N-Heterocyclic Carbene-Catalyzed Oxidative Annulations of α,β-Unsaturated Aldehydes with Hydrazones: Selective Synthesis of Optically Active 4,5-Dihydropyridazin-3-ones and Pyridazin-3-ones

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.

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.

N-Heterocyclic Carbene-Promoted [4+2] Annulation of α-Chloro Hydrazones with α-Chloro Aliphatic Aldehydes to Access Enantioenriched Dihydropyridazinones

An expeditious protocol for the assembly of chiral 4,5-dihydropyridazin-3(2H)-ones from α-chloro hydrazones and α-chloro aliphatic aldehydes via N-heterocyclic carbene (NHC) catalysis is outlined. These in situ-generated 1,2-diaza-1,3-dienes undergo asymmetric [4+2] annulation with NHC-bound enolates to afford the desired products bearing a stereogenic center at the C4 position. The notable features of this approach include good to excellent enantioselectivities, high functional group tolerance, mild reaction conditions, simple operating procedures, and compatibility with gram-scale synthesis.

Synthesis of multi-substituted 1,2,4-triazoles utilising the ambiphilic reactivity of hydrazones

The synthesis of N-alkyl-1H-1,2,4-triazoles from N,N-dialkylhydrazones and nitriles via formal [3+2] cycloaddition including the C-chlorination/nucleophilic addition/cyclisation/dealkylation sequence was developed. This sequential reaction utilising the in situ generation of hydrazonoyl chloride based on the ambiphilic reactivity of hydrazones afforded a variety of multi-substituted N-alkyl-triazoles in high yields. The synthetic utility of multi-substituted triazoles was also demonstrated by further transformations.

Multicomponent Enantioselective Synthesis of Tetrahydropyridazinones Employing Chiral α,β-Unsaturated Acylammonium Salts

An enantioselective three-component reaction was developed for the synthesis of tetrahydropyridazinones employing chiral α,β-unsaturated acylammonium salts, malonates, and azodicarboxylates. An initial α-amination of a malonate with an azodicarboxylate and a subsequent chiral Lewis-base-catalyzed Michael/proton transfer/lactamization process delivered optically active tetrahydropyridazinones (up to 99:1 er). Subsequent transformations of these adducts were explored, revealing some unexpected rearrangements, and the use of an allyl methyl malonate enabled a subsequent deallylative decarboxylation and the introduction of a second stereocenter.

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.

Carbene-catalyzed enantioselective annulation of dinucleophilic hydrazones and bromoenals for access to aryl-dihydropyridazinones and related drugs

4,5-Dihydropyridazinones bearing an aryl substituent at the C6-position are important motifs in drug molecules. Herein, we developed an efficient protocol to access aryl-dihydropyridazinone molecules via carbene-catalyzed asymmetric annulation between dinucleophilic arylidene hydrazones and bromoenals. Key steps in this reaction include polarity-inversion of aryl aldehyde-derived hydrazones followed by chemo-selective reaction with enal-derived α,β-unsaturated acyl azolium intermediates. The aryl-dihydropyridazinone products accessed by our protocol can be readily transformed into drugs and bioactive molecules.

Polarity inversion of arylidene hydrazones to react with bromoenals via carbene organic catalysis is disclosed. The reaction enantioselectively affords 6-aryl-4,5-dihydropyridazinones and related drugs with proven commercial applications.

Oxidative N-heterocyclic carbene-catalyzed [3 + 3] annulation reaction of enals with benzofuran-3-ones: efficient access to benzofuran-fused δ-lactones

A facile route to benzofuran-fused δ-lactones was developed via an N-heterocyclic carbene-catalyzed [3 + 3] annulation reaction, giving the expected products in high yields (up to 99%) with excellent enantioselectivities (up to 98% ee).

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.

