Catalytic oxidation by polymer-supported copper(II)–l-valine complexes

Abstract

Anchoring of an amino acid l-valine on cross-linked styrene–divinyl benzene was carried out in presence of a base. Reaction of cupric acetate with the polymeric ligand resulted in chelate formation with Cu(II) ion. The immobilized Cu(II) catalysts were characterized by elemental analyses, IR, UV-Vis, SEM, ESR and thermal analysis. Physico-chemical properties like surface area, apparent bulk density, pore volume, etc. have been determined. The supported Cu(II) complexes behave as versatile catalysts in the oxidation of various substrates such as benzyl alcohol, cyclohexanol and styrene in presence of t-butyl hydroperoxide as oxidant. The effect of reaction conditions on conversion and selectivity to products has been studied in detail. Preliminary kinetic experiments reveal that the Cu(II) complexes attached to polymer matrix can be recycled about four times with no major loss in activity.

Introduction

The potential applications of polymer–metal complexes in organic transformations has been reviewed in detail by many groups [1], [2]. The advantages derived from the use of functionalized macromolecules being simplification of the workup, ease of separation of products from reaction mixture, recovery and recycling of catalysts, etc. [3], [4], [5]. Ligands which have been commonly anchored to polystyrene prior to complexation with metal ions include dipyridylamine [6], phosphines [7], β-diketones [8], Schiff bases [9] and multidentate amines [10]. Anchoring of (N, O)-containing ligand such as an α-amino acid directly on a polymer backbone has not been well studied. One of the major problems associated with binding of optically active amino acids is the presence of reactive amino and carboxylic groups. Synthetic sequence commonly involves protection–deprotection strategy of the desired end group to overcome racemization and separation problems which are in addition sensitive to pH of reaction medium. One of the earliest instance of an α-amino acid supported Merrifield’s resin was reported by Petit and Jozefonvicz [11]. This study describes the methods for preparation of Cu(II) complexes with different polymer-bound amino acids. Catalytic activity, however, was not investigated. More recently zeolite encapsulated copper(II)–histidine complexes were shown to exhibit catalytic activity in the oxidation of olefins and alcohols with high turnovers [12]. The interest in these complexes stems from the fact that Cu(II)–amino acids resemble the active center of Cu-enzymes, which are known to participate in selective catalytic oxidation reaction in nature [13], [14].

In continuation of our work on the catalytic application of bidentate Schiff base ((N, O) donor) bearing polymer-supported catalysts [15], [16] we have in the present work devised a convenient synthetic strategy for anchoring an amino acid such as l-valine (l-val) on moderately cross-linked chloromethylated poly(styrene–divinyl benzene) (poly(S-DVB)) support followed by isolation of the corresponding Cu(II) complexes. These newly synthesized compounds have been evaluated as catalysts for the oxidation of benzyl alcohol, cyclohexanol and styrene in presence of t-butyl hydroperoxide (TBHP) as the oxidant under mild conditions. The effect of different reaction parameters on the kinetics of oxidation of benzyl alcohol as model substrate has been examined in detail. A brief study on the recyclability of Cu catalysts has been undertaken.