Metal-catalyzed C–H functionalization involving isocyanides

Synthesis of imidazo[1,2-c]thiazoles through Pd-catalyzed bicyclization of tert-butyl isonitrile with thioamides

Building new biological molecules is challenging. Herein, imidazo[1,2-c]thiazoles were synthesized as a new class of heterobicyclic analogs through Pd-catalyzed cascade bicyclization from isonitriles with thioamides. The bicyclic scaffolds were constructed by inserting three molecules of isonitrile into two molecules of thioamide and then cyclizing them in a one-pot procedure. In vitro antitumor studies of these new compounds were conducted by using the MTT assay, and compound 3c showed excellent inhibitory effects against HepG2 at 7.06 ± 0.68 μM.

Silver-Mediated Synthesis of Substituted Benzofuran- and Indole-Pyrroles via Sequential Reaction of ortho-Alkynylaromatics with Methylene Isocyanides

A silver-mediated reaction between 2-ethynyl-3-(1-hydroxyprop-2-yn-1-yl)phenols or 2-ethynyl-3-(1-hydroxyprop-2-yn-1-yl)anilines and methylene isocyanides has been developed. A sequential 5-endo-dig cyclization and [3 + 2] cycloaddition process is proposed. This synthetic strategy is atom- and step-efficient and applicable to a broad scope of substrates, allowing the synthesis of valuable substituted benzofuran- and indole-pyrroles in moderate to high yields.

A rhodium(iii)-catalyzed tunable coupling reaction of indole derivatives with alkylidenecyclopropanes via C-H activation

We herein report a rhodium(iii)-catalyzed cross coupling of indole derivatives with alkylidenecyclopropanes (ACPs) in the presence of KCl, affording the alkene products exclusively with E-selectivity via C-H bond activation. The beta-H elimination to afford the conjugated diene derivatives has been suppressed by the addition of KCl. A plausible reaction mechanism has been proposed along with derivatization of the obtained product to demonstrate the practical usefulness of this synthetic protocol.

One-Pot Synthesis of N-Imidoyl-(1 H)-indoles via Palladium-Catalyzed Oxidative Insertion Domino Reaction with Isocyanide and Arylboronic Acid

Efficient one-pot synthesis of N-imidoyl-(1 H)-indoles has been described, which is achieved by the palladium-catalyzed oxidative insertion of 2-(phenylethynyl)aniline, arylboronic acid, and isonitrile. This method provides a new way to synthesize N-imidoyl-(1 H)-indoles, which has a wide substrate scope, good functional group tolerance, and mild reaction condition.

Porous Organic Polymer-Derived Nanopalladium Catalysts for Chemoselective Synthesis of Antitumor Benzofuro[2,3- b]pyrazine from 2-Bromophenol and Isonitriles

An efficient strategy for the synthesis of benzofuro[2,3- b]pyrazines was developed. These tricyclic scaffolds were formed through a multistep cascade sequence, which includes double insertion of isonitriles and chemoselective bicyclization. In this reaction, a nanopalladium was used as a recyclable catalyst. Product 3w exhibited excellent anticancer activity toward T-24 (IC₅₀ = 12.5 ± 0.9 μM) and HeLa (IC₅₀ = 14.7 ± 1.6 μM) cells. We also explored the action mechanism of 3w on T-24 cells.

Pd-Catalyzed C(sp2)-H Imidoylative Annulation: A General Approach To Construct Dibenzoox(di)azepines

A general method to construct the scaffolds of dibenzooxazepine and dibenzodiazepine, through Pd-catalyzed isocyanide insertion and intramolecular C(sp²)-H activation, has been developed. This is the first example of seven-membered heterocycle formation by C-H imidoylative annulation.

Silver-Mediated [3 + 2] Cycloaddition of Alkynes and N-Isocyanoiminotriphenylphosphorane: Access to Monosubstituted Pyrazoles

A silver-mediated [3 + 2] cycloaddition of "CNN" and "C≡C" for constructing pyrazoles has been described. The "CNN" building block used is N-isocyanoiminotriphenylphosphorane, which is a stable, safe, easy-to-handle, and odorless solid isocyanide. The reaction is characterized by its mild conditions, broad substrate scope, and excellent functional group tolerance.

Visible-light-induced radical trifluoromethylthiolation of N-(o-cyanobiaryl)acrylamides

An efficient and general visible-light-mediated trifluoromethylthiolation of N-(o-cyanobiaryl)acrylamides has been successfully accomplished using N-trifluoromethylthiosaccharin as an effective source of SCF3 radicals. The reaction was proposed to proceed via a domino radical trifluoromethylthiolation/cyano insertion/cyclization to afford the corresponding SCF3-containing ring-fused phenanthridine derivatives in moderate to good yields.

