Electrodimerization

Electroanalytical Chemistry and lnterJaeial Electrochemistry 42 (1973) 189-221

189

~?, Elsevier Sequoia S.A., Lausanne - Printed in The Netherlands

ELECTRODIMERIZATION V. MECHANISM OF THE ELECTROHYDRODIMERIZATION OF ACTIVATED OLEFINS IN MEDIA OF LOW ACIDITY

E. LAMY, L. NADJO and J. M. SAVI~ANT

Laboratoire d'Electrochimie de l'Universit# de Paris VII, 2 place Jussieu, 75--Paris (5e) (France) (Received 27th June 1972; in revised form 20th September 1972)

Since the early paper by Baizer ~ on electrolytic hydrodimerization (EHD) of activated olefins and its application to the commercial conversion of acrylonitrile (AN) into adiponitrile (ADN), a considerable amount of synthetic and mechanistic work has been devoted to this reaction by Baizer himself and by other workers (for a recent review, see ref. 2). The first problem to be solved was to define the conditions for optimizing the selectivity of the hydrodimerization reaction: + e + H+ --~ "~

,/"-x

X[

X

I H

[ N

I X

(X: activating group)

against hydrogenation of the double bond: "-, / /'--'~X

+ 2e + 2 N+ ~

H}

1.41X

and against oligomerization and polymerization. A large part of the earlier work in the field was devoted to the EHD of AN in water owing to the key importance of ADN in the synthesis of 66 Nylon and to the commercial desirability of water as a solvent. This was the case for the mechanistic work too, although AN is presumably one of the more difficult activated olefins to study by electrochemical kinetic techniques. Despite the discouraging two-electron behavior of AN in water, involving a total conversion 3 to propionitrile (PN), Baizer 1"4 was able to produce ADN, in almost quantitative yield, by macroscale electrolysis of AN in water. This was feasible through the use of large quantities of tetra-alkylammonium-p-toluene-sulfonates as a supporting electrolyte. The merit of this is to raise the solubility of AN in the aqueous solution and thus allow the use of large concentrations of AN. In such conditions the dimerization process is favored regardless of its exact mechanism. Furthermore; for a given AN concentration, the specific role of large organic cations, such as quaternary ammonium, v e r s u s alkaline cations in raising the yield of ADN has been amply demonstrated experimentally 4-8. The same kind of a study with diethyl maleate 9, the reduction potential of which is distinctly anodic to the discharge potentials of the cations considered, shows clearly that the specific effect of tetra-alkylammoniums on the reduction of AN is not related to a possible mixed reduction of alkaline cations and AN as initially supposed 4. It is rather related to the

E. LAMY,L. NADJO, J. M. SAVEANT

190

well-known specific adsorption of these large organic cations which leads to the establishment of a water-poor zone in the vicinity of the cathode surface 9-12. The same kind of effects may likewise explain the higher ADN yield in amalgam reduction of AN in the presence of tetraethylammonium-p-toluene sulfonate 9,13. In other words, a kind of a non-aqueous medium of relatively low proton availability exists close to the electrode although water is the solvent. The situation can thus be fruitfully parallelled to reduction in non-aqueous "aprotic" solventslO.14 such as dimethylformamide (DMF). In this connection the results of Feoktistov and co-workers1 o are particularly interesting. They show that D M F with 5~o water is roughly equivalent to the system water-tetra-alkylammonium-p-toluenesulfonat~ as concerns proton availability in the medium surrounding the electrode. EHD has also been shown to be applicable to compounds other than AN provided they are good Michael acceptors 1' 15-22. As with AN an almost exclusive /~-/? coupling is observed in all cases (a small amount of e-/? coupled products has been detected with metacrylonitrile and crotononitrile23). Furthermore, mixed coupling is obtained when a mixture of two activated olefins A and B is electrolyzed17-20.24-31. If A and B are reducible in the same electrode potential region a mixture of the symmetric hydrodimers HA-AH, HB-BH and of the, mixed coupled product HA-BH is obtained 24'17-2°. If A is markedly more reducible than B, the electrolysis in the potential region where A is reduced and B is not, leads to a mixture of one symmetric hydrodimer HA-AH and of the mixed coupled product HA-BH, whereas no HB-BH can be detected 24. However the yield of HA-BH is generally low even when B is in large excess over A, and falls to zero if the difference in the reduction potentials of A and B is too large (say more than 0.4 V) 3°'31. Furthermore, the ratio HA-BH/HA-AH increases as the reduction potential is made more negative 3°. Definitive conclusions are not easy to draw from these observations owing to the practical difficulty of ensuring that the reduction potential is constant during macro-scale electrolysis and the same at any point of the working electrode 3°. As soon as the electrolysis potential is shifted to the region where B can be reduced the three coupling products are again obtained3k From these results, however, evidence remains that in some favorable cases it is possible to obtain, even in low yield, the mixed coupled product in a potential region where B is not reduced at the electrode. This has been one of the main arguments for rejecting 11'24'a2 the possibility of radical coupling as the main mechanistic path to the formation of the hydrodimer in the reduction of a single activated olefin. Such a mechanism is depicted by the following equations: S C H E M E (I)

A+e- ~ A ~ A =-+ TH ~:e AH, + T -

(TH : proton donor)

