(5-ETHOXYCARBONYLMETHYLIDENE-4- OXOTHIAZOLIDINE-2-ILYDENE)-1-PHENYLETANONE IN APROTIC MEDIA: ELECTROCHEMICAL AND SPECTRAL STUDY Isidora Ceki ć -Laskovi ć 1,2, Dragica Mini ć 1,2, Rade Markovi ć 2,3, Elena Volanschi 4 1 Faculty of Physical Chemistry, Studentski trg 12, Beograd, University of Belgrade, Serbia, 2 Center for Chemistry ICTM, P.O. Box 473, Belgrade, 3 Faculty of Chemistry, University of Belgrade, Studentski trg 16, Belgrade and 4 Faculty of Chemistry, University of Bucharest, Romania PHYSICAL CHEMISTRY 2010 ABSTRACT Push-pull alkenes, consisting of one or two electron-donating groups (EDG) at the terminus of the C=C bond and one or two electron-withdrawing groups (EWG) at the other terminus, have been widely studied on the account of their low rotational barrier around the C-C double bond. This is attributed to the high degree of polarization, or in valence-bond language, to the importance of zwitterionic limiting forms of the push-pull alkenes as convenient description of their ground states. Previous electrochemical studies of selected 5-substituted 2-alkylidene-4-oxothiazolidines, as a typical representatives of push-pull alkenes, in aprotic polar solvents gave valuable insight on electrochemical behavior of these compounds. The aim of the present work is to study (5-etoxycarbonylmethylidene-4- oxotiazolidine-2-ylidene)-1-phenylethanone (1), synthesized as mixture of (2E,5Z)- and (2Z,5Z)-1 isomers (molar ratio 90/10%), in order to assess the role of the C-C double bond at C(5) position, as well as EWG substituent on the electrochemical behaviour of selected 4-oxothiazolidines in terms of the reduction mechanism and the reactivity of the intermediate species. The comparison between experimental data and theoretical curves, calculated by means of the DigiSim software, indicates an ECECE reaction sequence as a major reaction pathway. It consists of monoelectronic reduction of the investigated compound to the anion radical (E), followed by deprotonation of the substrate by the anion radical to form the anion (C) and reduction of the anion to dianion radical (E). The dianion radical of predominant isomer (2E,5Z)-1 obtained at the second reduction step is observed by both, optical and EPR spectra. OBJECTIVES Electrochemical and spectral investigation of the reduction of (5-etoxycarbonylmethylidene-4-oxotiazolidine-2- ylidene)-1-phenylethanone in DMSO Elucidation of the reduction mechanism Determination of the reaction intermediate species Investigation of the influence of the C(5)=C(5 ' ) bond, as well as electron withdrawing substituent on the electrochemical behaviour of selected 4-oxothiazolidines MATERIALS AND METHODS Cyclic and linear voltammetry with stationary and rotating disc electrode (RDE) Pt working electrode and Pt counter electrode Ag/Ag + reference electrode Solvent: DMSO Numerical simulation, accomplished by the software DigiSim 3.03 Bioanalytical Systems Inc. UV-Vis absorption spectroscopy EPR spectroscopy Semiempirical calculation (PM3 method, HyperChem-7) Fig. 2. RDE curves of the cathodic waves of 1 solution (c = 4 mM) in 0.1 M TBAHFP/DMSO at rotating rates rpm; insert: plot of the limit current density in function of the square root of the rotation rate Fig. 1. Cyclic voltammogram of compound 1 (c = 4 · M) starting with reduction, in 0.1 M TBAHFP/DMSO, in the range -1.6 to 1.25 V, v = 0.1 Vs -1 ; insert: potential range -1.6 to 0.1 V, different scan rates v = 0.1 Vs -1 ; insert: potential range -1.6 to 0.1 V, different scan rates Fig. 5. EPR spectrum obtained by in situ electrochemical reduction of 1 in 0.1 M TBAHFP/DMSO at the potential of the second wave on the voltammogram (a) experimental; (b) simulated spectrum Fig. 4. (a) Absorption spectra registered on electrochemical reduction of compound 1 in 0.1 M TBAHFP/DMSO at the potential in between the first and second reduction wave, (curves 1-7) (b) UV-Vis spectra of 1 at c TBOH /c Substrate molar ratios from 0:1 to 1.5:1 (curves 1-16) (b) UV-Vis spectra of 1 at c TBOH /c Substrate molar ratios from 0:1 to 1.5:1 (curves 1-16) Fig. 6. SOMO at the optimized geometry of dianion radical (2E,5Z) 2 ˉ-1 in DMSO Proposed reduction mechanism: ( E) (2E,5Z) -1 + eˉ (2E,5Z)ˉ˙-1 E o = V, α = 0.5, k s = 1 ּ cms -1 (C) (2E,5Z)ˉ˙-1 + (2E,5Z)-1 (2E,5Z)˙-1 + (2E,5Z)ˉ-1 K eq = 160, k f = 10 6 M -1 s -1 (E) (2E,5Z) eˉ (2E,5Z) 2 ˉ˙-1 E o = -1.3 V, α = 0.5, k s = 0.05 cms -1 (C) (2E,5Z) 2 ˉ˙-1 (2Z,5Z) 2 ˉ˙-1 K eq = 5, k f = 0.2 M -1 s -1 (E) (2E,5Z)˙-1, (2E,5Z)ˉ-1 Pox + eˉ E o = V, α = 0.5, k s = 1 ּ cms -1 Fig. 3. (a) Experimental and (b) simulated cyclic voltammogram curves of 1 in 0,1 M TBAHPF/DMSO, in the potential range -1.6 to 0.1 V, c = 4 · M, v = 0.1 Vs -1, room temperature CONCLUSION The electrochemical results point to an ECECE reaction sequence. Unlike the previously studied related compound (5-etoxycarbonylmethylidene-4-oxothiazolidine-2-ylidene)-N phenylethanamide, where the chemical step following the first ET is E/Z isomerisation, the chemical step in this case is a rapid proton transfer between the electrogenerated base (EGB) anion radical and the substrate i.e. a self-protonation reaction. The proposed ECECE sequence is supported by DigiSim simulations, EPR and UV-Vis spectroelectrochemistry in absence and presence of exogeneous base, which outline the role of the anion radical as EGB. Gas phase and solvent dependent semi-empirical PM3-MO calculations allow the characterization of all intermediate species evidenced by experimental data, in terms of their electronic structure and reactivity. ELECTROCHEMICAL RESULTS SPECTROELECTROCHEMICAL RESULTS AND THEORETICAL CALCULATIONS in DMSO