Rhodium-Catalyzed Coupling–Cyclization of Alkenyldiazoacetates with o-Alkenyl Arylisocyanides: A General Route to Carbazoles

Synthesis of Functionalized 4-Hydroxy Carbazoles and Carbazole Alkaloids via Ring Expansion of Indole Cyclopentanone

The exploration of a ring expansion reaction from indole cyclopentanone to generate a range of diversely functionalized 4-hydroxyl carbazole frameworks, representing the core structure of numerous carbazole alkaloids, has been conducted under mild reaction conditions. This approach exhibits broad functional group tolerance and moderate to good yields. The practical applicability of this strategy has been demonstrated through the concise syntheses of carbazomycins A, D, and G.

DBU-Promoted Tandem Cyclization of Ynones and Diazo Compounds: Direct Synthesis of Eight-Membered Cyclic Ethers

A novel DBU-promoted tandem cyclization reaction of ynones with diazo compounds as the N-terminal electrophiles has been developed. The reaction provides a simple and efficient method for the synthesis of fused eight-membered oxocino[2,3-c] pyrazoles from readily available acyclic starting materials in a single step. This reaction allows the formation of four new bonds and two rings in a highly regio- and diastereoselective manner, where two adjacent stereocenters are created simultaneously in an atom-economic manner.

Iron-Catalysed Carbene Transfer to Isocyanides as a Platform for Heterocycle Synthesis

An iron-catalysed carbene transfer reaction of diazo compounds to isocyanides has been developed. The resulting ketenimines are trapped in situ with various bisnucleophiles to access a range of densely functionalized heterocycles (pyrimidinones, dihydropyrazolones, 1H-tetrazoles) in a one-pot process. The electron-rich Hieber anion ([Fe(CO)₃ NO]^- ) facilitates efficient catalytic carbene transfer from acceptor-type α-diazo carbonyl compounds to isocyanides, providing a cost-efficient and benign alternative to similar noble metal-catalysed processes. Based on DFT calculations a plausible reaction mechanism for activation of the α-diazo carbonyl carbene precursor and ketenimine formation is provided.

Lewis Acid Mediated Conjugate Addition of Isocyanides to β-Hydroxy-α-diazo Carbonyls: Synthesis of β-Carboxamido-α-diazo Carbonyl Compounds

A Lewis acid mediated conjugate addition of isocyanides to β-hydroxy-α-diazo carbonyls has been developed for the first time. The reaction realizes the efficient construction of quaternary carbon centers and provides a novel and efficient strategy for the synthesis of β-carboxamido-α-diazo carbonyls that would be otherwise difficult to form in a single step. Further applications, including synthesis of methylenecyclohexane, spiro-β-lactam, and nitrogen-bridged tricyclic β-lactam, demonstrated the tremendous potential of the coupling reaction.

Transition metal-catalysed carbene- and nitrene transfer to carbon monoxide and isocyanides

Transition metal-catalysed carbene- and nitrene transfer to the C1-building blocks carbon monoxide and isocyanides provides heteroallenes (i.e. ketenes, isocyanates, ketenimines and carbodiimides). These are versatile and reactive compounds allowing in situ transformation towards numerous functional groups and organic compounds, including heterocycles. Both one-pot and tandem processes have been developed providing valuable synthetic methods for the organic chemistry toolbox. This review discusses all known transition metal-catalysed carbene- and nitrene transfer reactions towards carbon monoxide and isocyanides and in situ transformation of the heteroallenes hereby obtained, with a special focus on the general mechanistic considerations.

Rhodium-catalyzed coupling-cyclization reaction of isocyanides and 2-azidophenyloxyacrylates: synthesis of N-(3-substituted benzo[d]oxazol-2(3H)-ylidene)amines and dihydrobenzo[d]oxazoles

A rhodium-catalyzed coupling cyclization of isocyanides with 2-azidophenyloxyacrylates has been developed. The reaction provides a new method for the synthesis of N-(3-substituted benzo[d]oxazol-2(3H)-ylidene)amines and dihydrobenzo[d]oxazoles.

Rhodium-catalyzed coupling-cyclization of o-alkynyl/propargyl arylazides or o-azidoaryl acetylenic ketones with arylisocyanides: synthesis of 6H-indolo[2,3-b]quinolines, dibenzonaphthyridones and dihydrodibenzo[b,g] [1,8]-naphthyridines

The developed rhodium-catalyzed coupling-cyclization provides a new strategy for the assembly of 6H-indolo[2,3-b]quinolines, dibenzonaphthyridones and dihydrodibenzo[b,g] [1,8]-naphthyridines.

Mn(III)-Mediated Radical Cyclization of o-Alkenyl Aromatic Isocyanides with Boronic Acids: Access to N-Unprotected 2-Aryl-3-cyanoindoles

The synthesis of N-unprotected 2-aryl-3-cyanoindoles was realized via the Mn(III)-mediated radical cascade cyclization of o-alkenyl aromatic isocyanides with boronic acids. A possible mechanism involving a sequential intermolecular radical addition, intramolecular cyclization, and cleavage of the C-C bond under mild reaction conditions is proposed. Mechanism studies show that H₂O or O₂ might provide the oxygen source for the elimination of benzaldehyde.