Synthesis of iminoisoindolinones via a cascade of the three-component Ugi reaction, palladium catalyzed isocyanide insertion, hydroxylation and an unexpected rearrangement reaction

A robust ligand-free palladium-catalyzed cascade reaction for the synthesis of diversely substituted iminoisoindolinones has been developed. The cascade reaction involves isocyanide insertion into Ugi-3CR adducts, accompanied by unexpected hydroxylation and rearrangement.

Bond Forming Reactions Involving Isocyanides at Diiron Complexes

The versatility of isocyanides (CNR) in organic chemistry has been tremendously enhanced by continuous advancement in transition metal catalysis. On the other hand, the urgent need for new and more sustainable synthetic strategies based on abundant and environmental-friendly metals are shifting the focus towards iron-assisted or iron-catalyzed reactions. Diiron complexes, taking advantage of peculiar activation modes and reaction profiles associated with multisite coordination, have the potential to compensate the lower activity of Fe compared to other transition metals, in order to activate CNR ligands. A number of reactions reported in the literature shows that diiron organometallic complexes can effectively assist and promote most of the “classic” isocyanide transformations, including CNR conversion into carbyne and carbene ligands, CNR insertion, and coupling reactions with other active molecular fragments in a cascade sequence. The aim is to evidence the potential offered by diiron coordination of isocyanides for the development of new and more sustainable synthetic strategies for the construction of complex molecular architectures.

Palladium-Catalyzed Multicomponent Reaction of Alkynes, Carboxylic Acids, and Isocyanides: A Direct Approach to Captodative Olefins

A palladium-catalyzed multicomponent reaction of alkynes, carboxylic acids, and isocyanides has been developed with the assistance of silver salt under mild conditions. Highly functionalized captodative olefins are synthesized efficiently by this method, which can find many applications as versatile synthons in organic synthesis.

Cascade double isocyanide insertion and C-N coupling of 2-iodo-2'-isocyano-1,1'-biphenyls

A palladium-catalyzed double isocyanide insertion using 2-iodo-2'-isocyano-1,1'-biphenyls followed by copper-catalyzed intramolecular C-N coupling, delivering a unique heterocyclic structure containing both phenanthridine and carbazole scaffolds, has been developed. In this cascade process, four chemical bonds, including two C-C, one C-O, and one C-N bonds are formed consecutively without isolating an intermediate. The strategy of using a functionalized isocyanide in double insertion provides a quick approach for constructing heterocyclic systems with high bond-forming efficiency.

Mn(III)-Mediated Cascade Cyclization of 3-Isocyano-[1,1'-biphenyl]-2-carbonitrile with Arylboronic Acid: Construction of Pyrrolopyridine Derivatives

A Mn(III) mediated cascade cyclization of new designed multifunctionalized 3-isocyano-[1,1'-biphenyl]-2-carbonitrile with arylboronic acid to construct pyrrolopyridine derivatives is developed. A series of pyrroloporidine compounds have been constructed through the formation of two new C-C bonds and one C-N bond via a radical pathway.

Electrochemical Transition-Metal-Catalyzed C-H Bond Functionalization: Electricity as Clean Surrogates of Chemical Oxidants

Transition-metal-catalyzed C-H activation has attracted much attention from the organic synthetic community because it obviates the need to prefunctionalize substrates. However, superstoichiometric chemical oxidants, such as copper- or silver-based metal oxidants, benzoquinones, organic peroxides, K₂ S₂ O₈ , hypervalent iodine, and O₂ , are required for most of the reactions. Thus, the development of environmentally benign and user-friendly C-H bond activation protocols, in the absence of chemical oxidants, are urgently desired. The inherent advantages and unique characteristics of organic electrosynthesis make fill this gap. Herein, recent progress in this area (until the end of September 2018) is summarized for different transition metals to highlight the potential sustainability of electro-organic chemistry.

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.

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.

Nickle Catalysis Enables Access to Thiazolidines from Thioureas via Oxidative Double Isocyanide Insertion Reactions

An efficient synthesis of thiazolidine-2,4,5-triimine derivatives was developed via Ni-catalyzed oxidative double isocyanide insertion to thioureas under air conditions, in which thioureas play three roles as a substrate, a ligand, and overcoming isocyanide polymerization. The reaction is featured by employing a low-cost and low loading Ni(acac)₂ catalyst, without any additives, and high atom economy. This is the first example to directly apply a Ni(II) catalyst in oxidative double isocyanide insertion reactions.

