Enantioselective Allylic Oxidation of Cycloolefins Using Chiral Heterogeneous Catalysts Based on Aliphatic Substituted Oxazoline Ligands

Asymmetric oxidation of allylic C-H bonds of cycloolefins and synthesis of chiral allylic esters, is one of the most important reactions in asymmetric synthesis, that unlike the hydroxylation and epoxidation, the second functional group forms while double bond remains unchanged [1,2]. Despite high selectivity of chiral homogeneous catalysts, because of easy separation of trace amounts of catalyst from the end product, stability under different reaction conditions, reducing metal productions pollution and being consistent with the goals of green chemistry, heterogeneous catalysts have attracted enormous interest. Herein we have focused on the synthesis of recyclable chiral heterogeneous catalysts and their application in copper-catalyzed asymmetric allylic oxidation [3,4]. We first immobilized the chiral amino oxazoline ligands with aliphatic substituent groups ((S)-4-isopropyl-4, 5-dihydrooxazol-2-amine or (S)-4-isobutyl-4, 5-dihydrooxazol-2-amine) on nanoporous silica SBA-15. To probe the catalytic capability of the chiral heterogeneous catalysts, we used them in copper-catalyzed esterification of allylic sp3 C–H bonds of cyclic olefins with variously substituted peresters. In continuation, we optimized the conditions for these reactions by changing some parameters such as temperature, solvents and additives. The chiral allylic esters were achieved in good yields and enantioselectivities.