Investigation of a new oxidative catalytic system involving Jacobsen's catalyst in the absence of organic solvents

Abstract

In this work, the commercially available Jacobsen's catalyst, Mn(salen), was occluded in a hybrid polymeric membrane based on poly(dimethylsiloxane) (PDMS). This system was used as a catalytic barrier between two different phases: an organic substrate phase (cyclohexane, cyclooctene, cyclohexene or styrene) in the absence of solvent, and an aqueous solution of either t-BuOOH or H₂O₂. Such system was characterized by UV–vis spectroscopy, TGA, DTA, DSC, and SEM techniques. The occluded complex proved to be an efficient catalyst for the oxidation of alkanes and alkenes when t-BuOOH was used as oxidant. However, the hybrid polymeric membrane was a barrier against H₂O₂, preventing this oxidant from reaching the bulk of the membrane and oxidizing the substrate.

Introduction

The development of catalysts based on non-porphyrinic transition metal complexes for the oxidation of organic substrates is a subject of current research interest. In this context, salen appears as being the most versatile and thoroughly studied ligand [1], [2]. Its reportedly simple synthesis, easy handling, commercial availability, and high activity and selectivity are some of the advantages that have led researchers to use Mn(salen) complexes in the oxidation of a wide range of organic compounds [3]. The most widely studied salen complex, the Jacobsen's catalyst, is active in the asymmetric epoxidation synthesis of unfunctionalized olefins. This reaction is considered to be a progress toward the highly enantioselective asymmetric epoxidation synthesis of simple olefins conjugated with alkenyl, alkynyl or aryl groups [4].

In the case of both porphyrinic and non-porphyrinic catalysts, homogeneous catalysis often provides the best results in terms of product yield, whereas heterogeneous catalysis offers advantages such as easy product purification and potential catalyst recycling [5], [6], [7]. Such advantages have led both organic and industrial chemists to develop heterogenized catalysts by using suitable supports [8], [9]. Manganese complexes based on Schiff's base are known to be active homogeneous catalysts in oxidation reactions in the presence of a variety of oxidants [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], and their immobilization on solid supports combines easy product separation with the good selectivity of the catalytic reaction. Therefore, a variety of supports have been studied, such as clays [20], silica [4], [21], [22] and zeolites [23], [24], [25], [26], [27], [28], [29], [30]. The immobilization of Mn(salen) complexes on polymeric supports has also received considerable attention [31], [32], [33], [34].

Elastomeric and dense membranes based on poly(dimethylsiloxane) (PDMS) exhibit good thermal and chemical stabilities. They have been successfully applied to the immobilization of oxidation catalysts, such as metalloporphyrins, metallophthalocyanines, and Mn(salen) [35], [36], [37], [38]. These membranes can also be used to keep the two liquid reagent phases apart, thus completely eliminating the use of a solvent.

In a previous work, our group compared the catalytic activities of three ironporphyrin generations, Fe(TPP)Cl ¹, Fe(TDCPP)Cl ² and Fe(PCl₈)Cl ³, occluded in a hybrid polymeric membrane based on poly(dimethylsiloxane) (PDMS) [35]. These systems gave rise to added benefits when compared to the conventional supported catalytic systems, especially when the great ability of the polymeric support to concentrate the reagents next to the catalyst in diluted conditions is concerned. Because these hybrid membranes consist of hydrophobic PDMS chains crosslinked by polar organic clusters [39], their design is an alternative approach to the production of multi-functional materials with polar and apolar nano-domains, leading to different intrinsic properties, and thus to a vast number of applications.

In this work, we used hybrid polymeric membranes based on poly(dimethylsiloxane) (PDMS) as supports for the occlusion of Jacobsen's catalyst (Fig. 1). The resulting system was used as a catalytic barrier between an aqueous solution of either t-BuOOH or H₂O₂, and an organic substrate phase (cyclohexane, cyclooctene, cyclohexene or styrene). This triphasic system containing a cheap and easily available catalyst allowed substrate oxidation and easy product separation. We also investigated the role of this membrane support on the reactivity of the encapsulated Jacobsen's catalyst.