The reduction of some o-Nitrophenylazo dyes with thiourea S,S-Dioxide (Formamidinesulfinic acid): a general synthesis of 2-Aryl-2H-benzotriazoles and their 1-Oxides

Abstract

Thiourea S,S-dioxide (formamidinesulfinic acid) reacts with o-nitrophenylazo dyes in ethanolic alkali to give, depending on the reaction conditions, either the corresponding 2-aryl-2H-benzo-triazoles or their 1-oxides. The reagent is particularly effective for preparing in high yield 2-(2'H- benzotriazol-2'-yl)phenols, which are important ultraviolet absorbers. The corresponding reductions involving ammonium sulfide and sodium dithionite were also examined. The former reagent was unsatisfactory; the latter reagent behaved erratically and was a less effective reducing reagent than thiourea dioxide. Nevertheless, conditions were established whereby sodium dithionite furnished the 2-(2'H-benzotriazol-2'-yl)phenols in high yield and purity.