Recent advances in the chemistry of aryltriazene

Triazene is one of the most versatile building blocks in organic synthesis. Generally, it is viewed as a safe equivalent of diazonium salt, thus immediately finding numerous applications in preparative chemistry and medicinal chemistry. Besides, it can be used as a removable directing group in C-H functionalization or play a smart role as a precursor for aryl cation/radical generation. In this review, we will highlight recent noteworthy developments in this field.

Dirhodium(ii)-catalysed cycloisomerization of azaenyne: rapid assembly of centrally and axially chiral isoindazole frameworks

Described herein is a dirhodium(ii)-catalyzed asymmetric cycloisomerization reaction of azaenyne through a cap-tether synergistic modulation strategy, which represents the first catalytic asymmetric cycloisomerization of azaenyne. This reaction is highly challenging because of its inherent strong background reaction leading to racemate formation and the high capability of coordination of the nitrogen atom resulting in catalyst deactivation. Varieties of centrally chiral isoindazole derivatives could be prepared in up to 99 : 1 d.r., 99 : 1 er and 99% yield and diverse enantiomerically enriched atropisomers bearing two five-membered heteroaryls have been accessed by using an oxidative central-to-axial chirality transfer strategy. The tethered nitrogen atom incorporated into the starting materials enabled easy late-modifications of the centrally and axially chiral products via C–H functionalizations, which further demonstrated the appealing synthetic utilities of this powerful asymmetric cyclization.

Rh(ii)-catalyzed asymmetric cycloisomerization of azaenyne through a cap-tether synergistic modulation strategy was described. Diverse centrally and axially chiral isoindazoles were prepared and directed C–H late-stage modifications were developed.

Recent advances in the synthesis of indoles from alkynes and nitrogen sources

This review highlights ten years of success in the synthesis of indoles using alkynes and nitrogen sources as substrates.

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.

Biaryl synthesis with arenediazonium salts: cross-coupling, CH-arylation and annulation reactions

The rich legacy of arenediazonium salts in the synthesis of unsymmetrical biaryls, built around the seminal works of Pschorr, Gomberg and Bachmann more than a century ago, continues to make important contributions at various evolutionary stages of modern biaryl synthesis. Based on in-depth mechanistic analysis and design of novel pathways and reaction conditions, the scope of biaryl synthesis with arenediazonium salts has enormously expanded in recent years through applications of transition metal/photoredox-catalysed cross-coupling, thermal/photosensitized radical chain CH-arylation of (hetero)arenes and arylative radical annulation reactions with alkynes. These recent developments have provided facile synthetic access to a wide variety of unsymmetrical biaryls of pharmaceutical, agrochemical and optoelectronic importance with green scale-up options and created opportunities for late-stage modification of peptides, nucleosides, carbon nanotubes and electrodes, the details of which are captured in this review.

One-Pot Synthesis of Trisubstituted Triazenes from Grignard Reagents and Organic Azides

A simple and versatile method for the preparation of linear, trisubstituted triazenes is reported. The procedure is based on the reaction of Grignard reagents with 1-azido-4-iodobutane or 4-azidobutyl-4-methylbenzenesulfonate. These organic azides enable the regioselective formation of triazenes via an intramolecular cyclization step. The new method can be used for the preparation of aryl, heteroaryl, vinyl, and alkyl triazenes. The synthetic utility of vinyl triazenes is demonstrated by acid-induced C-N, C-O, C-F, C-P, and C-S bond-forming reactions.

Rhodium-catalyzed redox-neutral coupling of phenidones with alkynes

A switchable synthesis ofN-substituted indole derivatives from phenidonesviarhodium-catalyzed redox-neutral C–H activation has been achieved.

Cu/Pd cooperatively catalyzed tandem C–N and C–P bond formation: access to phosphorated 2H-indazoles

An unprecedented Cu/Pd cooperatively catalyzed tandem C–N/C–P bond formation reaction for the synthesis of phosphorated 2H-indazoles has been developed.

Oxidation of diazenyl-protected N-heterocycles – a new entry to functionalized lactams

A strategy for the synthesis ofN-aryldiazenyllactams, offering the direct access to protected lactam derivatives, is based on the oxidation of the corresponding cyclic amines with a ruthenium-based catalyst in CCl₄/CAN/phosphate buffer.

Three-step synthesis of 2,5,7-trisubstituted indoles from N-acetyl-2,4,6-trichloroaniline using Pd-catalyzed site-selective cross-coupling

A facile three-step synthesis of 2,5,7-trisubstituted indoles was realized by using Cy-DHTP as an effective ligand for site-selective cross couplings.

Palladium-catalyzed carbonylative Sonogashira coupling between aryl triazenes and alkynes

We developed a palladium-catalyzed carbonylative Sonogashira reaction with aryl triazenes and alkynes as substrates and methanesulfonic acid as the additive. A series of α,β-ynones were synthesized by this alternative procedure. Notably, bromides, iodides and hydroxyl groups could be well-tolerated under these reaction conditions.

Advances in the development of catalytic tethering directing groups for C–H functionalization reactions

Inspired by the development of organocatalysis, transition-metal-catalyzed C–H functionalization reactions using tethering groups to accomplish site-selectivity catalytically have been demonstrated.

Silver-catalysed reactions of alkynes: recent advances

Silver is a less expensive noble metal. Superior alkynophilicity due to π-coordination with the carbon-carbon triple bond makes silver salts ideal catalysts for alkyne-based organic reactions. This review highlights the progress in alkyne chemistry via silver catalysis primarily over the past five years (ca. 2010-2014). The discussion is developed in terms of the bond type formed with the acetylenic carbon (i.e., C-C, C-N, C-O, C-Halo, C-P and C-B). Compared with other coinage metals such as Au and Cu, silver catalysis is frequently observed to be unique. This critical review clearly indicates that silver catalysis provides a significant impetus to the rapid evolution of alkyne-based organic reactions, such as alkynylation, hydrofunctionalization, cycloaddition, cycloisomerization, and cascade reactions.

Non-precious metals catalyze formal [4 + 2] cycloaddition reactions of 1,2-diazines and siloxyalkynes under ambient conditions

Copper(I) and nickel(0) complexes catalyze the formal [4 + 2] cycloaddition reactions of 1,2-diazines and siloxyalkynes, a reaction hitherto best catalyzed by silver salts. These catalysts based on earth abundant metals are not only competent, but the copper catalyst, in particular, promotes cycloadditions of pyrido[2,3-d]pyridazine and pyrido[3,4-d]pyridazine, enabling a new synthesis of quinoline and isoquinoline derivatives, as well as the formal [2 + 2] cycloaddition reaction of cyclohexenone with a siloxyalkyne.

Triflic acid-promoted cycloisomerization of 2-alkynylphenyl isothiocyanates and isocyanates: a novel synthetic method for a variety of indole derivatives

A unique method for synthesis of structurally diverse types of indoles involving a triflic acid-promoted cycloisomerization and a Wagner–Meerwein-type rearrangement is described.