of the enal from nucleophilic to electrophilic, and thus facilitates the subsequent addition of carboxylate.

Catalytic Access to Chiral δ-Lactams via Nucleophilic Dearomatization of Pyridine Derivatives

Nitrogen-bearing rings are common features in the molecular structures of modern drugs, with chiral δ-lactams being an important subclass due to their known pharmacological properties. Catalytic dearomatization of preactivated pyridinium ion derivatives emerged as a powerful method for the rapid construction of chiral N-heterocycles. However, direct catalytic dearomatization of simple pyridine derivatives are scarce and methodologies yielding chiral δ-lactams are yet to be developed. Herein, we describe an enantioselective C4-dearomatization of methoxypyridine derivatives for the preparation of functionalised enantioenriched δ-lactams using chiral copper catalysis. Experimental ¹³ C kinetic isotope effects and density functional theory calculations shed light on the reaction mechanism and the origin of enantioselectivity.

Synthesis of 3,4-Dihydropyridin-2-ones via Domino Reaction under Phase Transfer Catalysis Conditions

3,4-dihydropyridin-2-ones are of considerable importance due to the large number of these core structures exhibiting a diverse array of biological and pharmacological activities. The Michael-type addition of 1,3-dithiane-2-carbothioates to α,β-unsaturated N-tosyl imines, followed by intramolecular annulation driven by a sulfur leaving group, provides a practical reaction cascade for the synthesis of a variety of substituted 3,4-dihydropyridin-2-ones. In this work, the reaction was carried out under solid–liquid phase transfer catalysis (SL-PTC) conditions at room temperature, in short reaction times in the presence of cheap Bu4N+HSO4– and solid KOH. The new PTC method exhibited adequate functional group tolerance, proving to be a green and reliable method and easy to scale up to furnish rapid access to 3,4-dihydropyridin-2-ones after desulfurization from simple, readily available starting materials.

Kinetic Resolution Approach to the Synthesis of C-N Axially Chiral N-Aryl Aminomaleimides via NHC-Catalyzed [3 + 3] Annulation

Chiral NHC-catalyzed kinetic resolution of N-aryl aminomaleimides allowing the synthesis of C-N axially chiral N-aryl aminomaleimides via remote chirality control is presented. The catalytically generated α,β-unsaturated acylazoliums from 2-bromoenals underwent selective [3 + 3] annulation with one of the enantiomers of maleimide to furnish fused-dihydropyridinone (bearing axial/central chirality, up to 6:1 dr, >99:1 er) leaving the enantioenriched opposite enantiomer (up to >99:1 er). Studies on C-N bond rotation barrier and dependence on temperature are also provided.

Electroredox carbene organocatalysis with iodide as promoter

Oxidative carbene organocatalysis, inspired from Vitamin B1 catalyzed oxidative activation from pyruvate to acetyl coenzyme A, have been developed as a versatile synthetic method. To date, the α-, β-, γ-, δ- and carbonyl carbons of (unsaturated)aldehydes have been successfully activated via oxidative N-heterocyclic carbene (NHC) organocatalysis. In comparison with chemical redox or photoredox methods, electroredox methods, although widely used in mechanistic study, were much less developed in NHC catalyzed organic synthesis. Herein, an iodide promoted electroredox NHC organocatalysis system was developed. This system provided general solutions for electrochemical single-electron-transfer (SET) oxidation of Breslow intermediate towards versatile transformations. Radical clock experiment and cyclic voltammetry results suggested an anodic radical coupling pathway.

N-Heterocyclic Carbene-Catalyzed Atroposelective Synthesis of Pyrrolo[3,4-b]pyridines with Configurationally Stable C-N Axial Chirality

The first atroposelective synthesis of pyrrolo[3,4-b]pyridines catalyzed by N-heterocyclic carbene has been achieved. A wide range of chiral atropisomers of pyrrolo[3,4-b]pyridines were obtained in high yields with excellent enantioselectivities (96-99% enantiomeric excess). The experimental results and density functional theory calculations showed that the C-N axial chirality of the product had high thermal stability.

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.

Carbene catalyzed C(sp3)–Cl activation of chlorinated solvents for benzyne chlorination

A new mode of N-heterocyclic carbene (NHC) organocatalysis was discovered, in which the normally inert chlorinated solvents were activated by carbene to realize the chlorination of hexa-dehydro-Diels-Alder derived benzynes.

