Carbene-Catalyzed Enantioselective Addition of Thioamides to Bromoenals for Access to Thiazinone Heterocycles

Three-Component Domino Reaction of Thioamide, Isonitriles, and Water: Selective Synthesis of 1,2,4-Thiadiazolidin-3-ones and (E)-N-(1,2-Diamino-2-thioxoethylidene)benzamides

The three-component domino reaction of thioamides, benzyl isocyanide, and water in the presence of a catalytic amount of both Pd(dppf)Cl2 and Cu(OAc)2 afforded novel 1,2,4-thiadiazolidin-3-one cyclic compounds, whereas the same reaction with tertiary alkylisonitriles in the presence of rare earth metal salt [La(OTf)3] resulted in (E)-N-(1,2-diamino-2-thioxoethylidene)benzamide open-chain products. This divergent reaction enabled the one-pot construction of five (N-S, C-S, C-O, and two C-N) or four (C-S, C-N, C-O, and C-C) new chemical bonds. Mechanism studies indicate that the oxygen atom of the product was derived from H2O.

NHC-Catalyzed Enantioselective Transformations Involving α-Bromoenals

α-Haloenals, especially, α-bromoenals considered as one of the important building blocks in organic synthesis. They can participate in various (3+2)-, (3+3)-, (3+4)-, and (2+4)-annulation reactions with other organic molecules in the presence of an NHC catalyst to produce enantioenriched carbo-, and heterocyclic compounds. Herein, we have described NHC-catalyzed enantioselective transformations of α-bromoenals in the synthesis of various heterocycles, and carbocycles, as well as acyclic organic compounds.

Direct Transamidation of Thioamides with Amines via Acetophenone-Promoted Enamine Catalysis under Metal-Free Conditions

Herein, we developed a highly selective, efficient, and simple method for direct transamidation of thioamides with amines, promoted by commercially available acetophenone under metal-/solvent-free conditions. The reaction tolerated a wide range of functional groups and substrates, including single- or double-thioamides, benzylamines, or alkyl/cycloalkyl-substituted aliphatic amines. The present protocol can be applied to gram-scale in good yields. In addition, the Pt-/Ni-complexes of double-transamidation products were obtained in good to excellent yields. The investigation of photophysical properties indicated that the fluorescence spectra of Pt-complexes showed an emission band centered at 550-750 nm and exhibited red fluorescence when irradiated by a UV lamp (365 nm).

N-Heterocyclic Carbene-Catalyzed Aza-Michael-Mannich-Lactamization Cascade for the Enantioselective Synthesis of Pyrazoloquinolin-3-ones

The enantioselective synthesis of functionalized pyrazoloquinolin-3-ones via N-heterocyclic carbene-catalyzed cascade reaction of α-bromoenals with 2-aminoaryl N-tosyl hydrazones is reported. The in situ-generated α,β-unsaturated acylazoliums underwent an aza-Michael-Mannich-lactamization sequence to afford the tricyclic products bearing three contiguous stereocenters, including a sterically demanding quaternary stereocenter with high enantioselectivity. The unprotected amine-triggered aza-Michael pathway over the competing amidation pathway is noteworthy.

Base-promoted highly efficient synthesis of nitrile-substituted cyclopropanes via Michael-initiated ring closure

A convenient and efficient annulation reaction has been developed for the general synthesis of dinitrile-substituted cyclopropanes in moderate to excellent yields. A variety of 2-arylacetonitriles and α-bromoennitriles were compatible under the standard conditions. The reaction was achieved through tandem Michael-type addition followed by intramolecular cyclization. The preliminary application of this method was confirmed by the synthesis of the 2,4-dioxo-3-azabicyclo[3.1.0]hexane scaffold.

Carbene-Catalyzed Activation of Formyl-phenylacetic Esters for Access to Chiral Dihydroisoquinolinones

A carbene-catalyzed reaction to synthesize chiral dihydroisoquinolinones via an o-quinodimethane (o-QDM) intermediate is disclosed. o-QDM reacts with cyclic sulfonic imines via annulation to afford highly enantioenriched dihydroisoquinolinone products. ESI-HRMS studies suggest a stepwise Mannich addition and acylation reaction pathway, and the pathways of the catalytic and uncatalyzed background reactions are evaluated via DFT calculations.