Enantioselective Alkylation of N-Arylhydrazones Derived from α-Keto Esters and Isatin Derivatives through Asymmetric Phase-Transfer Catalysis

The phase-transfer-catalyzed asymmetric alkylation reactions of N-arylhydrazones derived from α-keto-esters and isatin derivatives afford enantioenriched azo compounds that bear a tetra-substituted carbon stereocenter in good yields with high chemo- and enantioselectivity. The alkylation products can be readily converted into chiral amino esters, hydrazine derivatives, and aza-β-lactams without loss of enantiopurity.

NHC-catalyzed regiodivergent syntheses of difunctionalized 3-pyrazolidinones from α-bromoenal and monosubstituted hydrazine

A formal [3 + 2] annulation of α-bromoenal with monosubstituted hydrazine could give 1,5 or 2,5-difunctionalized 3-pyrazolidinone regiodivergently by tuning the structure of the N-Heterocyclic Carbene (NHC) catalyst. Moderate to high yields, mild reaction conditions, good regioselectivity and potential biological significance of the final product have made this protocol attractive for the assembly of 3-pyrazolidinone.

Formal [3 + 4] Annulation of α,β-Unsaturated Acyl Azoliums: Access to Enantioenriched N-H-Free 1,5-Benzothiazepines

An unprecedented formal [3 + 4] annulation of α,β-unsaturated acyl azoliums with 2-aminobenzenethiols has been utilized to synthesize enantioenriched N-H-free 1,5-benzothiazepines, which are recognized as privileged structures in numerous biologically active scaffolds. This protocol offers a rapid and direct pathway to access the target compounds with high enantioselectivities and has been applied in the concise synthesis of chiral drug (R)-thiazesim.

N-Heterocyclic Carbene/Lewis Acid Dual Catalysis for the Divergent Construction of Enantiopure Bridged Lactones and Fused Indenes

The chiral triazole carbene and Ti(OPr-i)₄ cocatalyzed reaction between α,β-unsaturated aldehydes and 2-(aroylvinyl)benzaldehydes was systematically studied. A divergence in reaction pathways was observed under different reaction conditions. In benzene solvent and at ambient temperature, the reaction produced 4,5-dihydro-1,4-methanobenzo[c]oxepin-3-ones, the bridged caprolactones, as the major products in moderate yields with excellent enantioselectivity. The same reaction in dichloroethane and at 50 °C, however, gave 2,8-dihydrocyclopenta[a]indenes as the major products in most cases. The application of the method developed was demonstrated by the transformation of the bridged lactone products into enantiopure 4-hydroxy-1,2,3,4-tetrahydronaphthalene-2-carboxylic acids.

N-Heterocyclic carbene-catalyzed [3 + 3] cyclocondensation of bromoenals with hydrazones: highly enantioselective synthesis of dihydropyridazones

The N-heterocyclic carbene-catalyzed [3 + 3] cyclocondensation of bromoenals and hydrazones is developed to give the corresponding chiral 4,5-dihydropyridazones in good yields with excellent enantioselectivities.

Chiral N-heterocyclic carbene/Lewis acid cooperative catalysis of the reaction of 2-aroylvinylcinnamaldehydes: a switch of the reaction pathway by Lewis acid activation

Chiral N-heterocyclic carbene/Lewis acid co-catalyzed reaction of 2-aroylvinylcinnamaldehydes produced good yields of indeno[1,2-c]furan-1-ones with excellent enantioselectivity. A switch of intramolecular to intermolecular reaction was achieved by the cooperative catalysis of NHC/Ti(OPr-i)₄ catalysts.

Formal [3 + 3] annulation of isatin-derived 2-bromoenals with 1,3-dicarbonyl compounds enabled by Lewis acid/N-heterocyclic carbene cooperative catalysis

Several novel isatin-derived 2-bromoenals were applied for formal [3 + 3] annulation with 1,3-dicarbonyl compounds enabled by NHC/Lewis acid cooperative catalysis.