¹ H NMR Dynamic study of thermal Z/E isomerization of 5-substituted 2-alkylidene-4-oxothiazolidine derivatives: Barriers to rotation about C{double bond, long}C bond

The rotational barriers between the configurational isomers of two structurally related push-pull 4-oxothiazolidines, differing in the number of exocyclic C{double bond, long}C bonds, have been determined by dynamic ¹ H NMR spectroscopy. The equilibrium mixture of (5-ethoxycarbonylmethyl-4-oxothiazolidin-2-ylidene)-1-phenylethanone (1a) in CDCl ₃ at room temperature to 333 K consists of the E- and Z-isomers which are separated by an energy barrier ΔG ^# 98.5 kJ/mol (at 298 K). The variable-temperature ¹ H NMR data for the isomerization of ethyl (5-ethoxycarbonylmethylidene-4-oxothiazolidin-2-ylidene)ethanoate (2b) in DMSO-d ₆ , possessing the two exocyclic C{double bond, long}C bonds at the C(2)- and C(5)-positions, indicate that the rotational barrier ΔG ^# separating the (2E,5Z)-2b and (2Z,5Z)-2b isomers is 100.2 kJ/mol (at 298 K). In a polar solvent-dependent equilibrium the major (2Z,5Z)-form (>90%) is stabilized by the intermolecular resonance-assisted hydrogen bonding and strong 1,5-type S · · · O interactions within the S{single bond}C{double bond, long}C{single bond}C{double bond, long}O entity. The ¹³ C NMR Δδ _C(2)C(2′) values, ranging from 58 to 69 ppm in 1a-d and 49-58 ppm in 2a-d, correlate with the degree of the push-pull character of the exocyclic C(2){double bond, long}C(2′) bond, which increases with the electron withdrawing ability of the substituents at the vinylic C(2′) position in the following order: COPh ∼ COEt > CONHPh > CONHCH ₂ CH ₂ Ph. The decrease of the Δδ _C(2)C(2′) values in 2a-d has been discussed for the first time in terms of an estimation of the electron donor capacity of the {single bond}S{single bond} fragment on the polarization of the C{double bond, long}C bonds.