An efficient oxidative coupling of naphthols catalyzed by Fe impregnated pillared montmorillonite K10
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Catalysis Letters Vol. 112, Nos. 1–2, November 2006 ( 2006) DOI: 10.1007/s10562-006-0162-3
An efficient oxidative coupling of naphthols catalyzed by Fe impregnated pillared montmorillonite K10 Malhari D. Bhor,* Nitin S. Nandurkar, Mayur J. Bhanushali, and Bhalchandra M. Bhanage Department of Chemistry, Institute of Chemical Technology, N. Parekh Marg, Matunga, Mumbai 400 019, India
Received 11 April 2006; accepted 29 June 2006
Montmorillonite K10 was modified with pillaring solutions of Fe. The catalyst was activated at different temperatures. The resultant heterogeneous catalyst was used for coupling of 2-naphthol and derivatives under mild operating conditions. The activity of catalyst was compared with Fe pillared bentonite, Fe-exchanged montmorillonite K10 and bentonite catalysts. Amongst them, K10-FePLS120 showed better activity and selectivity. The catalyst can be recycled without loss of activity. KEY WORDS: binol; Fe impregnated pillared; montmorillonite K10; oxidative coupling.
1. Introduction Binol (1,1¢-Bi-2-naphthol) has gained considerable interest as an important chiral auxiliary for asymmetric synthesis [1]. Therefore much attention has been paid to the development of binol derivatives, which are versatile sources of the various 1,1¢-binaphthalene skeleton. A convenient and economic way for obtaining the enantiomers is the resolution of racemic 1,1¢-binaphthalene-2,2¢-diol [2]. Various synthetic approaches to this compound have been developed. The typical method of preparation involves transition metals, which acts as a catalyst and/or oxidant. Frequently employed transition metals are Fe(III) and Cu(II); though use of Mn(III), Ti(IV), V(V), Ru(III) have also been reported [3]. This methodology suffers through disadvantages like catalyst/product separation, also demanding for sophisticated techniques such as high-pressure reactors to carry out the reaction when O2/air is used as an oxidant. In the laboratory scale synthesis as well as for the manufacture of large volume chemicals, ‘‘greener’’ catalytic ones with clean, safe and inexpensive oxidant should replace the environmentally unacceptable processes. In this regards few heterogeneous catalysts have been reported, which offers some advantages with regard to ease of isolation of products, however typically high volumes of organic solvents have been used for reaction (15–20 ml of either xylene or chlorobenzene per mmol of 2-naphtol). Solid phase oxidation with FeCl3 and Cu(II)-amine complexes, also various other metal complexes have been immobilized on solid supports for the same reaction [4]. For e.g. FeCl3 adsorbed on alumina, silica gel; CuSO4 on alumina, Fe(III) and Cu(II) exchanged zeolites, clays have also been used [5, 6]. *To whom correspondence should be addressed.
Non-conventional techniques such as microwave [7] and ultrasound [6a] have been reported for the coupling of naphthols. Recently, treatment of sodium-2-naphthoxide with bromine, molecular O2 as oxidant in presence of VO(acac)2 and Ru(OH)x has been reported [8]. The review of l
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