Metal-Free Blue Dye Synthesis: Oxidative Coupling of Benzylamines and N,N-Dimethylanilines to Yield 4,4'-Diaminotriarylmethanes in the Presence of Salicylic Acid as a Co-oxidant.

A convenient, novel, and metal-free method for the synthesis of 4,4'-diaminotriarylmethanes (DTMs) is described. This process is based on a one-pot condensation of benzylamines with N,N-dimethylaniline derivatives using 4,6-dihydroxysalicylic acid as a co-oxidant and N-iodosuccinimide as an oxidant. To the best of our knowledge, the present method provides the first reported synthesis of DTMs from benzylamines via oxidative C-N bond cleavage and subsequent double C-C bond formations. The obtained DTMs were then easily converted into a series of blue dyes upon treatment with tetrachloro-1,4-benzoquinone (chloranil). Although the production of triarylmethane dyes tends to require the use of toxic heavy metals, the present method is advantageous in that it is a metal-free and straightforward process.