Shape-Selective Oxidation of Benzylic Alcohols by a Receptor Functionalized with a Dicopper(II) Pyrazole Complex

A novel metallohost containing a substrate-binding site and two copper ions held by two bis-pyrazole ligand sets is described. The cavity of this molecule can bind dihydroxybenzene guests (association constants in chloroform are in the range K& = 2000-3000 M-1). In the presence of benzylic alcohols the Cu(II) centers of the metallohost are reduced to Cu(I). During this process the alcohols are oxidized to aldehydes. Benzylic alcohols possessing phenolic hydroxyl functions are extremely effective in the reduction reaction. It is believed that they are bound in the cavity of the metallohost and are oriented in the correct position with respect to the copper centers. This results in a rate enhancement of at least 4 orders of magnitude for the oxidation reaction. Introduction Scheme 1 An important theme in the field of homogeneous catalysis is the development of catalysts that display shape selectivity. An enzyme achieves this selectivity by a process of molecular recog­ nition: out of a mixture of substrate molecules one is selected because it has a shape complementary to the pocket of the enzyme. By a multipoint interaction the substrate is bound and oriented in the correct position with respect to a nearby catalytic center. After reaction the enzyme is regenerated because the binding pocket has a lower affinity for the product than for the substrate. The challenge of mimicking the selectivity of enzymes poses numerous problems to the chemist. A substrate usually possesses diverging binding sites . 1 The enzyme mimic must therefore be a concave molecule with converging binding sites. A molecule that has these properties is receptor molecule 1 (see Scheme 1 ), which was developed in our group . 2 It is based on the building block diphenylglycoluril and has the capability of binding neutral molecules. Extensive work has shown that a wide variety of dihydroxybenzenes and other guest molecules can be bound in the cavity of 1 through hydrogen bonding with the carbonyl units of the receptor and tttt stacking interactions with the receptor walls. 3 5 In this paper, we describe a derivative of 1 which is functionalized with two bis-pyrazole type of ligands (compound 2). The ligand arms of 2 can coordinate to two copper centers, yielding a dinuclear metallohost (3). We report that this metallohost displays high selectivity in the oxidation of benzylic