Umamaheswari, V.Cias, PawelPöppl, AndreasKaupp, MartinGescheidt, Georg2016-06-222016-06-222014https://depositonce.tu-berlin.de/handle/11303/5583http://dx.doi.org/10.14279/depositonce-5212Dieser Beitrag ist mit Zustimmung des Rechteinhabers aufgrund einer (DFG geförderten) Allianz- bzw. Nationallizenz frei zugänglich.This publication is with permission of the rights owner freely accessible due to an Alliance licence and a national licence (funded by the DFG, German Research Foundation) respectively.The stereoselective hetero Diels–Alder reaction between ethyl glyoxylate and cyclohexadiene catalyzed by [Cu(II)t-Bu-(box)](OTf)2 was investigated. The reaction was performed step-by-step and the geometry of the Cu(II) complexes formed in the course of the catalysis was analysed by EPR spectroscopy, advanced pulsed EPR methods (ENDOR, and HYSCORE) and DFT calculations. Our results show that one triflate counterion is directly coordinated to Cu(II) during the catalytic process (axial position). This leads to penta-coordinated Cu(II) complexes. Solvent molecules are able to alter the geometry of the Cu(II) complexes although their coordination is weak. These findings provide an explanation for the solvent and counterion effects observed in many catalytic reactions.en540 Chemie und zugeordnete WissenschaftenArticle1477-922624146076