This project is focused on the use of supramolecular catalysts in the H2O2-oxidation of aliphatic and aromatic C-H bonds. Such reaction is currently one of the most investigated chemical transformations in organic synthesis since it allows the direct installation of the desired function into the target molecule, with minimization of the number of synthetic steps and waste production. Furthermore the use of the environmentally friendly H2O2 offers the advantage of non-polluting reduction products.
In particular, in the first phase of the research, a complex based on White's PDP catalytic site, and endowed with benzo18crown6 moieties for the recognition of ammonium groups, already shown to possess unprecedented selectivity properties in the oxidation of primary ammonium ion with long aliphatic chain, will be tested as a supramolecular catalyst for the selective oxidation of substrates with biological interest such as derivatives of fatty acids and cholesterol, properly decorated with the ammonium anchoring group.
In the second phase of the research the effort of the team will be devoted to the realization of a new supramolecular catalyst, again based on a non-heme iron core as the catalytic site and on the benzo18crown6 ligand as the recognition site, but prepared with a different strategy. Using an innovative approach in the field of supramolecular catalysis, the possibility to mount the catalyst directly in the reaction mixture (in situ) without long and expansive synthetic procedure, will be investigated. The imine chemistry will be exploited both to i) assemble the iron catalytic core, an imine-based complex which is already known to efficiently catalyze the H2O2 oxidation of aliphatic and aromatic C-H bonds, ii) insert the recognition site for the binding of the ammonium anchoring group, which hopefully will confer to the catalyst the desired selectivity properties.
The supramolecular approach applied to the oxidation of C-H bonds catalyzed by non-heme iron or manganese complexes is certainly an innovative tool for the realization of unprecedented selectivities in this kind of catalytic processes. So far, a number of non-heme iron or manganese complexes have appeared in the literature, which were demonstrated to be efficient catalysts for the C-H bond oxidations, but no one of them, or very nearly so, is endowed with recognition sites for the binding of the substrate. As a consequence, these systems base their selectivity properties, that is the choice of the C-H bond to oxidize among those present in the substrate, on the relative electronic and steric features of such C-H bonds. In contrast, when the recognition event is involved in the catalytic process, the selectivity is mainly due to the positional factor, that is the relationship between the distance from the recognition and the active sites in the catalyst and the separation between the anchoring group and the C-H bond in the substrate. Supramolecular catalysts 1 and 6 could pave the way for oxidative procedures aimed at obtaining selectivities otherwise hardly reachable. This should be an important goal from a synthetic standpoint since the reaction can be also applied to substrates with biological interest.
The results that could be obtained by the supramolecular imine-based catalyst 6 are also innovative in the field of supramolecular catalysis since the attempt at an in situ assembly of a supramolecular catalyst is not a typical protocol. Most of the times supramolecular catalysts are prepared following long and expensive synthetic paths, while in this case a procedure consisting of no more than two or three synthetic steps would be required, starting from cheap and commercially available materials.
Since the imine-type catalytic core 5 is able to oxidize aromatic C-H bond without sensible deactivation from phenol products, in striking contrast with most of the amino-pyridine based catalytic complexes appeared in the literature so far, complex 6 could be also a supramolecular catalyst for the hardly obtainable selective oxidation of C-H bonds in aromatic compounds endowed with an ammonium anchoring function.