Intramolecular Acceleration of Asymmetric Epoxide Ring-Opening by Dendritic Polyglycerol Salen-CrIII Complexes

We present the synthesis of symmetrical (pyrrohdine-salen)Cr-III complexes immobilized on hyperbranched polyglycerol 1 through linkers of different lengths and their application in the asymmetric ring-opening of meso-epoxides. This reaction proceeded through a cooperative bimetallic mechanism and for the polymeric catalysts a positive dendritic effect with regard to the reaction rate was found. In addition, the introduction of long linkers (C6, C10, and C18) forced the favored head-to-tail orientation of two catalyst molecules and led to greater enantioselectivity with ee values of 48% (cyclohexene oxide) and 64% (cyclopentene oxide) for the ring-opening of meso-epoxides with TMSN3 catalyzed by hPG-C10-CrCl (13). The soluble polyglycerol-supported catalyst was recovered five times by dialysis to afford similar activities and a 10% increase in the enantioselectivity. ((C) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)