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On the Intermediates in Chiral Bis(oxazoline)copper( II )‐Catalyzed Enantioselective Reactions—Experimental and Theoretical Investigations
Author(s) -
Thorhauge Jacob,
Roberson Mark,
Hazell Rita G.,
Jørgensen Karl Anker
Publication year - 2002
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/1521-3765(20020415)8:8<1888::aid-chem1888>3.0.co;2-9
Subject(s) - enantioselective synthesis , oxazoline , copper , catalysis , chemistry , combinatorial chemistry , organic chemistry
The intermediates in the chiral bis(oxazoline)copper( II )‐catalyzed reactions have been investigated by means of experimental and theoretical investigations. It is shown that the absolute configuration of the hetero‐Diels–Alder adduct obtained from the reaction of ethyl glyoxylate with 1,3‐cyclohexadiene in the presence of the chiral bis(phenyloxazoline)copper( II ) is dependent on the solvent. In this case, a linear relationship between the enantiomeric excess ( ee ) and the dielectric constant of the solvent was observed. However, the enantiomeric excess for the adduct obtained with the chiral bis( tert‐ butyloxazoline)copper( II ) complex is independent of the solvent. The addition of different coordinating solvents to the chiral catalysts was investigated and no effect on the enantioselectivity of the reaction was observed. A series of chiral bis( tert ‐butyloxazoline)‐, bis(phenyloxazoline)‐, and bis(indaneoxazoline)copper( II ) complexes has been prepared and characterized by X‐ray analysis, and the similarity between the structures is discussed. For comparison, two related chiral bis( tert‐ butyloxazoline)‐ and bis(phenyloxazoline)zinc( II ) complexes were also prepared and characterized. A series of chiral bis(oxazoline)copper( II )–substrate (the substrate being glyoxal or methyl glyoxylate) complexes was investigated by means of ab initio calculations. Calculation of the total energy of the optimized structure of 17‐, 19‐, and 21‐electron bis(oxazoline)copper( II )–substrate complexes give the 17‐electron complex as the most stable and the most reactive complex, while the 21‐electron complex is less stable and also much less reactive. The optimized structures of both the 17‐electron bis( tert‐ butyloxazoline)‐ and bis(phenyloxazoline)copper( II )–substrate complexes show that the plane of the substrate molecule is twisted by ≈40–45° out of the bis(oxazoline)copper( II ) plane, in agreement with the X‐ray structures. On the basis of the experimental results, X‐ray structures, and ab initio calculations, the structure of the intermediate(s) and reactivity of the chiral bis(oxazoline)copper( II )–substrate complexes are discussed.