A Practical and Simple Method of Recycling Catalyst in Asymmetric Aminohydroxylation of Olefins

Abstract

The Os-catalyzed asymmetric aminohydroxylation (AA) of olefins provides a straightforward method for the enantioselective synthesis of a wide variety of protected vicinal aminoalcohols. The resulting chiral β-aminoalcohols group is the most abundant structural element in many biologically active molecules as well as the starting point in the design of many chiral ligands. Although AA reaction servers as a powerful method for the synthesis of a variety of products, its application has still been limited because of the high cost of osmium and chiral ligand. In order to explore the possibility of the repetitive use of ligand and/or osmium, several attempts to immobilize this catalytic system have been made. Nandanan group prepared highly crosslinked copolymers between ethylene glycol dimethacrylate (90 mol%) and a bis(quininyl)pyridazine derivative (10 mol%). This insoluble ligand was then used in AA reaction of various olefins in 52-65% yields and 34-54% ees. Up to now, many insoluble polymer-supported ligands have been successfully reused in AA reaction. Yang first reported an immobilized soluble PEG-bound bis-cinchona alkaloid ligand which could be recovered and reused in homogeneous AA reactions. Excellent yields and ees were obtained in homogeneous system. In most reported recycling methods, osmium component was hardly recovered and sometimes synthesis route of the polymer-supported ligands were complicated. Here we report a recyclable monomeric ligand 1 and its application in homogeneous AA reaction. In addition, poly(ethylene glycol) (PEG, MW 400) linked with the special encapsulating effect on osmium was successfully applied in AD reaction. Enlightened by this, we applied PEG in AA reaction for the recovery of osmium and achieved an amazing result that about 50% amount of osmium component could be efficiently recycled through very simple method.