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

Recent approaches for the synthesis of heterocycles from amidines via a metal catalyzed C-H functionalization reaction

Transition metal catalyzed C-H bond activation has become one of the most important tools for constructing new chemical bonds. Introducing directing groups to the substrates is the key to a successful reaction, these directing groups can also be further transformed in the reaction. Amidines with their unique structure and reactivity are ideal substrates for transition metal-catalyzed C-H transformations. This review describes the major advances and mechanistic investigations of the C-H activation/annulation tandem reactions of amidines until early 2024, focusing on metal-catalyzed C-H activation of amidines with unsaturated compounds, such as alkynes, ketone, vinylene carbonate, cyclopropanols and their derivatives. Meanwhile this manuscript also explores the reaction of amidines with different carbene precursors, for example diazo compounds, azide, triazoles, pyriodotriazoles, and sulfoxonium ylides as well as their own C-H bond activation/cyclization reactions. A bright outlook is provided at the end of the manuscript.

Understanding the Remarkable Stability of Well-Defined Dinuclear Copper(I) Carbene Complexes

The synthesis of a well-defined dicopper carbene complex supported by the PNNP (2,7-bis(di-tert-butylphosphaneyl)methyl-1,8-naphthyridine) expanded pincer ligand is reported. This carbene complex is remarkably stable, even in the presence of air and water. The reactivity of this complex was explored towards typical carbene transfer substrates and its electronic structure was investigated. Using a combination of experiments and DFT calculations, the principles that underly the stability of dinuclear carbene complexes are probed.

Iron(II) Phthalocyanine-Catalyzed Homodimerization and Tandem Diamination of Diazo Compounds with Primary Amines: Access to Construct Substituted 2,3-Diaminosuccinonitriles in One-Pot

We herein first report the homodimerization and tandem diamination of diazo compounds with primary amines catalyzed by the iron(II) phthalocyanine (PcFe(II)), which can construct one C-C bond and two C-N bonds within 20 min in one-pot. Compared to the traditional metal-catalyzed N-H insertion reaction between amines with diazo reagents, the developed reaction almost does not generate the N-H insertion product, but the homodimerization/tandem diamination product. The proposed mechanism studies indicate that primary amines play a crucial role in the homocoupling of diazo compounds via dimerization of iron(III)-acetonitrile radical generated from the reaction between diazoacetonitrile with PcFe(II) coordinated by bis(amines); the β-hydride elimination is involved, and then, the attack of primary amines toward the carbon atoms on the formed C-C bond is followed. Moreover, this novel reaction can be used to effectively prepare substituted 2,3-diaminosuccinonitriles with high yields and even up to >99:1 d.r., encouragingly these products contain both 1,2-diamines and succinonitrile motifs, which are two classes of important organic compounds with significant applications in many yields. This reaction is also suitable for the gram-scale preparation of 2,3-bis(phenylamino)succinonitrile (2a) with a yield of 84%. Therefore, the developed reaction represents a new type of transformation.

Nitrene-transfer from azides to isocyanides: Unveiling its versatility as a promising building block for the synthesis of bioactive heterocycles

Cross-coupling azide and isocyanide have recently gained recognition as ideal methods for efficiently synthesizing asymmetric carbodiimides. This reaction exhibits high reaction rates, efficiency, and favorable atom/step/redox economy. It enables the nitrene-transfer process, facilitating the formation of C-N bonds and providing a direct and cost-effective synthetic strategy for generating diverse carbodiimides. These carbodiimides are highly reactive compounds that can undergo in-situ transformations into various functional groups and organic compounds, including heterocycles. Developing one-pot and tandem processes in this field has significantly contributed to advancements in organic chemistry. Moreover, the demonstrated utility of these architectural motifs extends to areas such as chemical biology and medicinal chemistry, further highlighting their potential in various scientific applications.

Cyanation with isocyanides: recent advances and perspectives

Cyanation has attracted considerable attention in organic synthesis because nitriles are key structural motifs in numerous important dyes, agrochemicals, natural products and drug molecules. As the fourth generation of cyanating reagents, isocyanides occupy a prominent place in the synthesis of nitriles due to their favorable stability, easy operability and high reactivity. In recent years, three types of cyanation with isocyanides have been established: the cleavage of the C-NC bond of tertiary alkyl isocyanides (Type I), the rearrangement of aryl isocyanides with azides (Type II), and the reductive cyanation of ketones with α-acidic isocyanides (Type III). This review focuses on advances in cyanation with isocyanides with an emphasis on reaction scope, limitations and mechanisms, which could reveal their remarkable value and superiority for accessing various nitriles. In addition, the future development prospects of this specific field are also introduced. We believe that this feature article will serve as a comprehensive tool to navigate cyanation with isocyanides across the vast area of synthetic chemistry.

Visible-Light-Driven Iron-Catalyzed Intermolecular Benzylic C(sp3)-H Amination with 1,2,3,4-Tetrazoles

A visible-light-driven iron-catalyzed C(sp3)-H amination of diphenylmethane derivatives with 1,2,3,4-tetrazoles under mild conditions has been developed. The reaction proceeds with photosensitizer-free conditions and features satisfactory to good yields. Mechanistic studies revealed that the reaction proceeded via an iron-nitrene intermediate, and H atom abstraction was the rate-determining step. Computational studies showed that the denitrogenation of 1,2,3,4-tetrazole depends on the conversion of the sextet ground state of 1,2,3,4-tetrazole-bounding iron species to the quartet spin state under visible-light irradiation.

