Highly regioselective di-tert-amylation of naphthalene over reusable H-mordenite zeolite

Alaa K. Hashim Al-Khalaf BSc, MSc, PhD, MRSC

Abstract:

Highly regioselective di-tert-amylation of naphthalene using different alcohols can be achieved over a H-mordenite (HM) zeolite. For example, the tert-amylation of naphthalene using tert-amyl alcohol in cyclohexane over HM (Si/Al = 10) zeolite has been opti mised to give a 70% yield of 2,6-dialkylnaphthalenes, of which 2,6-di-tert-amylnaphthalene was produced in 46% yield along with 2-tertamyl-6-tert-butylnaphthalene (23%) and 2,6-di-tert-butylnaphthalene (1%). This has been achieved by varying the reaction time, temperature, pressure and amounts of tert-amyl alcohol and zeolite No 2,7-dialkylnaphthalenes were seen under the conditions tried. The zeolites can be easily regenerated by heating and then reused.

Introduction

There is considerable interest in the use of zeolites to control alkylation of naphthalene,1–14 primarily as a result of the commercial importance of 2,6-dimethylnaphthalene (DMN), a precursor of naphthalene-2,6-dicarboxylic acid (NDA), which is needed for production of poly(ethylene naphthalate) (PEN).

Current methods for the production of DMN are cumbersome and low yielding and require the separation of complex mixtures, including isomers,15 followed by extensive recycling in order to provide a reasonable quantity of product. Consequently, the utilisation of PEN is limited by the commercial availability and cost of DMN. A greener method for production of DMN or an alternative PEN precursor, involving fewer processes, higher yields of desired products, and less need for separation and recycling, is much needed. A convenient process for preparation of DMN would be selective dimethylation of naphthalene. Unfortunately, in addition to the usual problems of polyalkylation, the ?-positions of naphthalene are less reactive than the ?-positions and there are 10 possible isomeric dimethylnaphthalenes, so simple double methylation of naphthalene is not easy to control to give the specific compound required. Mixtures of isomers are produced and require separation.16 Since the 2,6-disubstituted compounds are the most “linear” isomers, zeolites offer a possible way to favour such products. It is well recognised that zeolites and other solid catalysts can play an important role in the development of greener organic syntheses through their abilities to act as recyclable heterogeneous catalysts, support reagents, entrain by-products, avoid aqueous work-ups and enhance product selectivities.17–22 For example, we have shown that zeolites or other solids can have advantages in nitration,23–28 bromination,29 chlorination,30 methanesulfonylation31 and acylation32,33 as well as alkylation34,35 of aromatic compounds. Indeed, a degree of ?-selectivity in dimethylation of naphthalene has been achieved over zeolite HZSM-5, whereas nonselective methylation was seen over the larger pore H-mordenite(HM) and HY zeolites.36 However, even using HZSM-5, a poor yield of the 2,6-isomer was obtained and the ratio of 2,6/2,7

isomers was approximately 1.Since other 2,6-dialkylnaphthalenes can in principle also be oxidised to NDA, attempts to gain greater control have been extended to the use of alternative alkylating agents. Isopropylation of naphthalene with propene,37 isopropyl alcohol38–42 and isopropyl bromide43 has been studied using a variety of zeolites and ?,?-selectivities of over 75%, with a 2,6/2,7 ratio of approximately 2, have been reported using H-mordenite.44 The best isopropylation procedure gave 2,6-di-isopropylnaphthalene in 54% yield, along with 14% of 2,7-di isopropylnaphthalene, i.e. a 2,6/2,7 ratio of 4.44 The best reported cyclohexylation makes use of HY zeolite, and results in the corresponding 2,6-dicyclohexylnaphthalene in 19% yield, along with 17% of 2,7-dicyclohexylnaphthalene, i.e. a 2,6/2,7 ratio of 1.1.45 The same authors also reported tert-butylation to give a 23% yield of 2,6-di-tert-butylnaphthalene, with around 4% of 2,7-di-tert-butylnaphthalene, i.e. a 2,6/2,7 ratio of 5.9, with the added advantage of easy separation of the 2,6-isomer by crystallisation.46,47 By studying the above reaction in more detail, we have previously shown that highly regioselective di-tert-butylation of naphthalene can be achieved over zeolite H-mordenite using tert-butyl alcohol as the alkylating agent under autoclave conditions. 34,35