Promoted activity of Cr(Salen) in a nanoreactor for kinetic resolution of terminal epoxides

Abstract

Cr(Salen) catalyzed kinetic resolution of terminal epoxides via asymmetric ring opening (ARO) with TMSN₃ is an important approach for the synthesis of enantiopure 1,2-amino alcohols, however, the high catalyst usage amount (1–2 mol%) impedes its practical applications. An efficient solid nanoreactor was constructed by encapsulation of Cr(Salen) and pyridine in the nanocages of mesoporous silica. This solid nanoreactor exhibits high activity (TOF: 1325 h⁻¹) and high enantioselectivity (91% ee) for the kinetic resolution of 1,2-epoxyhexane via ARO with TMSN₃ at a catalyst concentration as low as 0.002 mol%, whereas the homogeneous counterpart affords almost no conversion of epoxide under similar reaction conditions. The high activity of the solid nanoreactor is mainly attributed to the greatly intensified cooperative activation in the nanocages as evidenced by the sharply increased TOF in parallel with Cr(Salen) concentration in each nanocage. The increased nucleophilicity of Cr(Salen) after coordination to pyridine could also promote the catalytic activity. The solid nanoreactor can be easily separated and recycled. We demonstrated the possibility for designing an efficient solid nanoreactor for asymmetric catalysis by taking the advantages of the cooperative activation.