a. Catalysis by metals

3. ENANTIOSELECTIVE CATALYSIS

Metal salen complexes, such as the Jacobsen’s catalyst, have attracted much interest in the last few decades because of their unique catalytic activity, especially as non- functionalized olefin epoxidation catalysts, in the presence of terminal oxidants like iodosylbenzene (PhIO), sodium hypochlorite (NaOCl), and meta-chloroperoxybenzoic acid (m-CPBA) [1].

 
Salen ligands of Jacobsen’s catalyst bind manganese ions through four atoms, two nitrogen and two oxygen atoms. This tetradentate-binding motif is reminiscent of the porphyrin framework in the heme-based oxidative. Nonetheless, salen derivatives are more easily synthesized than porphyrins and their structures are more easily manipulated to create an asymmetric environment around the active metal site. However, this homogeneous catalyst cannot be separated from the reaction media and, subsequently, cannot be recycled.

Moreover, it suffers deactivation in homogeneous phase by either formation of dimeric μ-oxo-manganese (IV) species, which are inactive in the olefin epoxidation or oxidative degradation of the salen ligand through the imine group. The conventional ways to solve t On the other hand, the immobilized catalysts usually lead to partial loss of activity and/or enantioselectivity as compared to their analogous homogeneous catalysts hese problems are to immobilize Jacobsen-type catalysts onto solid supports. Reports on the heterogenization of Jacobsen-type catalysts have been centered on their covalent binding to organic polymers and on their encapsulation, entrapment, adsorption and covalent attachment to porous inorganic supports, such as zeolites, Si-MCM-41, Al-MCM-41 and clays, and also on activated carbon. Unfortunately, the catalytic activity of the recovered catalyst decreases during the catalytic tests of reuse.

It is found that isolating Mn(III) salen complexes onto a solid support increases the catalyst stability by suppressing the formation of inactive dimeric μ-oxo manganese(IV) species. However, the deactivation route by ligand oxidation cannot be avoided by anchoring the catalyst to a solid matrix, since it depends on the oxidation conditions. It is well known that in homogeneous phase, the catalyst acquires an appropriate geometric configuration that promotes both, the oxygen transfer from oxo-Mn(V) active species to the double bond of olefin and chiral induction [11].

In contrast, when the catalyst is not free to move due to the influence of the solid support, an inappropriate geometrical configuration can be obtained. In order to solve the previous drawbacks, in our group an alternative convenient and economical strategy is under development. It consists in adjust the solubility of homogeneous catalysts by varying the reaction conditions, resulting in the direct separation of the catalysts during the reaction. We found that using dimethyldioxirane (DMD) as oxidizing agent for the epoxidation of various olefins (cis-ethyl cinnamate, R-(+)-limonene, chromenes derivatives) with Jacobsen-type catalysts could facilitate product isolation and catalyst recovery by segregating the catalysts into a solid phase during the reaction. Now day, we are interesting to expand this approach to other olefin types (styrene derivatives), as well to increase the catalyst stability during the reuse. 

Información actualizada: 24 de mayo de 2017