Enantioselective electrophilic activation of aldehydes, esters and imines via N-heterocyclic carbene catalysis

We summarize in this review the recent key progress in the NHC-catalyzed electrophilic activation of carbonyl compounds for the regioselective functionalization of saccharides, enantioselective construction of heterocycles, and asymmetric synthesis of axially and planar chiral molecules.

Divergent Synthesis of Axially Chiral 2-Pyranones and Fused 2-Pyridones via N-Heterocyclic Carbene-Catalyzed Atroposelective [3 + 3] Annulation

An N-heterocyclic carbene-catalyzed atroposelective [3 + 3] annulation of alkynyl acylazoliums with benzothiazole derivatives has been developed for the divergent synthesis of axially chiral triaryl 2-pyranones and fused 2-pyridones. The regioselectivity of this protocol depends on the structure of benzothiazoles with three different nucleophilic centers. The obtained axially chiral frameworks represent a new class of arylheterocycle atropisomers, which may be potentially useful in medicinal chemistry.

N-Heterocyclic Carbene Catalyzed Reactions Involving Acetylenic Breslow and/or Acylazolium as Key Intermediates

N-heterocyclic carbene (NHC) organocatalysis has been developed as a powerful tool in modern synthetic chemistry. NHC catalytic activation of ynals and alkynoic acid derivatives provided versatile reactions that involve acetylenic Breslow and/or acylazolium as key intermediates, and diverse transformations have been established for access to molecules with unique skeletons in efficient fashions. Herein we summarize the recent achievements in NHC-catalyzed reactions involving acetylenic Breslow and/or acylazolium intermediates. Different reactions belonging to three catalytic modes, including (1) conjugate additions to acetylenic Breslow derived α,β-unsaturated acylazolium intermediates, (2) β-umpolung of ynals via acetylenic Breslow intermediates, and (3) conjugate additions to acetylenic acylazolium intermediates, are emphasized with examples and plausible mechanisms cited to guide a better understanding.

Chemodivergent Parallel Kinetic Resolution of Paracyclophanes: Enantiomer Fishing with Different Substrates

An unprecedented chemodivergent strategy for parallel kinetic resolution (PKR) is disclosed through which two planar chiral products bearing different structures were simultaneously afforded with opposite stereoselectivities. Two achiral esters are activated by one single chiral N-heterocyclic carbene (NHC) catalyst to react with the different enantiomers of the racemic imine substrate in a parallel fashion. Two products bearing distinct structures and opposite stereoselectivities are respectively afforded from the same reaction system in good to excellent yields, enantio- and diastereoselectivities. Control experiments and kinetic studies are carried out to probe the kinetic and dynamic properties during the reaction progress. The planar chiral pyridine and lactam products show interesting applications in both asymmetric synthesis and pesticide development.

Catalytic Synthesis of 1H-Benzoxazolo[3,2-a]pyridin-1-ones via Formal [3 + 3] Annulations of NHC-Generated Alkynyl Acylazoliums with Benzoxazolyl Acetates and Their Photophysical Studies

We disclose the synthesis of a tricyclic fused N-heterocycle via the NHC-catalyzed annulation of either a ynal or an alkynyl ester with readily accessible benzoxazolyl acetate. While the annulation with ynals requires an oxidant, the reaction with alkynyl esters proceeds via the direct generation of alkynyl acylazolium intermediates with an NHC. With the dearth of catalytic processes to access these 1H-benzoxazolo[3,2-a]pyridin-1-ones from simple starting materials, this method is especially important. The photophysical properties of the products have also been evaluated.

Organocatalytic diastereo- and atroposelective construction of N-N axially chiral pyrroles and indoles

The construction of N-N axially chiral motifs is an important research topic, owing to their wide occurrence in natural products, pharmaceuticals and chiral ligands. One efficient method is the atroposelective dihydropyrimidin-4-one formation. We present herein a direct catalytic synthesis of N-N atropisomers with simultaneous creation of contiguous axial and central chirality by oxidative NHC (N-heterocyclic carbenes) catalyzed (3 + 3) cycloaddition. Using our method, we are able to synthesize structurally diverse N-N axially chiral pyrroles and indoles with vicinal central chirality or bearing a 2,3-dihydropyrimidin-4-one moiety in moderate to good yields and excellent enantioselectivities. Further synthetic transformations of the obtained axially chiral pyrroles and indoles derivative products are demonstrated. The reaction mechanism and the origin of enantioselectivity are understood through DFT calculations.

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.

Construction of Functionalized 2H-Pyran-2-ones via N-Heterocyclic Carbene-Catalyzed [3 + 3] Annulation of Alkynyl Esters with Enolizable Ketones

A new synthesis of functionalized 2H-pyran-2-ones has been developed through N-heterocyclic carbene-catalyzed formal [3 + 3] annulation of alkynyl esters with enolizable ketones. The key to the success of this protocol relies on the use of an NHC instead of a tertiary amine as the catalyst. This protocol also features a broad substrate scope and mild metal-free conditions, offering simple and rapid access to the target molecules in a highly regioselective manner.

