Synthesis of a heterogeneous metalloenzyme mimic based on Er[III] chemistry

A bifuctional catalyst is a system characterized by cooperatively working general base and acid groups. The main natural example of a polyfunctional system are the enzymes. Metalloenzymes are an interesting subset of polyfunctional catalysts that employ metal ions as Lewis acids and/or redox centers, in conjunction with “organic” functional groups, to enhance reaction rates.1 Acids and bases are antagonists and the cooperative use of both of them in a spatial proximity could generate a self quenching reaction. This problem has been avoided choosing a right combination such as an hard metal ion and a soft base.2 A recent work of Tiseni and Peters3 suggests that the oxophilic lanthanide Er(III) triflate, which combine low price with a relatively small radius, should be advantageous to achieve a rigid transition state. In this work we present the synthesis (Scheme) of an asymmetric heterogeneous bifunctional catalyst characterized by the presence of Er(III) in spatial proximity with an amine group.The catalyst, after full Ft-IR, Raman, ICP-MS and porosimetric characterization, has been tested on the Michael reaction of diethyl malonate and cicloexanone. Data concerning its activity, enentioselectivity, TOF/TON values, recovery and recycling are discussed.