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.
Key words
alkylations - quinones - phase-transfer catalysis - crown compounds
