On the Intermediates in Chiral Bis(oxazoline)copper(II)-Catalyzed Enantioselective Reactions-Experimental and Theoretical Investigations

摘要

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 trelated 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 inito 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 approx=40-45deg 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 calcualtions, the structure of the intrmediate(s) and reactivity of the chiral bis(oxazolien)copper(II)-substrate complexes are discussed.