Nickel-Catalyzed [2+2+2] Cycloaddition of Alkyne-Nitriles with Alkynes Assisted by Lewis Acids: Efficient Synthesis of Fused Pyridines

Pd/Cu Dual Metal-Catalyzed Regioselective [2 + 2 + 2] Cycloaddition of Malononitriles with Alkynes to Densely Substituted Pyridines

Transition metal-catalyzed [2 + 2 + 2] cycloaddition of nitriles and two alkynes is an efficient method for assembling pyridines. However, examples employing palladium catalysis have rarely been disclosed, and the processes of reactivity and selectivity remain unclear. We report here a palladium/copper dual metal-catalyzed [2 + 2 + 2] cycloaddition of diynyl-tethered malononitriles and terminal alkynes to synthesize densely substituted pyridines. This method features a good substrate scope, synthetically useful yields, and perfect regioselectivity. The derivatization of the pyridine products demonstrates the potential application of this method in synthesizing heterocycles and as ligands in photocatalysis. Preliminary mechanistic studies suggest that the reaction undergoes aza-oxidative cycloaddition of Pd(0) with nitrile and alkyne, followed by alkyne insertion and reductive elimination. The presence of copper is crucial to its reactivity and regioselectivity.

Cu(II)/Base Synergistic Promoted [2 + 1 + 3] Cyclization of Cyclic Ketones with α,β-Unsaturated Aldehydes/Ketones and NH4I to Access Fused Pyridines

Herein, a practical three-component [2 + 1 + 3] cyclization of various cyclic ketones with α,β-unsaturated aldehydes/ketones and ammonium iodide (NH4I) to access highly functional fused pyridines has been developed. The features of this transformation include mild reaction conditions, readily available starting materials, and excellent chemoselectivity. This protocol is compatible with various functional groups, and the preliminary studies on the mechanism of the reaction are also provided.

Nickel-Catalyzed Reductive Alkyne Hydrocyanation Enabled by Malononitrile and a Formaldehyde Additive

The development of a nickel-catalyzed reductive alkyne hydrocyanation is described using 2-methyl-2-phenylmalononitrile (MPMN), a C-bound electrophilic transnitrilation reagent. Reproducibility issues led to the detection of oxidized hemiaminal impurities within N,N-dimethylacetamide. These impurities release formaldehyde in situ, which was ultimately identified as a critical reaction additive. A range of diaryl and aryl-alkyl alkynes underwent hydrocyanation. Mechanistic experiments revealed that formaldehyde and MPMN undergo a Ni-catalyzed reductive coupling of two π-components, leading to the controlled release of glycolonitrile as the active cyanating agent.

The Renaissance of Organo Nitriles in Organic Synthesis

In the arena of functional group-oriented organic synthesis, the nitrile or cyano functionality is of immense importance. The presence of nucleophilic N -atom, π-coordinating ability of the triple bond, and electrophilic C-center imparts unique and interesting reactivity. Owing to the ability of the nitrile to transform into various other functional groups or intermediates, the chemistry is very rich and diverse. In particular, the involvement of nitrile in numerous organic reactions such as inter- or intramolecular alkyne insertion, [2 + 2 + 2] cycloaddition with alkynes, [3 + 2] cycloaddition with azides, [4 + 2] cycloaddition with diene allow the synthesis of many important carbocycles and heterocycles. Furthermore, the nitrile serves as a directing group in many C-H bond functionalization reactions to introduce diverse functionality and participate as a radical acceptor in radical cascade strategies to obtain a large variety of functional molecules. This review mainly focuses on the reactivity and diverse synthetic application of the nitrile including C-H bond functionalization, alkyne insertion, cycloaddition, and thermal or photochemical cascade strategy. The objective of the current review aims at bringing out the striking collection of various nitrile-triggered organic transformations.