Phosphine-Catalyzed Annulations Based on [3+3] and [3+2] Trapping of Ketene Intermediates with Thioamides

With the aim of developing novel annulations via ketene intermediates, allenyl imide and alkynoates bearing good leaving groups are used for their function in a tandem conjugate addition-elimination reaction (SN2' type) promoted by nucleophilic phosphine catalysts. By utilizing thioamides as 1S,3N-bis-nucleophiles, [3+3] and [3+2] annulations have been established to allow rapid access to 1,3-thiazin-4-ones and 5-alkenyl thiazolones in high yields, respectively. Furthermore, the possible reaction mechanisms are proposed on the basis of deuterium labeling experiments and density functional theory calculations.

N-Heterocyclic Carbene-Catalyzed Asymmetric C-O Bond Construction Between Benzoic Acid and o-Phthalaldehyde: Mechanism and Origin of Stereoselectivity

A computational study was contributed to explore the origin of stereoselectivity of NHC-mediated cyclization reaction between benzoic acid and o-phthalaldehyde for asymmetric construction of phthalidyl ester. The most energetically favorable pathway mainly includes the following steps: (1) nucleophilic attack on carbonyl carbon of o-phthalaldehyde by catalyst NHC, (2) formation of Breslow intermediate, (3) oxidation by DQ, (4) asymmetric formation of dual C-O bonds, and (5) dissociation of catalyst with the product. The C-O bond formation was testified as the stereoselectivity-determining step, the R-configurational pathway is more energetically favorable than the S-configurational one. The non-covalent interaction (NCI) and atom-in-molecule (AIM) analyses were performed to reveal that the O-H ⋅⋅⋅ O and C-H ⋅⋅⋅ O hydrogen-bond interactions are the key factors for controlling the stereoselectivity. The detailed mechanism and origin of stereoselectivity give useful insights for understanding organocatalytic reactions for asymmetric construction of C-O bond.

Revealing the Similarities of α,β-Unsaturated Iminiums and Acylazoliums in Organocatalysis

The secondary amine-catalyzed reactions proceeding via α,β-unsaturated iminiums and the N-heterocyclic carbene (NHC)-catalyzed transformations taking place via α,β-unsaturated acylazoliums are the two widely used electrophilic intermediates in organocatalysis. Over the last two decades, these two intermediates are extensively utilized for the enantioselective construction of valuable molecules. Both intermediates are generated by the covalent binding of catalysts to the substrates leading to LUMO activation of α,β-unsaturated carbonyls. A variety of soft nucleophiles are known to add to the α,β-unsaturated iminiums and acylazoliums in a conjugate fashion, and in many cases, striking similarity in reactivity has been observed. Having said this, there are few cases where these intermediates exhibit difference in reactivity. This Minireview is aimed at highlighting the resemblances in reactivity between α,β-unsaturated iminiums and acylazoliums thereby shedding light on the unnoticed parallels of the two intermediates in organocatalysis.

N-Heterocyclic carbene-catalyzed [3 + 3] annulation of bromoenals with 2-aminochromones to access chromeno[2,3-b]pyridinones

N-Heterocyclic carbene-catalyzed [3 + 3] annulation of bromoenals with 2-aminochromones has been successfully developed. A structurally diverse set of chromeno[2,3-b]pyridinones was efficiently constructed in acceptable to excellent yields. The reaction features mild reaction conditions, a broad substrate scope, and easy scale-up.

N-Heterocyclic carbene catalyzed asymmetric [3 + 3] cycloaddtion of β,β-disubstituted, α,β-unsaturated carboxylic esters with 3-aminobenzofurans

An NHC-catalyzed β-carbon functionalization reaction to afford enantioenriched benzofuran fused δ-lactams bearing an all-carbon quaternary stereocenter is documented.

Generation of azolium dienolates as versatile nucleophilic synthons via N-heterocyclic carbene catalysis

The recent advances in N-heterocyclic carbene (NHC)-catalyzed generation of azolium dienolates from different precursors and their synthetic applications for the construction of various valuable molecules are summarized comprehensively in this review.

Carbene-Catalyzed Enantioselective Synthesis of γ-Keto-β-silyl Esters and Amides

A variety of γ-keto-β-silyl esters and amides, most with extremely high enantioselectivities, were efficiently prepared via a carbene-catalyzed formal [4 + 2] annulation followed by ring opening with nucleophiles. The resulting compounds from this one-pot strategy can be easily converted into enantioenriched β,σ-dihydroxyl esters.

Golden Jubilee of Singapore National Institute of Chemistry (1970-2020): Celebrating its Partnership with Wiley-VCH

This year Singapore National Institute of Chemistry (SNIC) is celebrating its golden jubilee (1970-2020). Wiley-VCH has been a steadfast partner accompanying the rapid rise of chemistry research in Singapore. In celebration of this golden jubilee, we highlight 50 significant papers published in Angewandte Chemie by scholars currently based in Singapore, covering the widest possible spectrum of chemistry research.