Isonitriles as Stereoelectronic Chameleons: The Donor-Acceptor Dichotomy in Radical Additions

Radical addition to isonitriles (isocyanides) starts and continues all the way to the transition state (TS) mostly as a simple addition to a polarized π-bond. Only after the TS has been passed, the spin density moves to the α-carbon to form the imidoyl radical, the hallmark intermediate of the 1,1-addition-mediated cascades. Addition of alkyl, aryl, heteroatom-substituted, and heteroatom-centered radicals reveals a number of electronic, supramolecular, and conformational effects potentially useful for the practical control of isonitrile-mediated radical cascade transformations. Addition of alkyl radicals reveals two stereoelectronic preferences. First, the radical attack aligns the incipient C···C bond with the aromatic π-system. Second, one of the C-H/C-C bonds at the radical carbon eclipses the isonitrile N-C bond. Combination of these stereoelectronic preferences with entropic penalty explains why the least exergonic reaction (addition of the t-Bu radical) is also the fastest. Heteroatomic radicals reveal further unusual trends. In particular, the Sn radical addition to the PhNC is much faster than addition of the other group IV radicals, despite forming the weakest bond. This combination of kinetic and thermodynamic properties is ideal for applications in control of radical reactivity via dynamic covalent chemistry and may be responsible for the historically broad utility of Sn radicals ("the tyranny of tin"). In addition to polarity and low steric hindrance, radical attack at the relatively strong π-bond of isonitriles is assisted by "chameleonic" supramolecular interactions of the radical center with both the isonitrile π*-system and lone pair. These interactions are yet another manifestation of supramolecular control of radical chemistry.

Radical Cyanotrifluoromethylation of Isocyanides: Step-Economical Access to CF3-Containing Nitriles, Amines, and Imines

A novel copper-catalyzed radical cyanotrifluoromethylation has been achieved through a multicomponent reaction of isocyanides, Togni's reagent, and trimethylsilyl cyanides, affording trifluoroacetimidoyl nitriles in good yields. This reaction demonstrates a unique feature of merging two valuable functional groups-trifluoromethyl (CF₃) and cyan (CN)-onto the same C atom. The transformation proceeds by the initial addition of the CF₃ radical to isocyanide and the subsequent intermolecular C-CN formation. The products can be successfully transformed to a series of CF₃-containing amines and imines that may serve in the synthesis of valuable pharmaceuticals and agrochemicals.

Cu-Catalyzed [3 + 3] Cycloaddition of Isocyanoacetates with Aziridines and Stereoselective Access to α,γ-Diamino Acids

We report herein an efficient Cu-catalyzed formal [3 + 3] cycloaddition of isocyanoacetates with readily available aziridines of different substitution patterns, which provides a practical access to valuable 1,4,5,6-tetrahydropyrimidine derivatives. In particular, the use of enantiopure aziridines delivers disubstituted tetrahydropyrimidines bearing a 1,3-diamino unit in good yields as a single stereoisomer (>20:1 dr, > 99% ee). The heterocyclic products can also be easily converted to synthetically useful amino alcohol derivatives or α,γ-diamino acids.

Synthesis of 2,3-Difunctionalized Benzofuran Derivatives through Palladium-Catalyzed Double Isocyanide Insertion Reaction

A novel palladium-catalyzed tandem cyclization of 1-(allyloxy)-2-ethynylbenzene derivatives with isocyanides in the presence of water has been developed. The key intermediates, benzofuran-3-α-carbonyl aldehydes, were obtained through a simple acid hydrolysis process and could serve as precursors for structurally diverse 2,3-difunctionalized benzofuran derivatives such as important 2-benzofurylquinoxalines, benzofuran-3-α-ketoesters and benzofuryl ynediones. This transformation features convenient operation, simple and commercially available starting materials, broad functional-group compatibility, and moderate to good reaction yields.

Palladium-Catalyzed Insertion of Isocyanides into the C-S Bonds of Heteroaryl Sulfides

Insertion of tert-butyl isocyanide into the C(sp² )-S bonds of heteroaryl sulfides is catalyzed by a palladium diphosphine complex. Thioimidates generated through this reaction could be readily hydrolyzed under acidic conditions to yield the corresponding thioesters, which are of synthetic use. This insertion is useful because starting heteroaryl sulfides were readily prepared by either conventional ways or through sulfur-specific extended Pummerer reactions.

Reaction of B2 (o-tol)4 with CO and Isocyanides: Cleavage of the C≡O Triple Bond and Direct C-H Borylations

The reaction of highly Lewis acidic tetra(o-tolyl)diborane(4) with CO afforded a mixture of boraindane and boroxine by the cleavage of the C≡O triple bond. ¹³ C labeling experiments confirmed that the carbon atom in the boraindane stems from CO. Simultaneously, formation of boroxine 3 could be considered as borylene transfer to capture the oxygen atom from CO. The reaction of diborane(4) with ^t Bu-NC afforded an azaallene, while the reaction with Xyl-NC furnished cyclic compounds by direct C-H borylations.