ELECTROCHEMICALLY FREE

RADICALS

COMMUNICATION REDUCTION IN DIME

GENERATED

7. MECHANISM OF ETHYLENIC

OF

THE

DICARBONYL

ELECTROCHEMICAL COMPOUNDS

THYLFORMAMIDE A. V. II'yasov, Yu. and V. Z. Kondranina

M.

UDC 541.124:541.138.3:541.515:547.442.8

Kargin,

The e l e c t r o c h e m i c a l reduction of dicarbonyl compounds with C = O groups conjugated through an ethylenic bond in aprotonic m e d i u m has been little studied. It has been shown [1] that t r a n s - d i b e n z o y l ethylene in d i m e t h y l f o r m a m i d e (DMFA) is reduced in three steps, the f i r s t of which is r e v e r s i b l e . E s t e r s of m a l e i c and f u m a r i c acids in pyridine a r e reduced in two o n e - e l e c t r o n steps with s a t u r a t i o n of the C = C bond [2]. Our p r e l i m i n a r y data [3] indicate that the reduction of ethylenic dicarbonyl compounds has a numb e r of p e c u l i a r i t i e s that distinguish this group of compounds f r o m a r o m a t i c dicarbonyl compounds [4, 5]. In this work we p r e s e n t the r e s u l t s of an investigation of the m e c h a n i s m of the reduction of c e r t a i n ethylenic dicarbonyl compounds in DMFA on a m e r c u r y e l e c t r o d e by a complex of e l e c t r o c h e m i c a l methods. EXPERIME

NTA

L

The a p p a r a t u s , conditions of the e x p e r i m e n t , and purification of the solvent w e r e d e s c r i b e d e a r l i e r [5, 6]. In all c a s e s except w h e r e o t h e r w i s e stipulated, the t e m p e r a t u r e was 25 ~ concentration of Et4NI 0.08 M, concentration of the d e p o l a r i z e r 1 9 10 -3 M, dropping period of the m e r c u r y e l e c t r o d e (forced) 0.5 sec, and r a t e of outflux of m e r c u r y f r o m the c a p i l l a r y 0.64 m g / s e c . Industrial p r e p a r a t i o n s of dimethyl e s t e r s of f u m a r i c and m a l e i c acids w e r e used. Diphenyl f u m a r a t e and diphenyl m a l e a t e w e r e synthesized a c c o r d i n g to the well-known methods of [7]. The p r e p a r a t i o n of t r a n s - d i b e n z o y l e t h y l e n e was the s a m e as that used e a r l i e r [8]. c i s - D i b e n z o y l e t h y l e n e (mp 131-133 ~ coincides with [9]) was produced by i r r a d i a t i o n of a solution of t r a n s - d i b e n z o y l e t h y l e n e in methanol with sunlight. The constants of all the s u b s t a n c e s a f t e r r e c r y s t a l l i z a t i o n o r v a c u u m redistillation c o r r e s p o n d e d to the l i t e r a t u r e data.

i,10: A

i. i0 v,A a

1

a

,

l

~

-V

Fig. 1. C l a s s i c a l p o l a r o g r a m s (1) and p o l a r o g r a m s switched a c cording to s c h e m e II (see [5, 6]) (2) for solutions in DMFA, M: a) t r a n s - d i b e n z o y l e t h y l e n e , 1.10"3; b) dimethyl f u m a r a t e , 1" 10-3; c) dimethyl m a l e a t e , 0.7 9 10 -3. A. E. Arbuzov Institute of Organic and Physical C h e m i s t r y , A c a d e m y of Sciences of the USSR. T r a n s l a t e d f r o m I z v e s t i y a Akademii Nauk SSSR, Seriya Khimicheskaya, No, 5, pp. 927-932, May, 1971. Original a r t i c l e submitted July 21, 1969. 9 1971 Consultants Bureau, a division of Plenum Publishing Corporation, 227 West 17th Street, New York, N. Y. 10011. All rights reserved. This article cannot be reproduced for any purpose whatsoever without permission of the publisher. A copy of this article is available from the publisher for $15.00.

