Synthesis of Dyes and Intermediates Involving Hydride Loss

Abstract

Mechanism of formation of triphenylmethane dyes, Malachite Green and Crystal Violet from the corresponding leuco bases was investigated. The latter are excellent hydride donors and are readily converted to TPM dyes by hydride transfer to acceptors such as trityl chloride or even to weak hydride acceptors such as acetic acid, its chloro derivatives and the corresponding ethyl esters, sulphuric acid and incipient carbonium ions such as protonated ethanol. Hydride transfer was affected by various parameters such as concentration, temperature, acidity, time, etc. and to some extent by dissolved air.
A series of new thiacyanine dyes have been prepared by cyclodehydration of β-arylmercaptoethyl methyl ketones by acids in presence of trityl chloride as a hydride acceptor.
It seems probable that in the synthesis of other cyanine dyes and several other basic dyes also, hydride loss is involved. In the Scholl-condensation, an important reaction for the preparation of dye intermediates, hydride loss is also involved.