H-1 NMR dynamic study of thermal Z/E isomerization of 5-substituted 2-alkylidene-4-oxothiazolidine derivatives: Barriers to rotation about C=C bond

Abstract

The rotational barriers between the configurational isomers of two structurally related push-pull 4-oxothiazolidines, differing in the number of exocyclic C=C bonds, have been determined by dynamic H-1 NMR spectroscopy. The equilibrium mixture of (5-ethoxy-carbonylmethyl-4-oxothiazolidin-2-ylidene)-1-phenylethanone (1a) in CDCl3 at room temperature to 333 K consists of the E- and Z-isomers which are separated by an energy barrier Delta G(#) 98.5 kJ/mol (at 298 K). The variable-temperature H-1 NMR data for the isomerization of ethyl (5-ethoxycarbonylmethylidene-4-oxothiazolidin-2-ylidene)ethanoate (2b) in DMSO-d(6), possessing the two exocyclic C=C bonds at the C(2)- and C(5)-positions, indicate that the rotational barrier Delta G(#) 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 ( gt 90%) is stabilized by the intermolecular resonance-assisted hydrogen bonding and strong 1,5-type (SO)-O-... interactions within the S-C=C-C=O entity. The C-13 NMR Delta delta(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)=C(2') bond, which increases with the electron withdrawing ability of the substituents at the vinylic C(2') position in the following order: COPh similar to COEt gt CONHPh gt CONHCH2CH2Ph. The decrease of the Delta delta(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 -S- fragment on the polarization of the C=C bonds. (c) 2006 Elsevier B.V. All rights reserved.