New chiral bidentate ligands containing thiazolyl and pyridyl donors for copper-catalyzed asymmetric allylic oxidation of cyclohexene

Chiral bidentate ligands 1–3, which contain a combination of thiazolyl and pyridyl donors units, were prepared. The syntheses are facile and being based on Kröhnke condensation of a pinene derivative to form the pyridine ring. Modification at the 8-position of the tetrahydroquinoline ring can be carried out by alkylation reaction with 2a and 3a but not 1a. The structure of a copper(II) perchlorate complex of 1a was characterized with X-ray crystallography, which reveals the binding of the pyridyl–thiazole as a N–N donors at the copper center. The copper(I) thiazolyl–pyridine complexes prepared in situ are active catalysts in the enantioselective allylic oxidation of cyclohexene using tert-butyl perbenzoate as the oxidant. The isolated yields of the allylic benzoate were up to 98%, and enantioselectivity was up to 62% e.e.

Graphical abstractChiral bidentate pyridyl–thiazole ligands were prepared. The structure of a copper(II) perchlorate ligand complex was characterized with X-ray crystallography. The copper(I) thiazolyl–pyridine complexes prepared in situ are active catalysts in the enantioselective allylic oxidation of cyclohexene.Figure optionsDownload full-size imageDownload as PowerPoint slide