Tert -butylation of naphthalene-2,6-diol and 6-methoxynaphthalen-2-ol

The products of t -butylation of naphthalene-2,6-diol and 6-methoxynaphthalen-2-ol with isobutylene and t - butanol were identified and characterized; the structures of those products were proved by single crystal X-ray structural analysis


Introduction
The t-butylation of α-and β-naphthols has been studied since the middle of 20 th century.There are three general methods for t-butylation reported in the literature: Friedel-Crafts alkylation with t-butyl halide in the presence of Lewis acid, alkylation with isobutylene or with t-butanol in the presence of protic acids.The assignment of the position of t-butyl groups presented major difficulties during past studies and led to many mistakes and non-reproducible results. 1,2Nowadays the chemistry of t-butylation of α-and β-naphthols is well established and the structures of the products have been proved by 2D NMR. 3,4Only the t-butylation of 1,5naphthalenediol 5 and 2,3-naphthalenediol 6,7 have been reported in literature to give 2,6-di-tertbutylnaphthalene-1,5-diol and 6-tert-butylnaphthalene-2,3-diol respectively (Scheme 1).Scheme 1. Published syntheses of tert-butyl naphthols.

Results and Discussion
We recently required 3,7-di-tert-butyl-6-methoxynaphthalen-2-ol as a building block for synthesis of soluble derivatives of peri-xanthenoxanthenes.We decided to use naphthalene-2,6-diol as the starting material and to try the conditions reported for t-butylation of 2-naphthol that involves isobutylene in dichloromethane in the presence of catalytic amount of triflic acid. 3The desired 3,7-di-tert-butylnaphthalene-2,6-diol (2) was obtained in this reaction with only 5% yield mainly due to poor solubility of naphthalene-2,6-diol (1) in dichloromethane.Starting naphthalene-2,6-diol (1) was recovered in 78% yield together with quinoidal products of oxidation besides the desired product 2 that has been isolated, and its structure confirmed by single crystal X-ray structural analysis (Scheme 2).
3,7-Di-tert-butylnaphthalene-2,6-diol (2) proved to be stable in TFA solution up to 40 o C and up to 70 °C in TFA in the presence of excess of t-butanol.At higher temperatures, it starts to lose t-butyl groups returning to naphthalene-2,6-diol (1).3,7-Di-tert-butylnaphthalene-2,6-diol (2) appeared to be unstable in solution to oxidation by air and quickly gave deep blue colored products upon day light irradiation.
2,6-Dimethoxynaphthalene was tested with and t-butanol under conditions listed above but did not give products of t-butylation.

Conclusions
t-Butylation of naphthalene-2,6-diol and 6-methoxynaphthalen-2-ol has been successfully developed and the structure of products obtained unambiguously determined by single crystal X-ray structural analysis.