(5-ETHOXYCARBONYLMETHYLIDENE-4-(5-ETHOXYCARBONYLMETHYLIDENE-4-OXOTHIAZOLIDINE-2-ILYDENE)-1-PHENYLETANONE OXOTHIAZOLIDINE-2-ILYDENE)-1-PHENYLETANONE

IN APROTIC MEDIA IN APROTIC MEDIA

ELECTROCHEMICAL AND SPECTRAL STUDYELECTROCHEMICAL AND SPECTRAL STUDYIsidora Cekić-Lasković 12 Dragica Minić 12 Rade Marković23 Elena Volanschi41Faculty of Physical Chemistry Studentski trg 12 Beograd University

of Belgrade Serbia2Center for Chemistry ICTM PO Box 473 11001 Belgrade

3Faculty of Chemistry University of Belgrade Studentski trg 16 11001 Belgrade and

4Faculty of Chemistry University of Bucharest Romania

PHYSICAL CHEMISTRY 2010PHYSICAL CHEMISTRY 2010

ABSTRACTABSTRACT 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 (2E5Z)- and (2Z5Z)-1 isomers (molar ratio 9010) 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 (2E5Z)-1 obtained at the second reduction step is observed by both optical and EPR spectraOBJECTIVESOBJECTIVES 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 METHODSMATERIALS AND METHODS Cyclic and linear voltammetry with stationary and rotating disc electrode (RDE)

bull Pt working electrode and Pt counter electrodebull AgAg+ reference electrodebull Solvent DMSO

Numerical simulation accomplished by the software DigiSim 303 Bioanalytical Systems Inc UV-Vis absorption spectroscopy EPR spectroscopy Semiempirical calculation (PM3 method HyperChem-7)

NH

S

O

H

O

CO2Et

5

25

1

1

-18 -15 -12 -09 -06 -03 00 03-200

-150

-100

-50

0

=3500rpm

1 2 3 4 5 6 7 8-180

-160

-140

-120

-100

-80

-60

-40

-20

i lIIc

A

cm-2

12 (rots)12

|c IIc

Fig 2Fig 2 RDE curves of the cathodic waves of RDE curves of the cathodic waves of 1 1 solution (c = 4 mM) in 01 M TBAHFPDMSO at solution (c = 4 mM) in 01 M TBAHFPDMSO at

rotating rates 100-3500 rpm insert plot of the limit rotating rates 100-3500 rpm insert plot of the limit current density in function of the square root of the current density in function of the square root of the

rotation raterotation rate

-16 -12 -08 -04 00 04 08 12

-20

-15

-10

-5

0

5

10

15

-18 -16 -14 -12 -10 -08 -06 -04 -02 00 02

-50

-40

-30

-20

-10

0

10

20

30

40

j

Ac

m2

E V vs AgAg+

01Vs 05Vs 1Vs

Fig 1 Fig 1 Cyclic voltammogram of compound Cyclic voltammogram of compound 11 (c = 410 (c = 410-3 -3 M) M) starting starting

with reduction in 01 M TBAHFPDMSO in the range -16 to with reduction in 01 M TBAHFPDMSO in the range -16 to 125 V125 V

v = 01 Vsv = 01 Vs-1-1 insert potential range -16 to 01 V different insert potential range -16 to 01 V different scan rates scan rates

-16 -12 -08 -04 00

-20

-15

-10

-5

0

5

E V vs AgAg+

j A

cm-2

-16 -12 -08 -04 00

-20

-15

-10

-5

0

5

E V vs AgAg+

j A

cm-2

1cm = 0778mT

(a)

H 1cm = 0778G

(b)

H

Fig 5 Fig 5 EPR spectrum obtained by EPR spectrum obtained by in situin situ electrochemical reduction of electrochemical reduction of 1 1 in 01 M in 01 M TBAHFPDMSO at the potential of the second wave on the voltammogram (a) experimental TBAHFPDMSO at the potential of the second wave on the voltammogram (a) experimental

(b) simulated spectrum (b) simulated spectrum

350 400 450 500 550

00

01

02

03

04

05 (1)

(7)

(1)

(7)

Abs

orba

nce

nm(a)

280 320 360 400 440 480 520

00

02

04

06

08

10

(b)

(16)

(1)

(16)

Ab

sorb

an

ce

nm

(1)

Fig 4 Fig 4 (a) Absorption spectra registered on electrochemical reduction of compound (a) Absorption spectra registered on electrochemical reduction of compound 11 in 01 in 01 M TBAHFPDMSO at the potential in between the first and second reduction wave (curves 1-M TBAHFPDMSO at the potential in between the first and second reduction wave (curves 1-

7)7) (b) UV-Vis spectra of (b) UV-Vis spectra of 1 1 at cat cTBOHTBOHccSubstrateSubstrate molar ratios from 01 to 151 (curves 1-16) molar ratios from 01 to 151 (curves 1-16)

Fig 6 Fig 6 SOMO at the optimized geometry of dianion radical (2SOMO at the optimized geometry of dianion radical (2EE5Z)5Z)22ˉ-ˉ-1 1 in DMSOin DMSO

Proposed reduction mechanism(E) (2E5Z) -1 + eˉ (2E5Z)ˉ˙-1 Eo = -075 V α = 05 ks = 110-5 cms-1

(C) (2E5Z)ˉ˙-1 + (2E5Z)-1 (2E5Z)˙-1 + (2E5Z)ˉ-1 Keq = 160 kf = 106 M-1s-1

(E) (2E5Z)--1+ eˉ (2E5Z)2ˉ˙-1 Eo = -13 V α = 05 ks = 005 cms-1

(C) (2E5Z)2ˉ˙-1 (2Z5Z)2ˉ˙-1 Keq = 5 kf = 02 M-1s-1

(E) (2E5Z)˙-1 (2E5Z)ˉ-1 rarr Pox + eˉ Eo = +065 V α = 05 ks = 110-4 cms-1

Fig 3 Fig 3 (a) Experimental and (b) simulated (a) Experimental and (b) simulated cyclic cyclic voltammogram curves voltammogram curves of of 11 in 01 M TBAHP in 01 M TBAHPFFDMSODMSO in in the potential range -16 to 01 Vthe potential range -16 to 01 V c = 410 c = 410-3 -3 M v =M v = 0 01 V1 Vss-1-1 room temperature room temperature

CONCLUSIONCONCLUSIONThe 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 EZ isomerisation the chemical step in this case is a rapid proton transfer between the electrogenerated base (EGB) anion radical and the substrate ie 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 RESULTSELECTROCHEMICAL RESULTS SPECTROELECTROCHEMICAL RESULTS AND SPECTROELECTROCHEMICAL RESULTS AND THEORETICAL CALCULATIONS in DMSOTHEORETICAL CALCULATIONS in DMSO

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