Probing the Mechanism of the Asymmetric Aminolysis of meso-Epoxides Catalyzed by a Proline-Based N,N′-Dioxide-Indium Tris(triflate) Complex

Abstract

An exploration of the mechanism of the aminolysis of meso-epoxides catalyzed by the proline-based N,N′-dioxide-indium tris(triflate) complex was performed using control experiments, UV-Vis spectroscopy, 1H NMR, electrospray ionization mass spectrometry (ESI-MS) and scanning electron microscopy (SEM). Control experiments disclosed that the ligand-to-indium tris(triflate) ratio, the catalyst loading, the concentration, and the presence of 4 A MS have dramatic effects on this reaction with regard to both the yield and the enantioselectivity. Combined with control experiments, UV-Vis spectroscopy, 1H NMR, ESI-MS and SEM analyses revealed that molecular sieves perform multiple functions in the catalysis. A plausible molecular sieves-assisted reaction pathway was proposed. In this pathway, molecular sieves perform (i) as desiccant to in situ dry the reaction system, (ii) as proton transfer agent to accelerate the catalysis, and (iii) as counter ion source to preserve the electroneutrality of the transition states. Besides, the generality of the substrate scope was further explored; excellent yields (up to 99%) and enantioselectivities (up to 99% ee) were obtained.