Substrate-Controlled Regiodivergent Synthesis of Fluoroacylated Carbazoles via Friedel-Crafts Acylation

A general, efficient, and substrate-controlled regiodivergent trifluoroacetylation of carbazoles has been developed through Friedel-Crafts acylation. This strategy was applicable to a wide scope of readily available substituted carbazoles at air atmosphere without using a metal catalyst, affording the corresponding trifluoroacetylated carbazoles in up to 99% yield. The divergency of the products and the orientation rules have been illustrated based on different substituents on carbazole rings. This method could also be extended to the synthesis of chlorodifluoroacetylated and pentafluoropropionylated carbazoles, which have been achieved for the first time.

Self-cyclization vs. dimerization of o-alkenyl arylisocyanides: chemodivergent synthesis of quinolines and pyrrolo-fused diindoles

Solvent-dependent chemoselective MBH-type self-cyclization and dimerization of o-alkenyl arylisocyanides were developed for the efficient and chemodivergent synthesis of various 3-acylquinoline and pyrrolo-fused diindole frameworks.

Metal-Free Indole-Phenacyl Bromide Cyclization: A Regioselective Synthesis of 3,5-Diarylcarbazoles

A metal-free, simultaneous triple C-C coupling cyclization reaction between phenacyl bromides and indoles is discovered in a highly regioselective fashion to furnish 3,5-diarylcarbazoles. DMAP is utilized as the only reagent for the unusual and rapid cyclization reaction to furnish all new carbazole compounds through installation of a great diversity of substituents. A plausible radical mechanism for the new reaction is predicted by conducting various control experiments, competitive reactions, furoindole formation, and ESI-MS analyses of the ongoing cyclization reaction.

Palladium-catalyzed cascade reactions of enynones and isocyanides: access towards functionalized ketenimine and its application

Palladium-catalyzed cascade reactions of enynones and isocyanides have been disclosed. This strategy allows for the synthesis of structurally unusual furan-incorporated ketenimines, which are stable and can undergo a formal [4+1] annulation with another isocyanide to yield indenone derivatives.

Rhodium-catalyzed homodimerization–cyclization reaction of two vinyl isocyanides: a general route to 2-(isoquinolin-1-yl)oxazoles

A novel rhodium-catalyzed homodimerization–cyclization reaction of two vinyl isocyanides has been developed for the synthesis of 2-(isoquinolin-1-yl)oxazoles by formation of three new bonds and two rings in a single step.

Synthesis of pyrazolo[1,5-c]quinazoline derivatives through the copper-catalyzed domino reaction of o-alkenyl aromatic isocyanides with diazo compounds

Various o-alkenyl aromatic isocyanides were prepared from readily available reactants for their double annulation with diazo compounds for a one-pot synthesis of pyrazolo[1,5-c]quinazolines under mild reaction conditions.

Rhodium/copper-cocatalyzed coupling-cyclization of o-alkenyl arylisocyanides with vinyl azides: one-pot synthesis of α-carbolines

A novel rhodium/copper-cocatalyzed coupling–cyclization reaction of o-alkenyl arylisocyanides with vinyl azides has been developed. The reaction provides a new route to α-carbolines by the formation of two C–C bonds, one C–N bond and two aromatic rings in a single step.

Copper-catalyzed cascade cyclization reaction of 3-aminocyclobutenones with electron-deficient internal alkynes: synthesis of fully substituted indoles

A novel copper-catalyzed cascade cyclization reaction of 3-aminocyclobutenones with electron-deficient internal alkynes has been developed. This reaction provides a new method for the synthesis of fully substituted indoles by formation of four new bonds and two rings in a single step.

Zn(OAc)2-catalyzed tandem cyclization of isocyanides, α-diazoketones, and anhydrides: a general route to polysubstituted maleimides

A zinc-catalyzed three-component reaction of isocyanides, α-diazoketones, and anhydrides has been realized as a novel and efficient method for the synthesis of polysubstituted maleimides.

A rhodium-catalyzed three-component reaction of arylisocyanides, trifluorodiazoethane, and activated methylene isocyanides or azomethine ylides: an efficient synthesis of trifluoroethyl-substituted imidazoles

A novel method for the synthesis of trifluoroethyl-substituted imidazoles is reported via a rhodium-catalyzed three-component reaction of isocyanides, 2,2,2-trifluorodiazoethane and activated methylene isocyanides or azomethine ylides.

Access to polyfunctionalized carbazoles through π-extension of 2-methyl-3-oxoacetate indoles

A structurally diverse set of polyfunctionalized carbazoles was efficiently synthesized in acceptable to excellent yields.