850

DISCUSSION

OF

RESULTS

Table 1 p r e s e n t s the p o l a r o g r a p h i e c h a r a c t e r i s t i c s of the e t h y l d i e a r b o n y I compounds studied.

r

-e,-:

O L) o +4-

~9 D O

I I+-~l I r

O

I f+-~.l I o

O

O

O

~2 O

m

|

q~

On the c l a s s i c a l p o l a r o g r a m s of t r a n s - d i b e n z o y l e t h y l e n e , t h r e e w a v e s a r e o b s e r v e d (Fig. l a ) , which was a l r e a d y noted in [1]. The f i r s t wave c o r r e s p o n d s to the r e v e r s i b l e (see F i g s . l a and 2c) t r a n s f e r of one e l e c t r o n to a m o l e c u l e , a p p a r e n t l y f o r m i n g an anion r a d i c a l . The n u m b e r of e l e c t r o n s , j u s t as b e f o r e [4], was e s t i m a t e d by a c o m p a r i s o n method. The r a t i o of the height of the anodic s w i t c h e d wave to the height of the cathodic wave ia/i c = 0.69 a g r e e s with the v a l u e s for other r e v e r s i b l e p r o c e s s e s with r e l a t i v e l y s t a b l e anion r a d i c a l s [5]. The second wave has a c o n s i d e r a b l y l o w e r height than the first, and, depending on the e x p e r i m e n t a l conditions, c o m p r i s e s ~30% of the height of the f i r s t o n e - e l e c t r o n wave. To d e t e r m i n e the n a t u r e of this p r o c e s s we s t u d i e d the influence of the c o n c e n t r a t i o n of the d e p o l a r i z e r , d r o p p i n g p e r i o d , and t e m p e r a t u r e on the height of the wave and a l s o s t u d i e d s w i t c h e d p o l a r o g r a m s . Although the r a t i o hl/t11/G* for the f i r s t wave is c o n stant within ~1.9~c when t 1 is v a r i e d f r o m 0.18 to 4.45, for the s e c ond wave a s u b s t a n t i a l d e c r e a s e in the value of h2/tl J~ is o b s e r v e d (from 30.7 to 6.84 when t 1 is i n c r e a s e d f r o m 0.18 to 2.58 see). The r a t i o h J C , in c o n t r a s t to hl/C, does not r e m a i n c o n s t a n t : for t r a n s d i b e n z o y I e t h y l e n e c o n c e n t r a t i o n s 0.5, 1.0, and 1.5 mM, it is equal to 22, 20, and 16.7, r e s p e c t i v e l y . The a c t i v a t i o n e n e r g y of the l i m i t i n g c u r r e n t of the f i r s t wave (3.02 k c a l / m o l e ) c o i n c i d e s with the a c t i v a t i o n e n e r g y for o t h e r p r o c e s s e s l i m i t e d by diffusion [6, 10]. The height of the s e c o n d wave i n c r e a s e s with d e c r e a s i n g ternp e r a t u r e . The switched p o l a r o g r a m t a k e s the c h a r a c t e r i s t i c f o r m (see Fig. l a ) . As soon as the a u x i l i a r y p o t e n t i a l r e a c h e s the v a l u e s of the p o t e n t i a l s of the s e c o n d wave, the anodie c u r r e n t due to o x i d a t i o n of the p a r t i c l e s f o r m e d on the e l e c t r o d e d r o p s . At the a u x i l i a r y p o t e n t i a l on the l i m i t i n g c u r r e n t of the s e c o n d wave, the p r o c e s s b e c o m e s e n t i r e l y i r r e v e r s i b l e . It should be e m p h a s i z e d that with a c e r t a i n c o m b i n a t i o n of conditions (dropping p e r i o d , ternp e r a t u r e ) , no change in the l i m i t i n g c u r r e n t in the r e g i o n of p o t e n t i a l s of the s e c o n d wave can be noted, and without a c o n s i d e r a t i o n of the s w i t c h e d p o l a r o g r a m s , the t r a n s f e r of the s e c o n d e l e c t r o n in this r e g i o n of p o t e n t i a l s m a y be l o s t sight of. The o b s e r v a t i o n s and d e p e n d e n c e s noted above can be e x p l a i n e d on the a s s u m p t i o n that at the p o t e n t i a l s of the s e c o n d wave t h e r e is a t r a n s f e r of an e l e c t r o n to the a n i o n r a d i c a l A ' , c o m p l i c a t e d by a c h e m i c a l r e a c t i o n of A = with the i n i t i a l m o l e c u i e A a c c o r d i n g to the s c h e m e f i r s t wave

