C-H functionalization enabled by multiple isocyanides

Past decades have witnessed significant advance of isocyanides as a class of versatile organic synthons as well as their broad applications in multi-component reactions (MCRs) and other tandem reactions. Reactions involving multiple isocyanides allow the construction of molecules with further diversification and complexity, while C-H functionalization emphasizes the advantages of high atom economy, broad substrate availability and great synthetic efficiency. This promising synergistic strategy of C-H functionalization involving multiple isocyanides provides a variety of valuable synthetic methods for organic chemists' toolbox and offers considerable potential in pharmaceutical chemistry and materials science as well. The present review outlines in detail various reaction types of C-H functionalization enabled by multiple isocyanides, and the relevant mechanistic rationale is discussed.

Rapid Access to Fused Tetracyclic N-Heterocycles via Amino-to-Alkyl 1,5-Palladium Migration Coupled with Intramolecular C(sp3)-C(sp2) Coupling

An unprecedented route for the preparation of fused tetracyclic N-heterocycles is presented through the palladium-catalyzed cyclization of isocyanides with alkyne-tethered aryl iodides. In this transformation, a novel amino-to-alkyl 1,5-palladium migration/intramolecular C(sp3)-C(sp2) coupling sequence was observed first. More importantly, isocyanide exhibited three roles, serving simultaneously as a C1 synthon, a C1N1 synthon, and the donor of C(sp3) for C(sp3)-C(sp2) coupling, and the reaction was the sole successful example that achieved C(sp3)-H activation of isocyanide.

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.

Palladium-catalyzed acetalization/cyclization of enynones with alcohols: rapid access to functionalized dihaloalkenyl dihydrofurans

A novel Pd-catalyzed acetalization/cyclization of enynones and alcohols for the construction of dihaloalkenyl dihydrofuran derivatives is described.

Synthesis of polysubstituted pyrroles via isocyanide-based multicomponent reactions as an efficient synthesis tool

The present review covers all synthetic methods based on isocyanide-based multicomponent reactions for the preparation of polysubstituted pyrroles as the parent cores of many essential drugs, biologically active compounds, and compounds with wide application in materials science.

Palladium-catalyzed double coupling reaction of terminal alkynes with isocyanides: a direct approach to symmetrical N-aryl dialkynylimines

A novel and efficient one-pot synthesis of symmetrical N-aryl dialkynylimines via palladium-catalyzed and copper-promoted isocyanide insertion, cross-coupling and elimination has been developed. This method features readily available starting materials, mild reaction conditions and high atom efficiency as well as simple one-pot operation, which make this strategy highly attractive. Moreover, 2-iodobenzo[f]quinoline derivatives can be obtained via electrophilic cyclization of N-aryl dialkynylimines.

Gold(iii) promoted formation of dihydroquinazolinones: double X-H activation by gold

An efficient 2-furyl gold-carbene promoted synthetic method was developed for the formation of dihydroquinazolinones from enynones by dual insertion of anthranilamides. In this organic transformation a new C-O and two C-N bond formations occurred and dihydroquinazolinones were obtained with a quaternary centre in moderate to very good yields in one-pot synthesis.

Palladium-Catalyzed Imidoylation-Triggered [2 + 2 + 1] Cyclization of Internal Alkyne with Isocyanides

In this work, a palladium-catalyzed [2 + 2 + 1] cyclization of internal alkynes with double isocyanides is described. This facile procedure is efficient for synthesizing various pyrrolo[3,2-c]quinolin-2-amines. The reaction worked well with a broad reaction scope. In the process, it is believed that sequential double isocyanide insertion, 6-exo-dig cyclization of alkyne, and addition of an imino group are involved.

Palladium-Catalyzed Cycloaddition of Alkynylimines, Double Isocyanides, and H2O/KOAc

In this work, a palladium-catalyzed cyclization of alkynylimines and double isocyanides is described. This facile procedure is efficient for synthesizing various 4-amidyl-2-aminopyrroles. Mechanism investigation indicates that a four-membered ring-fused pyrrole species is a key intermediate and the reaction involves [4 + 1] cycloaddition, protonation, nucleophilic addition, 1,4-addition of isocyanide, and rearomatization. Interestingly, the linear dipyrrole derivative is found to be an appropriate fluoride ion probe with a remarkable emission change, which could serve as a potential candidate for optoelectronic conjugated materials.

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.

Synthesis of Sulfamate-Fused 2-Aminopyrroles via an Isocyanide-Based Three Component [1+2+2] Annulation

An efficient preparation of sulfamate-fused 2-aminopyrroles was achieved through an isocyanide-based three-component [1+2+2] annulation of isocyanides, dialkyl acetylenedicarboxylates, and sulfamate-derived cyclic imines in good to excellent yields (up to 99 %). This reaction proceeds smoothly without any activation or modification of substances under neutral and metal-free conditions. The reaction could also be conveniently performed on a gram scale.

