Aqueous Na₂WO₄/H₂O₂: an efficient tunable catalytic medium for selective oxidation of β-naphthol to diverse products

The aqueous-phase oxidation of β-naphthol 1 with hydrogen peroxide in the presence of sodium tungstate dihydrate (Na₂WO₄) and various basic or acidic reagents demonstrated remarkable condition-dependent product selectivity. By simple adjustment of the conditions, the transformation of 1 would be directed towards one of the products of choice 2-6 in high yields. In the presence of NaOH, Lawsone 2 was obtained quickly and efficiently. In contrast, use of a mild acidic reagent (acetic acid) afforded 1,1'-bi-2-naphthol 3, selectively. Alternatively, higher concentrations of acetic acid promoted an oxidation-dimerization-cyclization process to give benzochromene 4. By the replacement of AcOH with formic acid or ClCH₂CO₂H, the selectivity of the process shifted towards the high yield formation of 5, and 6, respectively. The results emphasized that the pivotal role of aqueous media and the relative pH of the reaction provided access to high yields of structurally diverse products in short reaction times. At the same time, the operations were quite simple and capable of being processed at higher scales, as demonstrated for ten-gram synthesis of Lawsone 2. In all reactions, the catalyst could be successfully recycled and reused efficiently in the next reactions, while products are obtained directly without the need for cumbersome and costly chromatographic operations.