Novel routes to Heterocyclic Azo compounds

Abstract

The potential use of electron-deficient heterocyclic azo compounds as inkjet dyes was explored. 2-Nitrosopyridine could be used to form a series of azo compounds via the Mills’ reaction with electron-rich aromatic amines. Conditions for this process were optimised by varying solvent and pH. In the presence of ethers, 2-nitrosopyridine is quickly reduced to azoxypyridine. This reaction follows first order kinetics; diethyl and diisopropyl ether react at similar rates, whereas cyclic ethers such as THF are far slower. Organic bases such as Hunigs base were also found to promote formation of azoxypyridine. The mechanism of this reduction was studied. The electrochemistry of 2-nitrosopyridine and azoxypyridine was also explored. Using the optimised conditions for the Mills’ reaction, 2-nitrosopyridine not only reacts with electron-rich amines but also electron-deficient examples. The series was also expanded to include other heterocycles as well as pyridine via the corresponding heterocyclic nitroso compound. Other nitroso compounds prepared were 1- nitrosoisoquinoline, 2-nitrosopyrazine, 4-nitrosopyrimidine and 2-nitrosopyrimidine. The absorption maxima of azo compounds prepared from these precursors were found to correlate with the values for the corresponding azobenzenes. 2-Nitrosopyridine and 2-nitrosopyrimidine react with diamines to give monoazo products. These in turn could be diazotised and coupled with various components to give either bisazo or trisazo compounds. These dyes were tested for their ozone and light fastness properties. The bisazo examples were found to have good ozone fastness but poor light fastness. The pyrimidine examples only showed a slight improvement in ozone and light fastness compared to their pyridine analogue. 2,3-Phthalocyanines are important components in cyan dyes. New routes to precursors of these compounds were explored using flash vacuum pyrolysis (FVP).