D c~

o O

~ o o o o --

--

--

<

N ~ ~ O

A + es~_A-

(1)

A-" -]- e ~ --> __. A=

(2)

second wave

mA + A= -" B

(3)

Depending on the r a t e constant k and the s t o i c h i o m e t r y of the c h e m i c a l r e a c t i o n (3), we should e x p e c t a change in the s u m m a r y * h i is the height of the i - t h wave in a r b i t r a r y units; t 1 is the d r o p ping p e r i o d ; C is the d e p o I a r i z e r c o n c e n t r a t i o n .

85t

3

t

-0,5

I

-o,9

I

-~j

I,

-0,2

I

-o,6

I

[

-I,o

-4#

I

-oj

I

-o,g-E, v

Fig. 2. P o l a r o g r a m s switched according to scheme I (see [5, 6]) at a switching frequency of 10 Hz (the concentrations, M and E s u r g e, V are indicated): 1) dimethyl fumarate, 1.10-3; 2) dimethyl maleate, 0 . 7 . 1 0 -3, -1.19; 3) diphenyl fumarate, 1" 10-3, - 0 . 8 5 ; 4) dipheayl maleate; 1" 10-3, -0.85; 5) t r a n s - d i b e n z o y l e t h y l ene, 1" 10-3, - 0 . 6 ; 6) cis-dibenzoylethylene, 1 . 6 . 1 0 -3, -0.8. The dotted line denotes the line of the capacitance current. limiting c u r r e n t (h 1 + h 2) at potentials of the second wave from the 2e to the [2/(m + 1)]e level. The influence of the experimental conditions is s a t i s f a c t o r i l y explained by the different contribution of step (3) to the flux of p a r t i c l e s A to the s u r f a c e of the electrode. Prolonged e l e c t r o l y s i s in a m i c r o c e l l at the potentials of the limiting c u r r e n t of the second step leads to an i n c r e a s e in the height of the third wave. This supports the fact that the third wave c o r r e s p o n d s to the reduction of B, and not of A =. The interaction of A= with A may p r o c e e d according to a m e c h a n i s m of diene synthesis, in nucleophilic attack by the negatively charged oxygen atom of the dianion on the electrophilic carbon atom of the molecule o r the carbon atom of the ethylene group. In each of these c a s e s is formed that contains an e l e c t r o c h e m i c a l l y active - C O - Ph group, to the reduction of which wave apparently c o r r e s p o n d s . The last m e c h a n i s m , with the formation of the product

as well as carbonyl the product the third

Ph--C--CH--CH:C--Ph II I I O O O1 Ph--C~CH--CH:C--Ph !

