Mechanistic Insights into Stereoselective Catalysis—The Effects of Counterions in a Cu II –Bissulfoximine‐Catalyzed Diels–Alder Reaction

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 (CW‐EPR, 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 CuX 2 (X=Cl − , Br − , TfO − , SbF 6 − ) salts, the chiral bissulfoximine ligand ( S , S )‐ 1 , and N ‐(1‐oxoprop‐2‐en‐1‐yl)oxazolidin‐2‐one ( 2 ) as the substrate in CH 2 Cl 2 were investigated in frozen and fluid solution. In all cases, penta‐ or hexacoordinated Cu II centers were established. The complexes with counterions indicating high stereoselectivity ( TfO − and SbF 6 − ) 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 SbF 6 − .