Alteration of the cis: trans isomer ratio in N-methylformamide using lanthanide shift reagents. Determination of the equilibrium constants for complexation

Abstract

The cis–trans conformational equilibrium in a secondary amide is usually disrupted upon the binding of a lanthanide shift reagent (LSR). In this work, n.m.r. spectroscopy is used to monitor the changes in the cis : trans population ratio as well as to measure the lanthanide-induced shifts for each isomer at each concentration of LSR. A theory is developed which allows the determination of equilibrium constants and the limiting shifts for the 1 : 1 amide–LSR complexes from the experimental data. Application to the N-methylformamide–Eu(fod)₃–CCl₄ system reveals that the equilibrium constant for LSR binding to the cis-conformation of N-methylformamide is much larger than for binding to the trans-conformation. This causes the population of the cis-isomer to change from 8% in the absence of LSR to 44% when the molar ratio of LSR to amide is 0.5.