Os e e m s most probable to us; the product is also capable of playing the role of a nucleophilic reagent, attacking a new molecule A. Under the influence of proton donor impurities, ketonization can proceed c o m p a r a tively slowly. The reduction of dimethyl and diphenyl e s t e r s of fumaric acid p r o c e e d s essentially s i m i l a r l y to the reduction of t r a n s - d i b e n z o y l e t h y l e n e (Fig. lb). The f i r s t step c o r r e s p o n d s to the t r a n s f e r of approximately one e l e c t r o n p e r molecule (see Table 1). Its r e v e r s i b i l i t y was established by the switched method (see Figs. lb and 2a). The limiting c u r r e n t of the first wave of dimethyl fumarate is influenced by subsequent chemical reactions. The ratio hl/C d e c r e a s e s from 86 to 70.5 when the concentration is i n c r e a s e d from 0.5 to 2.0 raM. When the dropping period is i n c r e a s e d from 0.18 to 4.45 sec, the ratio hl/t~l/6 d e c r e a s e s monotonically from 100 to 95. This change is small and only somewhat exceeds the experimental e r r o r , but is quite reproducible. The activation e n e r g y of the limiting c u r r e n t of the first wave (2.55 kcal/mole) is appreciably below the value of 3.0 • 0.1 k c a l / m o l e , c h a r a c t e r i s t i c of purely diffusion p r o c e s s e s [6]. On the basis of the above-mentioned, it can be a s s u m e d that the chemical reaction, leading to a c e r t a i n dec r e a s e in the limiting c u r r e n t , noticeable at l a r g e concentrations and dropping periods, as well as to a dec r e a s e in the activation e n e r g y of the limiting c u r r e n t of the first wave, consists of an interaction of the anion radical f o r m e d with neutral molecules. The t r a n s f e r of a second electron to the anion radical of f u m a r a t e s , just as in the case of t r a n s - and cis-dibenzoylethylene, is complicated by a chemical reaction between the dianion and neutral molecules, reducing the limiting current. This effect is especially noticeable for dimethyl fumarate (see Fig. lb). I~teresting peculiarities are o b s e r v e d in the reduction of e s t e r s of maleic acid. The p o l a r o g r a m of dimethyl maleate has a m o r e complex shape {see Fig. lc) than for dibenzoyldiethylenes and fumarates. The

852

first wave corresponds to an approximately one-electron process (see Table 1). The switched polarogram (Fig.2b) is separated by the line of the capacitance current into cathodic and anodic waves, possessing different El/2. These c h a r a c t e r i s t i c s in general resemble the process with decelerated discharging - ionization, discussed in detail in [11]. However, the chemical structure of maleates, the coincidence of E1/(2 of the anodic waves of maleates and fumarates (Fig. 2a, b), as well as the angular coefficient of the first wave 5

AE/A lg i](id -- i) = 60 mV,

Fig. 3. Polarograms of a solution of 1- 10 -3 M diphenyl maleate in DMFA, switched according to scheme I, for Esurg e = -0.85 V at a switching frequency, Hz:

indicate that the subsequent conversion is more probable than decelerated transfer of an electron. It is evident that the transfer of an electron to a lower unoccupied level lowers the energy b a r r i e r to rotation around the central C. 9 9 C bond on account of an increase in the contribution of structures of the type

I) 5; 2) i0; 3) 25; 4) 50;

5) 75. The dotted lines denote the lines of the capacitance current.

\/

o ]! C--OR

I O/%C/ \

OR

As a result of the subsequent rapid trans-isomerizationof the primary cis-anion radical of dimethyl maleate, the component of another redox system is subjected to anodic oxidation on the electrode. ]it might be expected that at higher frequencies of switching, the eis-anion radical could be detected. According to the data of the switched method for dimethyl maleate, up to a switching frequency of 100 Hz, the cis-anion radical is not observed. For diphenyl maleate, a switched polarogram was obtained, the anodic portion of which has two components, coinciding in El/2 with the anodie waves of maleate and fumarate (Fig. 3). Moreover, the ratio hcis/htran a a s increases from 0 to 0.91 when the frequency is increased from 5 to 75 Hz. Decelerated trans-isomerizationof the anion radical of diphenyl fumarate in comparison with the dimethyl e s t e r may be associated with the large volume and mass of the phenyl radical. Such a separation of the switched polarogram into cathodic and anodie waves with different Ei/~ is also detected for cis-dibenzoylethylene. In this case El/2 of the anodic wave coincides with El/2 of trans - d i benzoylethylene (see Fig. 2). The aforementioned permits us to conclude that the transfer of an electron to a molecule of the cisisomers studied is followed by rapid trans-isomerizationof the cis-anion radical. The limiting current of the first wave of dimethyl maleate, just as in the case of dimethyl fumarate, is somewhat lowered and is not truly a diffusion current. This is evidenced by the increase in the ratio h l / t l I/6 from 63.3 to 69.5 when the dropping period is lowered from 4.45 to 0.18 see, as well as the increase in the ratio hl/C from 73 to 81 when the concentration of the depolarizer is lowered from 2 to 0.6 mM and the reduced value of the activation energy of the limiting current (2.6 kcal/mole). A certain lowering of the limiting current of the first wave of dimethyl maleate, as well as the above-mentioned influence of tl, C, and T, just as in the case of dimethyl fumarate, is most likely caused by the occurrence of a chemical r e action between the anion radical and the initial molecules. The peeularities of the chemical structure of e s t e r s of maleic and fumaric acids are responsible for the possibility of the occurrence of a number of subsequent chemical reactions. Some of them are presented in the scheme

