The Gewald reaction in dye chemistry

Abstract

This review presents the most elegant and promising set of synthetic routes for the synthesis of 2‐aminothiophenes by the Gewald reaction. This reaction has applications in several applied fields, such as pharmaceuticals/biomedicine, agrochemicals, carbohydrate conjugates, sugar cane ripeners, peptide analogues, dyestuffs, and electronic materials; however, the present review will focus on the use of Gewald chemistry in the synthesis of conventional and functional colourants. The Gewald reaction produces 2‐aminothiophene, which is a key intermediate for the diazotisation reaction and for the synthesis of azo dyes, and thus opened the door for new dye chemistry. The reaction produces electron‐withdrawing groups such as CN, COOEt, COOMe, CONH2, and COPh in the 3‐ and/or 5‐position, resulting in bathochromic shifts for the deeper colours of the dyes. The review further reveals the importance of this reaction in the synthesis of azo dyes, fluorescent dyes (coumarin dyes, styryl dyes, thieno[2,3‐d]pyrimidine dyes, bithiophene dyes), thiophene analogues of phthalocyanines, and fluorescent whitening agents.