Atroposelective Synthesis of Triaryl α-Pyranones with 1,2-Diaxes by N-Heterocyclic Carbene Organocatalysis

Atropisomers bearing multiple stereogenic axes are of increasing importance to the field of material science, pharmaceuticals, and catalysis. However, the atroposelective construction of multi-axis atropisomers remains rare and challenging, due to the intrinsical difficulties in the stereo-control of the multiple stereogenic axes. Herein, we demonstrate a single-step construction of a new class of 1,2-diaxially chiral triaryl α-pyranones by an N-heterocyclic carbene organocatalytic asymmetric [3+3] annulation of well-designed alkynyl acylazolium precursors and enolizable sterically hindered 2-aryl ketones. The protocol features broad substrate scope (>50 examples), excellent stereo-control (most cases >20 : 1 dr, up to 99.5 : 0.5 er), and potentially useful synthetic applications. The success of this reaction relies on the rational design of structurally matched reaction partners and the careful selection of the asymmetric catalytic system. DFT calculations have also been performed to discover and rationalize the origin of the high stereoselectivity of this reaction.

A Review on Generation and Reactivity of the N-Heterocyclic Carbene-Bound Alkynyl Acyl Azolium Intermediates

N-heterocyclic carbene (NHC) has been widely used as an organocatalyst for both umpolung and non-umpolung chemistry. Previous works mainly focus on species including Breslow intermediate, azolium enolate intermediate, homoenolate intermediate, alkenyl acyl azolium intermediate, etc. Notably, the NHC-bound alkynyl acyl azolium has emerged as an effective intermediate to access functionalized cyclic molecular skeleton until very recently. In this review, we summarized the generation and reactivity of the NHC-bound alkynyl acyl azolium intermediates, which covers the efforts and advances in the synthesis of achiral and axially chiral cyclic scaffolds via the NHC-bound alkynyl acyl azolium intermediates. In particular, the mechanism related to this intermediate is discussed in detail.

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 Formal [3+3] Annulation of Alkynyl Acylazoliums for the Synthesis of Benzofuro[3,2-b]pyridin-2-ones

Through β-activation of alkynoic acid esters with N-heterocyclic carbene catalysis, a formal [3+3] annulation of alkynyl acylazoliums with indolin-3-ones has been developed for the rapid construction of structurally interesting benzofuro[3,2-b]pyridin-2-ones with potential bioactivities. This protocol provides a highly efficient and simple method for the synthesis of the target molecules under mild reaction conditions with a wide substrate scope and excellent chemoselectivity. The synthetic utility of this protocol was also demonstrated by the versatile late-stage modifications.

Unexpected Substituent Effects in Spiro-Compound Formation: Steering N-Aryl Propynamides and DMSO toward Site-Specific Sulfination in Quinolin-2-ones or Spiro[4,5]trienones

A highly substituent-dependent rearrangement allows for the novel and SOCl₂-induced divergent synthesis of 3-methylthioquinolin-2-ones and 3-methylthiospiro[4.5]trienones through intramolecular electrophilic cyclization of N-aryl propyamides. DMSO acts as both solvent and sulfur source, and use of DMSO-h₆/d₆ enables the incorporation of SCH₃ or SCD₃ moieties to the 3-position of the heterocyclic framework. Different para-substituents trigger divergent reaction pathways leading to the formation of quinolin-2-ones for mild substituents and spiro[4,5]trienones for both electron-withdrawing and -donating substituents, respectively. On the basis of both computational and experimental results, a new mechanism has been put forward that accounts for the exclusive spirolization/defluorination process and the surprising substituent effects.

Atroposelective Synthesis of Axially Chiral 4-Aryl α-Carbolines via N-Heterocyclic Carbene Catalysis

The first catalytic asymmetric construction of axially chiral 4-aryl α-carboline skeletons has been accomplished through an N-heterocyclic carbene (NHC)-catalyzed atroposelective formal [3 + 3] annulation of 4-nitrophenyl 3-arylpropiolates with 2-sulfonamidoindolines. The synthetic utility of the title compounds has been demonstrated by the diverse late-stage structural modifications. Density functional theory calculations were also conducted to illuminate the key factors for controlling the origin of the enantioselectivity. This strategy not only provides an efficient pathway to access axially chiral α-carboline atropisomers but also offers a novel catalytic enantioselective mode for the construction of axially chiral heterobiaryls by using NHC-bound alkynyl acylazoliums.

