Cooperatively catalyzed Henry reaction through directed metal-amine interactions

Obtaining acid-base cooperative interactions between metal and organic amine sites in a heterogeneous synthetic system is a challenging task. There are three architecture levels needed to have a cooperative heterogeneous catalyst (a) the external surrounding that envelopes the catalytic domain, (b) the immediate chemical environment surrounding the catalytic sites and (c) the orientation of the catalytic sites and their respective distances. Flexible and rigid solid catalytic supports have been used to construct synthetic cooperative catalysts.

In this work, the cooperativity between acidic metal and basic amine sites is tested using two different synthetic systems (1) a flexible amine-containing polymer functionalized with metal sites and (2) rigid silica support functionalized with metal and amine sites. The incorporation of the metal active sites in the flexible system was done by grafting of Ti or Sn sites to the hydroxy groups of an amine-containing polymer called chitosan (CS). In the rigid system, Ti or Sn sites were grafted to the hydroxy groups present on the surface of aminopropyl functionalized silica.

In this work, the critical parameters involved in inducing cooperative catalytic interactions in a hybrid metal-CS system as opposed to the more rigid system of functionalized silica is studied. On the latter system, the effect of the properties of the grafted organic tether such as bulkiness and polarity on the cooperative interactions is discussed. The comparative analysis of the cooperative interactions between rigid and more flexible systems provides basic guidelines for catalyst design. The overarching conclusion indicates that synthetically it is easier to obtain a material, which promotes cooperative catalysis by introducing a controlled degree of flexibility to a rigid backbone rather than rigidifying a flexible backbone. All catalysts were characterized using N₂-physisorption, XRD, FTIR, XPS and HRTEM in conjunctions with kinetic analysis.