Configurational isomerization of push–pull thiazolidinone derivatives controlled by intermolecular and intramolecular RAHB: 1H NMR dynamic investigation of concentration and temperature effects

Abstract

Abstract1H NMR spectroscopy was used to investigate hydrogen bonding in the structurally related (Z)‐ and (E)‐5‐substituted‐2‐alkylidene‐4‐oxothiazolidines in polar and apolar solvents. The equilibrated mixtures of these typical push–pull alkenes consist of the intramolecularly H‐bonded E‐isomer and intermolecularly H‐bonded Z‐isomer in varying proportions which depend on the solvent polarity. For a representative of the series, (E)‐(5‐ethoxycarbonylmethyl‐4‐oxothiazolidin‐2‐ylidene)‐1‐phenylethanone (1), the lack of a concentration and temperature dependence of the large chemical NH shift (δ 12.06 ppm) in CDCl3 indicates strong intramolecular resonance‐assisted hydrogen‐bond formation (RAHB). The upfield chemical shifts of the NH proton of the (Z)‐1 isomer as a function of temperature increase and the large 1H NMR Δδ/ΔT value (−11.82 ppb°C−1, Z/E=60 : 40, or −10.33 ppb°C−1, Z/E=20 : 80) in CDCl3 are explained in terms of a decrease in intermolecular H‐bonding resulting in a greater amount of free or unassociated Z‐isomer. Copyright © 2004 John Wiley & Sons, Ltd.Additional material for this paper is available in Wiley Interscience