Photoredox radical cyclization reaction of o-vinylaryl isocyanides with acyl chlorides to access 2,4-disubstituted quinolines

We herein report an efficient photoredox radical cyclization reaction of o-vinylaryl isocyanides with acyl chlorides to access a wide range of 2,4-disubstituted quinolines. Preliminary mechanism experiment results suggested that this reaction was initiated by an acyl radical generated from acyl chlorides through a single-electron-transfer (SET) process. This transformation showed good substrate suitability and functional group compatibility at room temperature.

Dearomative Spirocyclization of Tryptamine-Derived Isocyanides via Iron-Catalyzed Carbene Transfer

Tryptamine-derived isocyanides are valuable building blocks in the construction of spirocyclic indolenines and indolines via dearomatization of the indole moiety. We report the Bu4N[Fe(CO)3NO]-catalyzed carbene transfer of α-diazo esters to 3-(2-isocyanoethyl)indoles, leading to ketenimine intermediates that undergo spontaneous dearomative spirocyclization. The utility of this iron-catalyzed carbene transfer/spirocyclization cascade was demonstrated by its use as a key step in the formal total synthesis of monoterpenoid indole alkaloids (±)-aspidofractinine, (±)-limaspermidine, (±)-aspidospermidine, and (±)-17-demethoxy-N-acetylcylindrocarine.

Iron-catalyzed β-hydroxymethylative carbonylation of styrene under photo-irradiation

This study describes an iron-catalyzed divergent oxidation of styrene into β-hydroxylmethylketone and ketone under photo-irradiation. This divergence is ascribed to the use of styrene with various substituents. More importantly, methanol is oxidized into formaldehyde in the reaction and serves as a C1 synthon. Mechanism investigations show that the reaction is initiated by oxidative SET to transfer styrene into the cation radical. The reaction pathway undergoes HAT and β-hydride elimination as well as a concerted cyclization. Particularly, several drug-like molecules, such as melperone analogue, lenperone analogue, and haloperidol analogue, are synthesized. In addition, this method is also applicable to the synthesis of natural product (R)-atomoxetine.

Coupling-Spirocyclization Cascade of Tryptamine-Derived Isocyanides with Iodonium Ylides and Despirocyclization Reactions

A cobalt(II)-catalyzed coupling-cyclization cascade reaction between tryptamine-derived isocyanides and iodonium ylides is investigated, which allowed for the synthesis of different types of spiroindoline compounds by variation of substituents at the N1- and C2-positions in the indole skeleton. More interesting is that the spiroindoline products could undergo despirocyclization in the presence of amines, enabling efficient construction of enamine compounds.

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.

Palladium-Catalyzed Tandem C(sp3)-H Insertion Cyclization of 2-(2-Vinylarene)acetonitriles with Isocyanides to Access Naphthalen-2-amines

A Pd-catalyzed cyclization reaction of 2-(2-vinylarene)acetonitriles and isocyanides has been documented. Various naphthalen-2-amines were obtained in moderate to good yields under mild conditions. The in vitro cytotoxicity of all products was evaluated by MTT assay against seven human cancer cell lines. The results indicated that compounds 3ea, 3ma, and 3ob exhibited effective anticancer activities against the tested cancer cells.

Synthesis of Polysubstituted Pyrimidines through Palladium-Catalyzed Isocyanide Insertion to 2H-Azirines

The domino process of the palladium-catalyzed coupling reaction of isocyanides with 2H-azirine provides various tetrasubstituted pyrimidines via one C-C bond and two C-N bond formations with satisfactory yields. The title compounds are obtained with good functional group tolerance, high atom economy, and broad substrate scopes.

A Cobalt Mediated Nitrene Transfer aza-Wittig Cascade Reaction To Access 1,3,4-Oxadiazole Scaffolds

A cobalt(II) mediated three-component synthesis of 5-substituted-N-sulfonyl-1,3,4-oxadiazol-2(3H)-imines using sulfonyl azides, N-isocyaniminotriphenylphosphorane (NIITP), and carboxylic acids has been developed. This one-pot tandem reaction starts with a nitrene transfer to NIITP, followed by addition of the carboxylic acid to the in situ formed carbodiimide and subsequent intramolecular aza-Wittig reaction. Both the steric constraints of carboxylic acid and the stoichiometry of the employed cobalt salt determine the selectivity toward the two products, i.e. 5-substituted-N-sulfonyl-1,3,4-oxadiazol-2(3H)-imine versus 5-substituted-4-tosyl-2,4-dihydro-3H-1,2,4-triazol-3-one.

Visible-Light-Induced Annulation of Iodonium Ylides and 2-Isocyanobiaryls to Access 6-Arylated Phenanthridines

A 1,2,3,5-tetrakis(carbazol-9-yl)-4,6-dicyanobenzene (4-CzIPN)-photocatalyzed cascade arylation/cyclization reaction of 2-isocyanobiaryls and iodonium ylides was established for the synthesis of 6-arylated phenanthridines. This is the first example of employing iodonium ylides as aryl radical sources in a visible-light-induced radical cascade cyclization reaction.

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.