Carbene-Catalyzed Atroposelective Annulation and Desymmetrization of Urazoles

An NHC-catalyzed atroposelective reaction between ynals and urazoles is disclosed. The reaction establishes a chiral C-N axis via an atroposelective [3 + 2] annulation/desymmetrization process. Our reaction allows efficient access to axially chiral and heteroatom-rich urazole derivatives with potential applications in bioactive molecules and catalysis.

Bifunctional Urea/Hg(OAc)2-Mediated Synthesis of 4-Aryl-6-oxycarbonyl-2-pyrones and 2-Pyridones from Dithiomalonate and β,γ-Unsaturated α-Keto Esters

Disubstituted 2-pyrones and 2-pyridones were obtained by bifunctional urea-catalyzed Michael addition/lactonization or lactamization followed by a Hg(OAc)₂- or Hg(OAc)₂/DBU-mediated hydrolysis/decarboxylation/dehydrogenation process. This one-pot two-stage protocol enabled the rapid synthesis of 4,6-disubstituted 2-pyrones and 2-pyridones from dithiomalonate and β,γ-unsaturated α-keto esters in practical yields under mild reaction conditions. Additionally, the obtained 2-pyridones were facilely transformed to 2,4,6-trisubstituted pyridines in excellent yields.

Recent Progress towards Organocatalyzed Asymmetric (Hetero)Arene Formation

Owing to the importance of arene moieties in organic chemistry, methods for arene construction attract great attention. Besides the traditional substitution strategy from pre-existing arenes, the straightforward formation of arene cores can also provide significant shortcuts towards a wide array of target molecules with different substitution patterns. Among direct arene formation reactions, applying environmentally benign organocatalysis to access arene moieties continues to attract increasing attention. This short review provides a brief summary of recent progress on organocatalyzed de novo (hetero)arene formation and applications in enantioselective synthesis.

1 Introduction

2 Arene Formation with Non-Covalent Organocatalysts

2.1 Arene Formation with Acid Organocatalysts

2.2 Arene Formation with Base Organocatalysts

2.3 Arene Formation with Hydrogen-Bonding Organocatalysts

3 Arene Formation with Covalent Organocatalysts

3.1 Arene Formation with Lewis Base Organocatalysts

3.1.1 Arene Formation with Secondary Amine Organocatalysts

3.1.2 Arene Formation with Tertiary Amine, Tertiary Phosphine and Sulfide Organocatalysts

3.1.3 Arene Formation with N-Heterocyclic Carbene Organocatalysts

3.2 Arene Formation with Lewis Acid Organocatalysts

4 Conclusion

NaOH-Promoted Chemoselective Cascade Cyclization of Cyclopropyl Esters with Unsaturated Imines: Access to Bioactive Cyclopenta[c]pyridine Derivatives

A chemoselective cascade cycloaddition reaction is developed for green and efficient access to cyclopenta[c]pyridine derivatives. Simple and inexpensive NaOH is used as the sole catalyst for this process. The δ-carbon of cyclopropyl ester is activated as a nucleophilic carbon to initiate highly chemoselective cascade reactions. Cyclopenta[c]pyridines bearing various substituents are afforded in excellent yields. Preliminary studies on the bioactivities of the afforded products show promising antibacterial activities for potential applications in plant protections.

Facile Synthesis of 2,2-Diacyl Spirocyclohexanones via an N-Heterocyclic Carbene-Catalyzed Formal [3C + 3C] Annulation

A novel strategy for the construction of 2,2-diacyl spirocyclohexanones 3 has been demonstrated on the basis of an NHC-catalyzed [3C + 3C] annulation of potassium 2-oxo-3-enoates with 2-ethylidene 1,3-indandiones. Furthermore, enantioenriched 3 was obtained in good to excellent yields with good enantioselectivities when chiral N-heterocyclic carbene (NHC) was employed. Notably, ring opening of the resulting 2,2-diacyl spirocyclohexanones 3 with hydrazine led to the formation of phthalazinones in good to excellent yields.

Lipophilic NHC assisted one-pot synthesis of syncarpamide analogues in aqueous medium

Lipophilic NHC catalysis in aqueous medium was reported for the synthesis of biologically relevant (a)symmetrically substituted and unsymmetrically substituted syncarpamide analogues.

N-Heterocyclic carbene-catalyzed formal [3 + 3] annulation of alkynoic acid esters with indolin-3-ones: access to functionalized pyrano[3,2-b]indol-2-ones

The structurally interesting pyrano[3,2-b]indol-2-ones were synthesized by an N-heterocyclic carbene-catalyzed formal [3 + 3] annulation of alkynoic acid esters.

N-Heterocyclic carbene-catalyzed annulation of ynals with amidines: access to 1,2,6-trisubstituted pyrimidin-4-ones

A thiazolium-catalyzed annulation of ynals and amidines has been reported to construct pyrimidin-4-ones.