N-tetradentate spanamine derivatives and their Mn^II-complexes as catalysts for epoxidation of alkenes

Five new tetradentate N-donor ligands with the spirobi(chroman) skeleton (either chiral or racemic) have been synthesized and fully characterized by NMR, ESI-MS, and optical polarimetry. These ligands have been coordinated to a series of manganese salts, affording a family of new chiral and racemic Mn II complexes, which have also been characterized through analytical and spectroscopic techniques. The crystallographic structures of five of them have been obtained by X-ray diffraction; all show a distorted octahedral geometry with the ligand adopting a cis-α conformation over the Mn ion. All the complexes have been tested in the catalytic epoxidation of styrene, showing that both conversion and selectivity values increase in the presence of additives in the catalytic media. Experimental results and computational calculations explain the stereospecificity for the cis epoxide in the epoxidation of cis-β-methylstyrene mediated by one of the complexes synthesized. A new family of tetradentate N-donor spiro ligands and the corresponding mononuclear manganese(II) complexes have been fully characterized. Their catalytic ability in styrene epoxidation in the presence and absence of additives is also described. The stereospecificity of one of the catalysts in the epoxidation of cis-β-methylstyrene has been corroborated by computational calculations. Copyright © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.