Rate–pH profile for the formation of 1-hydroxy-8-acetylaminonaphthalene from 2-methylnaphth[1,8-de]-1,3-oxazine in aqueous solution; acid catalysis and inhibition and comparison with the reaction of 1-amino-8-trifluoroacetylaminonaphthalene

Abstract

Formation of 1-hydroxy-8-acetylaminonaphthalene from 2-methylnaphth[1,8-de]-1,3-oxazine in aqueous solution shows a rate maximum at pH ca. 2. The results are compatible with a mechanism involving addition of solvent to the protonated oxazine to give a tetrahedral intermediate which collapses to product. In the range pH 6 to 2 the rate increases with acidity because an increasing fraction of the oxazine is converted into the more reactive protonated species (pK_aca. 2). The unprotonated species is unreactive towards nucleophilic addition of water. Below pH 2 the reaction is inhibited by acid because collapse of the tetrahedral intermediate to the protonated oxazine is catalysed by acid. At pH > 12 the reaction is first-order in hydroxide ion showing that addition of hydroxide ion to the unprotonated oxazine occurs. The favourable reaction of 2-methylnaphth[1,8-de]-1,3-oxazine to 1-hydroxy-8-acetylaminonaphthalene contrasts with the behaviour of 1-amino-8-trifluoroacetylaminonaphthalene for which the preferred direction of reaction is the elimination of water in favour of the cyclic product 2-trifluoromethylperimidine.