New pathways of stability for NHCs derived from azole, di-azole, n-tetrazole, and ab-tetrazole, by DFT

We have investigated the pathways of stability for NHCs derived from azole, di-azole, n-tetrazole, and ab-tetrazole (1_a, 2_a, 3_a, and 4_a, respectively), at the M06/6-311++G** level of theory. Optimization and vibrational frequency calculations of ground states (GS) and transition states (TS) are performed to identify Gibbs free energies and nature of stationary points, respectively. Two possible pathways of stability for 1_a-4_a are compared and contrasted which entail dimerization through hydrogen bonding (HB) and covalent bonding (CB). The CB pathway comprises head to head (HH) and head to tail (HT) dimerizations. Plausible reaction profiles are illustrated for 1_a-4_a along with the mechanism of each dimerization. Structures 1_a-3_a show one possibility for HB while 4_a represents two possibilities. Structures 1_a and 4_a display two HH dimers while 2_a and 3_a show one. Structures 1_a-4_a undergo HT dimerizations to yield three possible dimers which include trans, cis, and [2+3] isomers. Interestingly, for all 1_a-4_a, HB dimerization turns out as the most favorable stability pathway for showing no barrier of reaction. Structures 4_b and 4_c indicate the highest stability with respect to their initial 4_a compared to remaining HB dimers 1_b-3_b. In addition, the 1,2-H shift appears as a possible rearrangement for 1_a-4_a to yield their corresponding tautomers (1_i, 2_h, 3_h, and 4_k, respectively). The reaction profile of this rearrangement indicates that 1_a-4_a favor HB dimerization pathway more than 1,2-H shift, in terms of kinetic and thermodynamic. Graphical Abstract.

NHC-Catalyzed Enantioselective [3 + 3] Annulation to Construct 5,6-Dihydropyrimidin-4-ones

The unprecedented enantioselective NHC-catalyzed [3 + 3] annulation of α-bromoenals with amidines via a dual C-N bond formation is described. The protocol allows a rapid preparation of 5,6-dihydropyrimidinones in acceptable yields with good enantioselectivities.

Imines as acceptors and donors in N-heterocyclic carbene (NHC) organocatalysis

The synthetic potential of imines as electrophiles or as a source of nucleophilic coupling partner in N-heterocyclic carbene (NHC) catalysis for the synthesis of various nitrogen heterocycles and functionalized amines is highlighted in this Feature Article. Electrophilic imines are suitable candidates for intercepting the NHC-derived acyl anions, homoenolate equivalents, and (di)enolates for the synthesis of α-amino ketones and a variety of lactam derivatives. Moreover, enamines generated from imines bearing α-hydrogen could be trapped with α,β-unsaturated acylazoliums for the synthesis of functionalized dihydropyridinones. NHCs are also useful for the umpolung of imines for the generation of aza-Breslow intermediates thus leading to the synthesis of indoles, quinolines, dihydroquinoxalines etc. A concise account of the diverse reactivity of imines in NHC catalysis has been presented.

Prediction of NHC-catalyzed chemoselective functionalizations of carbonyl compounds: a general mechanistic map

Generally, N-heterocyclic carbene (NHC) complexed with carbonyl compounds would transform into several important active intermediates, i.e., enolates, Breslow intermediates, or acylazolium intermediates, which act as either a nucleophile (Nu) or an electrophile (E) to react with the other E/Nu partner. Hence, the key to predicting the origin of chemoselectivity is to compute the activity (i.e., electrophilic index ω for E and nucleophilic index N for Nu) and stability of the intermediates and products, which are suggested in a general mechanistic map of these reactions. To support this point, we selected and studied different cases of the NHC-catalyzed reactions of carbonyl compounds in the presence of a base and/or an oxidant, in which multiple possible pathways involving acylazolium, enolate, Breslow, and α,β-unsaturated acylazolium intermediates were proposed and a novel index ω + N of the E and Nu partners was employed to exactly predict the energy barrier of the chemoselective step in theory. This work provides a guide for determining the general principle behind organocatalytic reactions with various chemoselectivities, and suggests a general application of the reaction index in predicting the chemoselectivity of the nucleophilic and electrophilic reactions.

A novel index ω + N can be used to predict the chemoselectivity according to the general NHC-catalyzed reaction mechanism.