Carbene-Catalyzed Enantioselective Hydrophosphination of α-Bromoenals to Prepare Phosphine-Containing Chiral Molecules

Disclosed herein is the first carbene-organocatalyzed asymmetric addition of phosphine nucleophiles to the in situ generated α,β-unsaturated acyl azolium intermediates. Our reaction enantioselectively constructs carbon-phosphine bonds and prepares chiral phosphines with high optical purities. The phosphine products are suitable for transforming to chiral ligands or catalysts with applications in asymmetric catalysis. The diarylalkyl or trialkyl phosphine products from our catalytic reactions, air-sensitive and reactive in nature, can be trapped (and stored) in their sulfur-oxidized form for operational simplicities.

Highly Enantioselective Synthesis of Pyrroloindolones and Pyrroloquinolinones via an N-Heterocyclic Carbene-Catalyzed Cascade Reaction

In this work, the NHC-catalyzed Michael/Mannich/lactamization cascade reaction of enals with either indole-2-carboxaldehyde-derived aldimines or indole-7-carboxaldehyde-derived aldimines is described. This protocol enables the rapid assembly of optically active pyrroloindolones and pyrroloquinolinones derivatives under mild conditions with high yields, excellent enantioselectivities, and a broad substrate scope.

NHC-catalyzed covalent activation of heteroatoms for enantioselective reactions

Covalent activation of heteroatoms enabled by N-heterocyclic carbene (NHC) organic catalysts for enantioselective reactions is evaluated and summarized in this review. To date, sulfur, oxygen, and nitrogen atoms can be activated in this manner to react with another substrate to construct chiral carbon-heteroatom bonds with high optical enantioselectivities. The activation starts with addition of an NHC catalyst to the carbonyl moiety (aldehyde or imine) of substrates that contain heteroatoms. The key in this approach is the formation of intermediates covalently bound to the NHC catalyst, in which the heteroatom of the substrate is activated as a nucleophilic reactive site.

Carbene-catalyzed selective addition of isothioureas to enals for access to sulphur-containing 5,6-dihyropyrimidin-4-ones

A carbene-catalyzed highly regioselective and enantioselective 1,4-addition reaction between isothioureas and enals has been achieved under oxidative conditions.

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.

Two Approaches for the Synthesis of Fused Dihydropyridines via a 1,6-Electrocyclic Reaction: Fluorescent Properties and Prospects for Application

Reactions of penta-2,4-dienethioamides with acetylenedicarboxylic acid, methyl and ethyl esters, and methyl propiolate were systematically studied, and a number of new 2,3-dihydro-5H-thiazolo[3,2-a]pyridines (DTPs) and 4H,6H-pyrido[2,1-b][1,3]thiazines (PTZs) were prepared. A possible mechanism for a multistep domino transformation is suggested, and the key step is the 1,6-electrocyclic reaction. An additional alternative method for the synthesis of new heterocyclic systems was achieved. Evidence of the electrocyclic mechanism of a key step was collected from the analysis of the spatial structure of the synthesized bicyclic nonaromatic pyridines by X-ray diffraction and quantum chemical calculations, as well as from the thermodynamic quantities. DTPs exhibited yellow fluorescence in solution and yellow to red emissions in the solid state. Biological investigations demonstrated the ability of DTPs to penetrate living and fixed cells and presumably accumulate in lysosomes.

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.

Access to dihydropyrano[3,2-b]pyrrol-5-ones skeletons by N-heterocyclic carbene-catalyzed [3+3] annulations

A novel and efficient NHC-catalyzed [3+3] annulation of enals with pyrrol-4-ones was developed, thus providing the dihydropyrano[3,2-b]pyrrol-5-one core structures with broad scope and good to excellent enantioselectivities. Notably, this strategy could also expand to the synthesis of axially chiral compounds and polysubstituted indoles.

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.