I~OOCH~ + ' ~---~](~'/')'~ F,.~cO~ k j J ' "-~-J[-

\/coocH~]-'~l |~-----

I " ~ ~176

853

The m e c h a n i s m of e l e c t r o c h e m i c a l reduction according to the f i r s t wave s e e m s e x t r e m e l y complex and cannot be entirely elucidated on the basis of the data obtained. We can only conclude with a s s u r a n c e that both at the f i r s t and at the second steps, the s e c o n d a r y chemical reactions affect the initial d e p o l a r i z e r molecules. These r e a c t i o n s a r e especially noticeable at the second stage of reduction of the ethylenic dicarbonyl compounds studied here. The authors would like to e x p r e s s their gratitude to V. F. Toropova, Ya. A. Levin, and L. V. Nest e r o v for their useful discussions and A. P. Rakov for his synthesis of c e r t a i n preparations.

CONC LUSIONS 1. The electrochemical reduction of cis- and trans-dibenzoylethylene, dimethyl and diphenyl fumarates and maleates on a mercury electrode in dimethylformamide was studied by a complex of electrochemical methods. 2. After transfer of the first electron, the cis-isomers rapidly undergo trans-isomerizatiom The kinetics of the isomerization was studied for the anion radicals of diphenyl maleate. 3. The transfer of the second electron is accompanied by a rapid and irreversible chemical reaction of the dianion with the initial molecules. LITERATURE 1. 2. 3. 4. 5.

6. 7. 8. 9. 10. 11.

854

CITED

R . H . Philp (Jr.), R. L. F l u r r y , and R. A. Day (Jr.), J. E l e c t r o c h e m . Soc., 111, 328 (1964). K. Takahashi and P. J. Elving, E l e c t r o c h i m . Acta, 12, 213 (1967). Yu. M. Kargin, Ya. A. Levin, V. Z. Kondranina, and R. T. Satin, in: News of the E l e c t r o c h e m i s t r y of Organic Compounds [in Russian], Nauka, Moscow (1968), p. 38. A . V . II'yasov, Yu. M. Kargin, Ya. A. Levin, I. D. Morozova, N. N. Sotnikova, V. Kh. Ivanova, and R. T. Satin, Izv. Akad. Nauk SSSR, Set. Khim., 736 (1968). Yu. M. Kargin, V. Z. Kondranina, and N. L Semakhina, Izv. Akad. Nauk SSSR, Ser. Khim., 278 (1971). A.V. ll'yasov, u M. Kargin, Ya. A. Levin, L D. Morozova, N. N. Sotnikova, V. Kh. Ivanova, and N. A. Bessolitsyna, Izv. Akad. Nauk SSSR, Ser. Khim., 740 (1968). C.A. Bischoffand A. Hedenstrom, Ber., 35, 4087 (1902). Yu. M. Kargin, V. Z. Kondranina, and R.-T. Satin, Izv. Akad. Nauk SSSR, Set. Khim., 1264 (1970). B.M. Bogoslovskii, Zh. Obshch. Khim., 14, 993 (1944). A.V. ll'yasov, Yu. M. Kargin, Ya. A. Lev-in, and B. V. Mel'nikov, Izv. Akad. Nauk SSSR, Ser. Khim., 1979 (1970). A.G. Stromberg, Zh. Fiz. Khimii, 29, 409 (1955).