Mechanistic insights into stereoselective catalysis - The effects of counterions in a Cu-II-bissulfoximine-catalyzed Diels-Alder reaction

摘要

The initial steps of an enantioselective Diels-Alder reaction catalyzed by a Cu-II-bissulfoximine complex were followed by EXAFS (EXAFS= extended X-ray absorption fine structure), EPR (EPR electron paramagnetic resonance) spectroscopy (CWEPR, FID-detected EPR, pulse ENDOR, HYSCORE; CW=continuous wave; ENDOR=electron nuclear double resonance; HYSCORE-hyperfine sublevel correlation; FID = free induction decay), and UV-visible spectroscopy. The complexes formed between the parent CuX2 (X = Cl-, Br-, TfO-, SbF6-) salts, the chiral bissulfoximine ligand (S,S)-1, and N-(1-oxoprop-2-en-1-yl)oxazolidin-2-one (2) as the substrate in CH2Cl2 were investigated in frozen and fluid solution. In all cases, penta- or hexacoordinated Cu-II centers were established. The com-plexes with counterions indicating high stereoselectivity (TfO- and SbF6-) reveal one unique species in which substrate 2 binds to pseudoequatorial positions (via O atoms), shifting the counterions to axial locations. On the other hand, those lacking stereoselectivity (X = Cl- and Br-) form two species in which the parent halogen anions remain at equatorial positions preventing the formation of geometries compatible with those found for X = TfO- and SbF6-.