Construction of Dihydropyrido[2,3-d]pyrimidine Scaffolds via Aza-Claisen Rearrangement Catalyzed by N-Heterocyclic Carbenes

N-Heterocyclic carbenes (NHCs) catalyzing aza-Claisen rearrangement of α,β-unsaturated enals with cyclic vinylogous amides under oxidative conditions generating potentially biologically active dihydropyridinone-fused uracils have been developed. This strategy represents a unique NHC-activation-based path with the use of 6-aminouracils as stable α,β-diEWG cyclic vinylogous amides for the efficient synthesis of bicyclic N-unprotected lactams similar to those in many useful drugs.

Phosphine-Catalyzed [4+1] Cycloadditions of Allenes with Methyl Ketimines, Enamines, and a Primary Amine

Unprecedented phosphine-catalyzed [4+1] cycloadditions of allenyl imides have been discovered using various N-based substrates including methyl ketimines, enamines, and a primary amine. These transformations provide a one-pot access to cyclopentenoyl enamines and imines, or (chiral) γ-lactams through two geminal C-C bond or two C-N bond formations, respectively. Several P-based key intermediates including a 1,4-(bis)electrophilic α,β-unsaturated ketenyl phosphonium species have been detected by ³¹ P NMR and HRMS analyses, which shed light on the postulated catalytic cycle. The synthetic utility of this new chemistry has been demonstrated through a gram-scaling up of the catalytic reaction as well as regioselective hydrogenation and double condensation to form cyclopentanoyl enamines and fused pyrazole building blocks, respectively.

Enantioselective NHC-Catalyzed [3+3] Annulation of α-Bromoenals with 2-Aminobenzimidazoles

A chiral carbene-catalyzed [3+3] annulation of α-bromoenals with 2-aminobenzimidazoles providing pyrimido[1,2-a]benzimidazoles has been described. This protocol features a broad scope and good functional group tolerance. Biological studies indicated that the formed pyrimido[1,2-a]benzimidazole exhibited moderate cytotoxic activity against tumor cells.

Isothiourea-catalysed enantioselective Michael addition of N-heterocyclic pronucleophiles to α,β-unsaturated aryl esters

The isothiourea-catalysed enantioselective Michael addition of 3-aryloxindole and 4-substituted-dihydropyrazol-3-one pronucleophiles to α,β-unsaturated p-nitrophenyl esters is reported. This process generates products containing two contiguous stereocentres, one quaternary, in good yields and excellent enantioselectivities (>30 examples, up to > 95 : 5 dr and 99 : 1 er). This protocol harnesses the multifunctional ability of p-nitrophenoxide to promote effective catalysis. In contrast to previous methodologies using tertiary amine Lewis bases, in which the pronucleophile was used as the solvent, this work allows bespoke pronucleophiles to be used in stoichiometric quantities.

The isothiourea-catalysed enantioselective Michael addition of 3-aryloxindole and 4-substituted-dihydropyrazol-3-one pronucleophiles to α,β-unsaturated p-nitrophenyl esters is reported.

N-Heterocyclic Carbenes as Key Intermediates in the Synthesis of Fused, Mesoionic, Tricyclic Heterocycles

Coupling between 5-bromoimidazo[1,5-a]pyridinium salts and malonate or arylacetate esters leads to a facile and straightforward access to the new mesoionic, fused, tricyclic system of imidazo[2,1,5-cd]indolizinium-3-olate. Mechanistic studies show that the reaction pathway consists of nucleophilic aromatic substitution on the cationic, bicyclic heterocycle by an enolate-type moiety and in the nucleophilic attack of a transient free N-heterocyclic carbene (NHC) species on the ester group; the relative order of these two steps depends on the nature of the starting ester. This work highlights the valuable implementation of free NHC species as key intermediates in synthetic chemistry, beyond their classical use as stabilizing ligands or organocatalysts.

Highly Enantioselective Synthesis of Functionalized Glutarimide Using Oxidative N-Heterocyclic Carbene Catalysis: A Formal Synthesis of (-)-Paroxetine

A simple yet highly effective approach toward enantioselective synthesis of trans-3,4-disubstituted glutarimides from readily available starting materials is developed using oxidative N-heterocyclic carbene catalysis. The catalytic reaction involves a formal [3 + 3] annulation between enals and substituted malonamides enabling the production of glutarimide derivatives in a single chemical operation via concomitant formation of C-C and C-N bonds. The reaction offers easy access to a broad range of functionalized glutarimides with excellent enantioselectivity and good yield. Synthetic application of the method is demonstrated via formal synthesis of (-)-paroxetine and other bioactive molecules.