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چکیده
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Chiral bisoxazoline ligand-metal complexes have received a great deal of attention through their use in various metal-catalyst asymmetric reactions [1]. On the other hand, enantioselective allylic oxidation of alkenes using organic peresters and copper chiral complexes has been a subject of great interest during the last decade. In addition, this reaction provides an access to chiral allylic alcohols which are key intermediates in synthesis of natural products [2]. Herein, at first, we report several new methods for synthesis of chiral C2-symmetric bisoxazoline ligands (1-3) and synthesis of some new chiral C1-symmetric oxazoline ligands (4) in high yield and enantiomeric excess. Then, the chiral copper complexes of these ligands were prepared and their reactivity was investigated as homogenous catalysts in asymmetric allylic oxidation of cycloolefins. Interestingly, when biphenylbisoxazoline ligands (3) were coordinated to tetrakis(acetonitrile) copper(I) hexafluorophosphate, it resulted in the formation of only a single diastereomer complex (S,aS,S). Finally, the enantioselectivity, yield and rate of this reaction were optimized under different conditions, such as change of solvents, temperature and additives and also using various copper salts. Surprisingly, when SBA-15 nanoporous silica material was employed in the reaction, nearly complete enantioselectivities (up to 97% ee) and excellent yields (up to 99%) of the corresponding chiral allylic esters were obtained in a reasonably short period of time [3-5]. (Scheme 1)
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