Squaramide-catalyzed enantioselective Michael addition of nitromethane to 2-enoylazaarenes: synthesis of chiral azaarene-containing γ-nitroketones

Bifunctional chiral squaramide-catalyzed highly enantioselective Michael addition of nitromethane to diverse 2-enoylazaarenes was successfully performed. This protocol provided a set of chiral azaarene-containing γ-nitroketones with up to 98% yield and 98% ee in a solvent-free catalytic system under mild conditions. Furthermore, gram-scale synthetic utility was also showcased.

Catalysis of a Diels-Alder Reaction between Azachalcones and Cyclopentadiene by a Recyclable Copper(II)-PEIP Metal-Organic Framework

Metal-organic frameworks (MOFs) have attracted considerable interest as emerging heterogeneous catalysts for organic transformations of synthetic utility. Herein, a Lewis-acidic MOF, {[Cu3(PEIP)2(5-NH2-mBDC)(DMF)]·7DMF}∞, denoted as Cu(ΙΙ)-PEIP, has been synthesized via a one-pot process and deployed as an efficient heterogeneous catalyst for a Diels-Alder cycloaddition. Specifically, the [4 + 2] cycloaddition of 13 substituted azachalcone dienophiles with cyclopentadiene has been investigated. MOF-catalyzed reaction conditions were optimized, leading to the selection of water as the solvent, in the presence of 10% mol sodium dodecyl sulfate (SDS) to address substrate solubility. The Cu(II)-PEIP catalyst showed excellent activity under these green and mild conditions, exhibiting comparable or, in some cases, superior efficiency to a homogeneous catalyst often employed in Diels-Alder reactions, namely, Cu(OTf)2. The nature of the azachalcone substituent played a significant role in the reactivity of the dienophiles, with electron-withdrawing (EW) substituents enhancing conversion and electron-donating (ED) ones exhibiting the opposite effect. Coordinating substituents appeared to enhance the endo selectivity. Importantly, the Cu(II)-PEIP catalyst can be readily isolated from the reaction mixture and recycled up to four times without any significant reduction in conversion or selectivity.

NHC Catalysis for Umpolung Pyridinium Alkylation via Deoxy-Breslow Intermediates

Umpolung N-heterocyclic carbene (NHC) catalysis of non-aldehyde substrates offers new pathways for C-C bond formation, but has proven challenging to develop in terms of viable substrate classes. Here, we demonstrate that pyridinium ions can undergo NHC addition and subsequent intramolecular C-C bond formation through a deoxy-Breslow intermediate. The alkylation demonstrates, for the first time, that deoxy-Breslow intermediates are viable for catalytic umpolung of areniums.

NHC Catalysis for Umpolung Pyridinium Alkylation via Deoxy-Breslow Intermediates

Umpolung N-heterocyclic carbene (NHC) catalysis of non-aldehyde substrates offers new pathways for C-C bond formation, but has proven challenging to develop in terms of viable substrate classes. Here, we demonstrate that pyridinium ions can undergo NHC addition and subsequent intramolecular C-C bond formation through a deoxy-Breslow intermediate. The alkylation demonstrates, for the first time, that deoxy-Breslow intermediates are viable for catalytic umpolung of areniums.

An enantioselective assembly of naphthopyran via NHC-catalyzed [3 + 3] annulation of bromoenal with β-tetralone

An asymmetric assembly of naphthopyran was realized via the N-heterocyclic carbene (NHC)-catalyzed formal [3 + 3] annulation of bromoenal and β-tetralone. The key advantages of this protocol include ready availability of starting materials, mild reaction conditions, good yields.

An efficient synthesis of triazolium ion based NHC precursors using diaryliodonium salts and their photophysical properties

Copper-catalysed N-arylation of fused triazoles using diaryliodonium salts as an aryl source is described. This scalable protocol displayed good compatibility towards diverse sensitive functional groups like ester, alkyl and nitro groups and halogens (F, Cl, Br). The synthetic usefulness of the prepared triazolium salts was proved by preparing α-hydroxyketone through benzoin condensation. Photophysical studies of these compounds showed promising Stokes-shifted fluorescence emission in aqueous medium, so this molecular framework could be a proficient probe for biological applications.

Phosphine-mediated enantioselective [1 + 4]-annulation of Morita–Baylis–Hillman carbonates with 2-enoylpyridines

This work describes a phosphine-catalyzed asymmetric [1 + 4] annulation of Morita–Baylis–Hillman carbonates with 2-enoylpyridines for constructing the enantiomerically enriched 2,3-dihydrofuran motif. In the presence of (−)-1,2-bis[(2R,5R)-2,5-dimethylphospholano]benzene, a series of Morita–Baylis–Hillman carbonates reacted with 2-enoylpyridines smoothly to afford a wide range of optically active 2,3-dihydrofurans featuring pyridine motifs in high yields with excellent asymmetric induction.

This work describes a phosphine-catalyzed asymmetric [1 + 4] annulation of Morita–Baylis–Hillman carbonates with 2-enoylpyridines for constructing the enantiomerically enriched 2,3-dihydrofuran motif.

Enantioselective Synthesis of Functionalized β-Lactones by NHC-Catalyzed Aldol Lactonization of Ketoacids

N-Heterocyclic carbene (NHC)-catalyzed intramolecular aldol lactonization of readily available ketoacids leading to the enantioselective synthesis of cyclopentane-fused β-lactones is presented. The reaction proceeds via the generation of NHC-bound enolate intermediates formed from the ketoacids in the presence of the peptide coupling reagent HATU and NHC generated from the chiral triazolium salt. The functionalized β-lactones are formed under mild conditions in high yields and enantioselectivities.

Construction of Fused Pyrrolidines and β-Lactones by Carbene-Catalyzed C-N, C-C, and C-O Bond Formations

A carbene-catalyzed intermolecular C-N bond formation, which initiates a highly selective cascade reaction for the synthesis of pyrrolidine fused β-lactones, is disclosed. The nitrogen-containing bicyclic β-lactone products are obtained with good yields and excellent stereoselectivities. Synthetic transformations of the reaction products into useful functional molecules, such as amino catalysts, can be efficiently realized under mild reaction conditions. Mechanistically, this study provides insights into modulating the reactivities of heteroatoms, such as nitrogen atoms, in challenging carbene-catalyzed asymmetric carbon-heteroatom bond-forming reactions.

Asymmetric synthesis of cyclopentanes bearing four contiguous stereocenters via an NHC-catalyzed Michael/Michael/esterification domino reaction

An NHC-catalyzed Michael/Michael/esterification domino reaction via homoenolate/enolate intermediates for the asymmetric synthesis of tetrasubstituted cyclopentanes bearing four contiguous stereocenters is described. A variety of α,β-unsaturated aldehydes and 2-nitroallylic acetates react well with good domino yields and high stereoselectivities.

Diastereoselective synthesis of cyclopentanone-fused spirooxindoles by N-heterocyclic carbene-catalyzed homoenolate annulation with isatilidenes

NHC-catalyzed reaction of enals with N-substituted isatilidenes resulting in the diastereoselective synthesis of cyclopentanone-fused spirooxindoles proceeding via the homoenolate equivalent intermediates is reported.