Synthesis of oxaboranes via nickel-catalyzed dearylative cyclocondensation

We report Ni-catalyzed dearylative cyclocondensation of aldehydes, alkynes, and triphenylborane. The reaction is initiated by oxidative cyclization of the aldehyde and alkyne coupling partners to generate an oxanickelacyclopentene which reacts with triphenylborane to form oxaboranes. This formal dearylative cyclocondensation reaction generates oxaboranes in moderate-to-high yields (47–99%) with high regioselectivities under mild reaction conditions. This approach represents a direct and modular synthesis of oxaboranes which are difficult to access using current methods. These oxaboranes are readily transformed into valuable building blocks for organic synthesis and an additional class of boron heterocycles. Selective homocoupling forms oxaboroles, oxidation generates aldol products, and reduction and arylation form substituted allylic alcohols.

Oxaboranes are prepared via a nickel-catalyzed dearylative cyclocondensation reaction in up to 99% yield and excellent regioselectivity. These oxaborane products can be further transformed into a variety of synthetically useful building blocks.

A review for the assembly of multi-substituted pyridines via Co-catalyzed [2+2+2] cycloaddition with nitriles

In the recent years, many methods for the facile synthesis of pyridines and their derivatives have been developed. The [2+2+2] cycloaddition reaction of alkynes and nitriles catalyzed by transition metals...

Metal Free Access to Polysubstituted Pyrimidines via Nitrile Activation and [2+2+2] Cycloaddition

Tf₂ O mediated intermolecular / intramolecular [2+2+2] cycloaddition between alkynes and nitriles has been developed for efficient construction of polysubstituted pyrimidines and bicyclopyrimidines. In presence of Tf₂ O, aza-allene species were generated in situ through nitrile activation and subsequently participated in the [2+2+2] cycloaddition, which was fully supported by deuteration experiments. The reaction had good substrate extensibility with moderate to excellent yield including trimethylsilylalkynes. The method was utilized as a synthetic tool in the preparation of a luminescent metal complex.

Nickelocene as an Air- and Moisture-Tolerant Precatalyst in the Regioselective Synthesis of Multisubstituted Pyridines

Ni(COD)₂-catalyzed cycloaddition reactions to access pyridines have been extensively studied. However, this catalyst typically requires drying procedures and inert-atmosphere techniques for the reactions. Herein, we report operationally simple nickel(0) catalysis to access substituted pyridines from various nitriles and 1,6-diynes without the aid of air-free techniques. The Ni-Xantphos-based catalytic manifold is tolerant to air, moisture, and heat while promoting the [2 + 2 + 2] cycloaddition reactions with high reaction yields and broad substrate scope. In addition, we disclose that not only the steric effect but also the frontier molecular orbital interactions can play a critical role in determining the regiochemical outcome of nickel-catalyzed [2 + 2 + 2] cycloaddition for the synthesis of substituted pyridines.

Robust Cobalt Catalyst for Nitrile/Alkyne [2+2+2] Cycloaddition: Synthesis of Polyarylpyridines and Their Mechanochemical Cyclodehydrogenation to Nitrogen-Containing Polyaromatics*

The transition-metal-catalyzed [2+2+2] cycloaddition of nitriles and alkynes is an established synthetic approach to pyridines; however, these cycloadditions often rely on the use of tethered diynes or cyanoalkynes as one of the reactants. Thus, examples of efficient, fully intermolecular catalytic [2+2+2] pyridine synthesis, especially those employing unactivated nitriles and internal alkynes leading to pentasubstituted pyridines, remain scarce. Herein, we report on simple and inexpensive catalytic systems based on cobalt(II) iodide, 1,3-bis(diphenylphosphino)propane, and Zn that promote [2+2+2] cycloaddition of various nitriles and diarylacetylenes for the synthesis of a broad range of polyarylated pyridines. DFT studies support a reaction pathway involving oxidative coupling of two alkynes, insertion of the nitrile into a cobaltacyclopentadiene, and C-N reductive elimination. The resulting tetra- and pentaarylpyridines serve as precursors to hitherto unprecedented nitrogen-containing polycyclic aromatic hydrocarbons via mechanochemically assisted multifold reductive cyclodehydrogenation.

Rh(iii)-Catalyzed [3 + 2]/[4 + 2] annulation of acetophenone oxime ethers with 3-acetoxy-1,4-enynes involving C–H activation

A novel, synthetically simple, selective rhodium(iii)-catalyzed [3 + 2]/[4 + 2] annulation cascade reaction to construct complex azafluorenone frameworks has been developed.

