Selective Hydrogen Transfer in N-(Diphenylmethyl)-1-phenylethan-1-imine

Abstract

Proton transfer processes mediate many organic reactions. How to realize stereochemical control of this process has always been a challenging topic in the field of asymmetric catalysis. In this study, N-(diphenylmethyl)-1-phenylethan-1-imine (Schiff base derived from 2,2-diphenylethan-1-amine and acetophenone) was used as substrate, and different near neutral solvents and various chiral metal complex catalysts were used to carry out photoinduced C=N double bond transfer in the substrate under irradiation with a mercury ultraviolet lamp. The double bond transfer in the substrate molecule was highly selective. Solvents containing strong electronegative atoms like oxygen and chlorine, such as alcohols, aldehydes, and carbon tetrachloride, were more effective than other solvents under high light intensity. The 1,3-proton transfer process involves photoexcitation of the Schiff base and coordination of the latter to the central metal atom of the chiral catalyst, so that the substrate molecule is placed in a stable chiral environment to form a transition state. The strongly electronegative atom of the near-neutral solvent attracts the active α-hydrogen from the excited Schiff base molecule to form a negatively charged delocalized π-bond structure. The subsequent proton addition yields more stable molecular structure to complete the selective proton transfer process. Among the examined chiral catalysts, divalent tin porphyrin was the most effective, and the product yield and enantiomeric excess were 98% and 91.49%, respectively. The described photoinduced C=N double bond transfer in N-(diphenylmethyl)-1-phenylethan-1-imine is characterized by mild conditions (room temperature), high stereoselectivity, and simple operation.