7-alkyl-1,3,7-triazapyrenium salts: Rare event of oxidative hydroxylation under the conditions of acid catalysis

Abstract

The transformation of N-alkylpyridinium salts into the corresponding N-alkylpyridones under the treatment with an alkaline solution of K3[Fe(CN)6] discovered by Decker to the end of XIX century [1] became a standard preparation procedure for N-alkylpyridones [2]. This reaction was later extended to numerous other heteroaromatic cations [3, 4]. The potassium ferrocyanide remains the classic oxidant in this reaction, more seldom oxygen [5] and other oxidants [6] are employed. The general condition of the reaction is the alkaline water medium. We formerly reported on the synthesis and oxidative hydroxylation of 1and 7-alkyl-1,3,7-triazapyrenium salts [7]. In the system KOH–K3[Fe(CN)6]–H2O they formed 2and 6-oxoderivatives respectively. We report here on an uncommon reaction of oxidative hydroxylation of 7-alkyl-1,3,7-triazapyrenium salts Ia–Ic under acid catalysis. It was found that at the addition of K3[Fe(CN)6] to the solution of salts Ia–Ic in 60% sulfuric acid the reaction readily proceeded at room temperature yielding the products of oxidative hydroxylation IIa–IIc. The probable mechanism of this reaction involves the protonation of salt I, the nucleophilic attack on dication A of a water molecule, and the subsequent oxidation of the product of the covalent hydration, cation of pseudobase B (or of the pseudobase itself). In such reactions occurring in alkaline medium the anion of the pseudobase