1H NMR Dynamic study of thermal Z/E isomerization of 5-substituted 2-alkylidene-4-oxothiazolidine derivatives: Barriers to rotation about CC bond
✍ Scribed by R. Marković; M. Baranac; N. Juranić; S. Macura; I. Cekić; D. Minić
- Publisher
- Elsevier Science
- Year
- 2006
- Tongue
- English
- Weight
- 414 KB
- Volume
- 800
- Category
- Article
- ISSN
- 0022-2860
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✦ Synopsis
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 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 DG # 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@C bonds at the C(2)-and C(5)-positions, indicate that the rotational barrier DG # 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 AE AE AE O interactions within the SAC@CAC@O entity. The 13 C NMR Dd C(2)C(2 0 ) 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 0 ) bond, which increases with the electron withdrawing ability of the substituents at the vinylic C(2 0 ) position in the following order: COPh $ COEt > CONHPh > CONHCH 2 CH 2 Ph. The decrease of the Dd C(2)C(2 0 ) values in 2a-d has been discussed for the first time in terms of an estimation of the electron donor capacity of the ASA fragment on the polarization of the C@C bonds.