Synthesis of Cr(III) Salen Complexes as Supramolecular Catalytic Systems for Ring-Opening Reactions of Epoxides

The synthesis of two conformationally restricted Cr(III) salen complexes, 2 and 3, is described. Together, they constitute a supramolecular hydrogen-bonding catalytic system for the recently reported asymmetric ring-opening reactions of epoxides by a dynamic supramolecular catalyst. The synthesis involves state-of-the art transformations in frontline synthetic chemistry applied to heterocyclic chemistry. Hence, palladium-catalyzed reactions were employed, including carbonylative annelation and Suzuki cross-coupling reactions, for the formation of one of the heterocyclic rings (quinolone) and the functionalization of the formed rings. For the construction of the second heterocyclic ring (isoquinolone), a Curtius rearrangement was employed. The corresponding salen ligands were then prepared by Schiff-base reactions, yielding the final complexes after metal insertion. For reference purposes the less conformationally restricted Cr(III) complexes 4 and 5 were also synthesized.