An easily recoverable and recyclable homogeneous polyester-based Pd catalytic system for the hydrogenation of α,β-unsaturated carbonyl compounds

Highlights

• trans-[Pd(OAc)₂(L)₂] was synthesized from Pd(OAc)₂ and the poly(l-lactide) (L).

• trans-[Pd(OAc)₂(L)₂] was used as catalytic precursor in hydrogenations.

• trans-[Pd(OAc)₂(L)₂] is a homogeneous catalytic precursor.

• The homogeneous catalyst can be easily recovered from the reaction mixture.

• The catalytic system maintained its activity in recycle experiments.

Abstract

Homogeneous catalysis is an efficient tool to carry out hydrogenation processes but the major drawback is represented by the separation of the expensive catalyst from the product mixture. In this view we prepared a polyester-based Pd catalytic system that offers the advantages of both homogenous and heterogeneous catalyses: efficacy, selectivity and recyclability. Here its application in the hydrogenation of selected α,β-unsaturated carbonyl compounds is described.

Introduction

The selective hydrogenation of α,β-unsaturated carbonyl compounds is an important process in the manufacture of some useful fine chemicals as intermediates for the synthesis of pharmaceuticals, additives for food flavors and valuable building blocks for fragrances [1]. The selective reduction of α,β-unsaturated carbonyl compounds by means of molecular hydrogen in the presence of catalysts still remains an intriguing challenge in catalysis [2]. The selectivity, including enantioselectivity, strongly depends on the nature of the active metal catalyst [3] and on the reaction conditions [4]. Homogeneous catalysts often present good activity and selectivity, but the major drawback of homogeneous processes is the separation of the precious catalyst from the product mixture that requires an energy intensive process such as distillation. Hence several attempts have been accomplished in order to facilitate the separation of the catalyst from the reaction medium like the use of biphasic systems [5] or supported metal complexes [6]. In the realm of Pd-based catalysts the hydrogenation reaction has been carried out in the presence of numerous systems [7]. Interesting results have been achieved with polymer-bound palladium complexes in heterogeneous reactions [8], [9], [10]. Recently, an innovative approach to recycle a catalytic system has been proposed. It is based on a homogeneous system consisting in a polyester-based Pd-macrocomplex that can be recovered by its precipitation and easy separation from the reaction mixture. The catalyst is prepared by reaction of Pd(OAc)₂ with 4-pyridinemethylene-end-capped poly(l-lactide) (macroligand L), as shown in Scheme 1 [11].

The resulting Pd-pyridine poly(l-lactide) macrocomplex, trans-[Pd(OAc)₂(L)₂], has been applied by some of us to catalyze the aerobic oxidation of selected primary and secondary alcohols [12]. Here we report on the capability of trans-[Pd(OAc)₂(L)₂] to be used as catalyst in the hydrogenation reaction of selected α,β-unsaturated carbonyl compounds, precisely 2-cyclohexen-1-one (I), (3E)-4-phenylbut-3-en-2-one (IV), (3E)-4-(6-methoxy-2-naphthyl)but-3-en-2-one (VII), (2E)-3-phenylprop-2-enal (X) and (2E)-3-(1,3-benzodioxol-5-yil)-2-methyl-prop-2-enal (XIII). The macrocomplex is soluble under the applied catalytic conditions but, upon addition of a hydrocarbon solvent or preferably methanol, the polymer-anchored Pd catalyst precipitates allowing the separation from the reaction solution and, thus, it is easily recyclable. This system therefore combines the activity and selectivity of a homogeneous catalyst with the easy recovery and recyclability of a heterogeneous one without making any change to the catalyst structure.