Efficient Preparation of Nitrosoarenes for the Synthesis of Azobenzenes^†

Reaction conditions are described for the oxidation of anilines furnishing nitrosoarenes and the synthesis of unsymmetrically substituted azobenzenes. In a comparative study, the catalytic oxidation of methyl 4-aminobenzoate by hydrogen peroxide was investigated, and SeO₂ proved to be superior or equal to methyl trioxorhenium (MTO) and Na₂WO₄, respectively. Nevertheless, the application of the inexpensive, environmentally friendly, Oxone in a biphasic system proved to be more efficient, and a variety of useful nitrosoarenes for the synthesis of azo compounds were prepared in high yield and purity on a large scale.