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

Abstract

The initial steps of an enantioselective Diels-Alder reaction catalyzed by a CuII-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 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 CuII centers were established. The complexes 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-.

Influence of counterions on the structure of bis(oxazoline)copper(ii) complexes; an EPR and ENDOR investigation

“…Bolm et al, 32,33 previously reported the influence of the counterion and choice of starting metal salt for Diels-Alder reactions using the structurally similar Cu II bis(sulfoximine) complexes. The EPR spectra of the copper-bissulfoximine complexes showed significant differences depending on the starting Cu IIsalt (CuCl 2 , CuBr 2 , Cu(OTf ) 2 and CuCl 2 -AgSbF 6 ).…”

“…One of the first groups to recognise how advanced EPR techniques can be used to probe these counterion (TfO − , SbF 6 − , Cl − , Br − ) effects for the related Cu IIbissulfoximine complexes in the Diels-Alder reaction was Bolm and Gescheidt. 32,33 Therefore in the current investigation we describe the detailed characterisation of the paramagnetic Cu II BOX complexes (shown in Scheme 1) using EPR and ENDOR spectroscopy, with specific emphasis on the influence of the counterion (TfO − or Cl − ) on the structure of the complex in solution.…”

Ligand spheres in asymmetric hetero Diels–Alder reactions catalyzed by Cu(ii) box complexes: experiment and modeling

“…42,43 For Cu(II) bis(sulfoximine) complexes, we have shown that penta coordination occurs in the course of (hetero) Diels-Alder reactions, with counterions and solvent molecules participating in the first coordination sphere. 44,45 A compatible geometry was found by X-ray analysis in the complex [Cu(II)t-Bu(box)(OH 2 ) 2 ](OTf ) 2 . 10 The question is whether such an arrangement of ligands ( penta-coordination) represents a general concept within the course of Cu(II) catalyzed Diels-Alder reactions.…”

Enantioselective Halogenation of β‐Oxo Esters Catalyzed by a Chiral Sulfoximine–Copper Complex

“…Interestingly, these two salts gave opposite enantiomers (Entries 11 and 12) with copper being superior in terms of enantioselectivity (41 % ee). [d] Cu(BF 4 ) 2 71 23 17 [d] Cu(PF 6 ) 2 79 0 18 [d] Cu(SbF 6 ) 2 93 6 19 [d] Cu (ClO 4 ) In previous studies it was shown that counterions are involved in the resulting sulfoximine-copper(II) complex, [25] and consequently their effect on the given catalysis (yielding 9a from 7a) was also studied in this work. Entries 12-19 of Table 3 reveal that their influence on both yield and enantioselectivity was significant with noncoordinating anions such as perchlorate, tetrafluoroborate, and triflate being the best.…”