Synthesis of 2,3-Ring Fused Pyrroles via Cu-Catalyzed 5-exo-dig Annulation of Alkyne-Tethered Enaminones

A copper-catalyzed annulation of alkyne-tethered enaminones for the synthesis of 2,3-ring fused pyrroles is reported. The 5-exo-dig cyclization/olefin migration reaction delivers the multisubstituted pyrroles in 59-99% yields with 16 examples. This strategy features easily available starting materials, mild reaction conditions, and a cheap ligand-free copper catalyst. The atom-economic transformation provides a simple access to a variety of synthetic useful pyrroles and their derivatives.

Rh(III)-Catalyzed C-H Activation/Cycloisomerization of N-Phenoxyacetamides with Enynones for One-Pot Assembly of Furylated 2-Alkenylphenols

An efficient and practical procedure for one-pot assembly of furylated 2-alkenylphenols has been achieved via the Cp*^CyRh-catalyzed regioselective redox-neutral C-H activation/5-exo-dig cyclization cascade using N-phenoxyacetamides and enynones as the viable substrates. The synthetic application of such a protocol has also been demonstrated to highlight the versatility of this transformation.

Palladium-Catalyzed Diarylation of Isocyanides with Tetraarylleads for the Selective Synthesis of Imines and α-Diimines

Using tetraaryllead compounds (PbAr₄) as arylating reagents, isocyanides undergo selective diarylation in the presence of palladium catalysts such as Pd(OAc)₂ or Pd(PPh₃)₄ to afford imines and/or α-diimines based on the isocyanide employed. With aliphatic isocyanides, imines are obtained preferentially, whereas α-diimines are formed in the case of electron-rich aromatic isocyanides. The differences in imine/α-diimine selectivity can be attributed to the stability of imidoylpalladium intermediates formed in this catalytic reaction. Compared with other arylating reagents, tetraaryllead compounds are excellent candidates for use in the selective transformations to imines and/or α-diimines, especially in terms of inhibiting the oligomerization of isocyanides, which results in a lower product selectivity in many transition-metal-catalyzed reactions of isocyanides.

A bicyclization reaction with two molecular allenyl ketones and isocyanides: synthesis of a lactone-containing azaspirocycle derivative

A novel bicyclization reaction of two molecular allenyl ketones and isocyanides has been disclosed. This strategy allows for the construction of structurally complex spirocyclic lactam–lactone systems in an efficient manner.

Phosphine-mediated sequential annulations of allenyl ketone and isocyanide: a bicyclization strategy to access a furan-fused eight-membered ring and a spirocycle

Phosphine-mediated cascade annulations of allenyl ketone and isocyanide have been disclosed. This strategy enables the efficient synthesis of a furan-fused eight-membered ring and a spirocycle.

A formal (5+1) annulation reaction from heterodimerization of two different isocyanides: stereoselective synthesis of 2H-benzo[b][1,4]oxazin-2-one

A novel (5+1) annulation reaction of functionalized 2-isocyanophenyloxyacrylate and aromatic, aliphatic isocyanides has been disclosed. This strategy allows for a quick access to many 2H-benzo[b][1,4]oxazin-2-one derivatives.

Cascade cyclization and acyl migration of propargylic esters with isocyanides: rapid access to substituted furans

A novel rearrangement of ester-activated propargylic acetate with isocyanide has been disclosed. This protocol enables a quick synthesis of polysubstituted furan derivatives.

Access to 2,3-Fused Pyrroles via Visible Light Driven Coupling of α-Azidochalcones with 1/2-Naphthols, or 2-Hydroxy-1,4-Naphthoquinone

α-Azidochalcones were coupled with 1/2-naphthols or 2-hydroxy-1,4-naphthoquinone using Ru(bpy)₃(PF₆)₂ as a photocatalyst under blue LED light irradiation to yield 2,3-fused pyrroles in high yields (68-84%). The overall transformation involves photosensitized decomposition of α-azidochalcones into highly reactive 2 H-azirines which are trapped by 1/2-naphthols or 2-hydroxy-1,4-naphthoquinone, leading to the construction of two new C-N and one new C-C bonds.

Carbon-nitrogen bond cleavage of pyridine with two molecular substituted allenoates: access to 2-arylpyrimidin-4(3H)-one

A DABCO-catalyzed annulation reaction of pyridin-2-amine and substituted allenoates has been disclosed. This strategy allows for the ring-opening of a pyridine ring system and the formation of two new rings including a pyrimidinone ring and a benzene ring in an efficient manner.

A domino reaction of 2-isocyanophenyloxyacrylate and aryne to synthesize arenes with vicinal olefin and benzoxazole

An unusual domino reaction of 2-isocyanophenyloxyacrylate and aryne has been disclosed. The present strategy experiences nucleophilic addition, Michael addition, carbon–oxygen cleavage, and cyclization, thus enabling the quick aryne vicinal difunctionalization by the installation of a benzoxazole and an olefin.