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

Abstract

1H 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.