De Novo Protocol for the Construction of Benzo[a]fluorenes via Nitrile/Alkene Activation

Unprecedented chemo- and regioselective synthesis of benzo[a]fluorenes and naphthamide-substituted benzo[a]fluorenes were constructed from the reaction of (E)-2-aroyl-3-(2-(arylalkynes/alkenes)aryl)acrylonitrile scaffolds under metal-free conditions via the activation of nitriles and alkenes, respectively. A tentative reaction mechanism was proposed for this homofunctionalization of nitriles. Control experiments showed that the reaction proceeds via selective nitrile or alkene protonation, depending upon the substrates. Additionally, we demonstrated an alternative expeditious route for the synthesis of disubstituted benzo[a]fluorenes in the presence of TfOH alone.

Functionalization of remote C(sp3)-H bonds enabled by copper-catalyzed coupling of O-acyloximes with terminal alkynes

Transition metal catalyzed Sonogashira cross-coupling of terminal alkynes with aryl(vinyl) (pseudo)halides has been successfully extended to alkyl halides for the synthesis of functionalized internal alkynes. The direct alkynylation of remote unfunctionalized sp3 carbon by terminal alkynes remains difficult to realize. We report herein an approach to this synthetic challenge by developing two catalytic remote sp3 carbon alkynylation protocols. In the presence of a catalytic amount of Cu(I) salt and a tridentate ligand (tBu3-terpyridine), O-acyloximes derived from cycloalkanones and acyclic ketones are efficiently coupled with terminal alkynes to afford a variety of γ- and δ-alkynyl nitriles and γ-alkynyl ketones, respectively. These reactions proceed through a domino sequence involving copper-catalyzed reductive generation of iminyl radical followed by radical translocation via either β-scission or 1,5-hydrogen atom transfer (1,5-HAT) and copper-catalyzed alkynylation of the resulting translocated carbon radicals. The protocols are applicable to complex natural products.

Divergent Synthesis of Densely Substituted Arenes and Pyridines via Cyclotrimerization Reactions of Alkynyl Triazenes

Densely substituted fused aromatic triazenes can be prepared by [2 + 2 + 2] cyclotrimerization reactions of 1-alkynyl triazenes. The Cp*Ru-catalyzed cyclization proceeds well with both simple alkynyl triazenes and tethered 1-diynyl triazenes. Attractively, the methodology can be extended to pyridine synthesis by replacing an alkyne with a nitrile. The reaction is regioselective and yields the sterically more hindered product. The triazene group precisely installed on the synthesized aryl and pyridyl ring is a highly versatile moiety, which is effortlessly converted into the most important and frequently used functional aryl substituents, including fluorides. It is also suited for intramolecular transformations to afford a variety of valuable heterocycles. The coordination chemistry of alkynyl triazenes and Cp*RuCl was studied and led to the structural characterization of a Cp*RuCl(η²-alkyne) complex, a Cp*RuCl(η⁴-cyclobutadiene) complex, and an unusual dinuclear Ru complex with a bridging tetramethylfulvene ligand. Complexes of this type are potentially involved in catalyst deactivation pathways.

Ruthenium-Catalyzed [2 + 2 + 2] Cycloaddition Reaction Forming 2-Aminopyridine Derivatives from α,ω-Diynes and Cyanamides

A novel, efficient, and mild synthetic route for the preparation of 2-aminopyridines via ruthenium-mediated [2 + 2 + 2] cycloaddition of α,ω-diynes and cyanamides has been developed. This atom-economical catalytic process demonstrated remarkable regioselectivities to access pyridine derivatives of high synthetic utility.

Solvent-free ruthenium trichloride-mediated [2 + 2 + 2] cycloaddition of α,ω-diynes and cyanamides: a convenient access to 2-aminopyridines

A convenient access to functionalized 2-aminopyridinesviaa solventless Ru-catalyzed [2 + 2 + 2] cycloaddition reaction of α,ω-diynes and cyanamides is described.