Synthesis of azobenzenes: the coloured pieces of molecular materials

Abstract

Azobenzenes are ubiquitous motifs very important in many areas of science. Azo compounds display crucial properties for important applications, mainly for the chemical industry. Because of their discovery, the main application of aromatic azo compounds has been their use as dyes. These compounds are excellent candidates to function as molecular switches because of their efficient cis-trans isomerization in the presence of appropriate radiation. The classical methods for the synthesis of azo compounds are the azo coupling reaction (coupling of diazonium salts with activated aromatic compounds), the Mills reaction (reaction between aromatic nitroso derivatives and anilines) and the Wallach reaction (transformation of azoxybenzenes into 4-hydroxy substituted azoderivatives in acid media). More recently, other preparative methods have been reported. This critical review covers the various synthetic methods reported on azo compounds with special emphasis on the more recent ones and their mechanistic aspects (170 references).

A potential role of a substrate as a base for the deprotonation pathway in Rh-catalysed C–H amination of heteroarenes: DFT insights

“…[1][2][3][4] Among them, the direct aryl amination reactions have attracted much attention from the synthetic chemistry community because aryl and hetero aryl amines are ubiquitous in a large number of natural products, pharmaceuticals, and functional organic materials. 5 The use of substrates containing 'directing groups' that coordinate to a transition metal catalyst, followed by proximal C-H bond activation, has become a common strategy to control the site selectivity. In current synthetic methodologies for the construction of C-N bonds, azides have been widely used as convenient aminating reagents in C-H activation protocols.…”

Gold catalysts for the synthesis of aromatic azocompounds from nitroaromatics in one step

“…Aromatic azocompounds are widely used [1] as dyes [2] , food additives [3] and drugs [4] . A major industrial process for their synthesis involves the azocoupling reaction between a diazonium salt and an activated arene [5] .…”

A bifunctional palladium/acid solid catalyst performs the direct synthesis of cyclohexylanilines and dicyclohexylamines from nitrobenzenes

“…Production is based on two different processes: dehydroxylative amination of alcohols for manufacturing alkyl amines, 1,2 and reductive amination of ketones. 3,4 The latter includes the multiton/year production of 15 aniline 3 from nitrobenzene 2 shown in Scheme 1. Cyclohexylaniline 5 and dicyclohexylamine 6 are not produced by the downstream hydrogenation route 2 →→ 5 → 6 because of the lack of efficient and selective catalysts for these transformations.…”