Phase-transfer catalysts in the O-alkylation of 2-hydroxynaphthoquinones

A mild procedure has been developed for the O-alkylation of 2-hydroxynaphthoquinones by using phase-transfer catalysts. Optimal yields were obtained when 18-crown-6 (10 mol%) and tetrabutylammonium iodide (10 mol%) were used as cocatalysts in refluxing tetrahydrofuran containing potassium carbonate. Variability in the product yields (0-82%) was attributed to differences in the reactivity of the alkylating agent and to resonance stabilization of the 2-hydroxynaphthoquinone anion. Comparisons of the UV-visible spectra suggest that the degree of electron delocalization has a marked effect on the outcome of the reaction. Quinoid π-electrons were more delocalized in the absence of a stabilizing counteraction, resulting in lower yields. In the presence of tetrabutylammonium iodide, reduced levels of electron delocalization were detected, possibly as a result of ion pairing between the tetrabutylammonium cation and the C-2 oxygen anion. Enhanced reactivity correlates to a lower level of delocalization, and a combination of both catalysts is necessary to facilitate the O-alkylation.