2 AH" ---* DH 2

(DH2: hydrodimer) TH

A ~ + A H --~ D H - (---~ DH2) TIt

2A ~ ~ D z- ("-* DH2) Similar reasoning derives from the study of the intramolecular cyclizing

ELECTRODIMERIZATION

MECHANISM

OF ACTIVATED OLEFINS

191

coupling in the reduction of compounds bearing two identical or different activated olefinic groups in the same molecule 33-35. More arguments have been invoked to rule out the radical coupling mechanism, e.#. the lack of influence of radical scavengers such as hydroquinone on the EHD process and the absence of polymers in the reduction products 4. In fact the competition between these reactions and radical coupling, if any, would depend on their relative rates so that a negative test does not provide unequivocal evidence against such a mechanism. On the other hand, the correlation of the yield of dimeric product with the Michael acceptor ability of the olefin has underlined 34 the idea that the coupling reaction was indeed a Michael type reaction. Accepting this, the first mechanistic proposal 1'4't°'36"37 for the EHD reaction was a coupling between the di-anion deriving from the initial olefin through a direct .two-electron transfer, which is a strong Michael donor and the substrate molecule which is a good Michael acceptor: SCHEME(ii) / - - \ X + 20-

~-.~X

~

(A) \

(A2-)

/ X

X

(A)

X

X

(A2-)

-:1

I

I

X

17

+

(D 2- )

2TN

HI

~

X

l

I

I M+2T-

X

X

(D2-)

(DN 2)

or:

"--/

/--"X (A)

-x__/

+

2o +

-H

--'"

t H+T

X (AH-)

+

-:I

X

-F-M

--*

X

(A)

-:1

rH

-~(

(AM-)

I

I

t M + rH --*X

I

I

I M

X

X

(OH-)

MI ×

I

I

4 H*T-

X

The lack of polymer formation despite the highly nucleophilic character of the monomer and dimer di-anions (A 2- and D 2-) was accounted for by assuming that these relatively small species exist in the "aprotic" layer surrounding the electrode whereas longer molecules would have at least one end in a more protic medium and so undergo a protonation that stops the polymerization propagation t°. This mechanism was thought to imply that the second electron transfer to the

192

E. LAMY,L. NADJO, J. M. SAVEANT

AN molecule is energetically easier than the first. This assumption was based both on experimental and theoretical evidence. The experimental argument was that in water 3 and apparently t4 also in DMF, AN exhibits a two-electron wave. As recognized later 3~, this does not mean that a direct two-electron transfer leading to A 2- is possible since the polarographic wave represents the conversion of AN into PN and not into A 2-, owing to the low concentration of AN and the relatively high proton availability of the medium. The theoretical evidence derives from LCAO-MO calculations l"t. The results seem rather surprising owing to the large coulombic repulsion energy in the di-anion which could hardly be compensated for by electronic rearrangement and solvation effects (note further-more that this last factor is not taken into account in quantum mechanical calculations on the free molecules and ions). Indeed, recent experimental results 38 show that in a series of ethylenes increasingly substituted by identical p-nitrophenyl groups, four of these groups are required in order to obtain a single two-electron reversible wave in DMF leading to the di-anion. It seems thus bighly improbable that such a possibility would exist in a mono-activated olefin such as AN. However it must be emphasized that the mechanism proposed does not imply in fact the di-anion of AN to be produced in one step at the electrode. Indeed, the following situation could be envisioned: (i) in the absence of any coupling step let us assume that the two electrons are actually transferred successively; (ii) the effect of the fast coupling step (A + A2 -) could be the transformation of these two successive one-electron waves into a single one-electron wave corresponding to the formation of the dimer. This ought to be the case if the dimer formation is quantitative, for this mechanism as well as for any other, Indeed, in watermlioxan mixtures with N(CH3)"tBr as supporting electrolyte the reduction pattern obtained using a streaming mercury electrode tends to exhibit a one-electron behavior as the AN concentration increases, approaching the concentration range used in macroscale experiments 36. Electrochemical kinetic studies of the AN reduction using Tafel line analysis 8,39 have shown that the rate-determining step is the transfer of the first electron and of a proton. No kinetic analysis of the dimerization process which follows the rate-determining step can therefore be derived from this result. Within the same working hypothesis of a Michael type coupling, this experiment led to the assumption of the intermediacy of the mono-protonated di-anion A H - as the Michael donor s. ~~ 39, which is more likely to survive in the water-poor quarternary ammonium layer than A 2-. In order to account for the absence of c~-fl coupled products, the structure of A H - was assumed to be: CH2-CH2-CN and not CH3-~I-:I--CN despite the second appearing to be more stable than the first 3"t. This assumption was-justified by considerations of the difference in the proton donating power of the first and second ionic layers from the electrode 1~, In fact, such considerations are not easy either to confirm or to reject experimentally. On the basis of the unlikelihood of a direct twoheleltron formation of the dianion A 2-, Inother mechanistic proposal was then made which assumed that the dimerization step is a reaction of the radical anion initially generated on the substrate 32.,to:

ELECTRODIMERIZATION MECHANISM OF ACTIVATED OLEFINS

193

S C H E M E (III)

A + e- ~=~A ~A T +A --* D ~ 2 TH

D-+e-

---* D 2- (

' DH2)

The same mechanism was also proposed on the basis of an experimental study4~'42 of di-activated olefins in DMF. This mechanism has been considered to fit the data obtained in the study of the electrohydrocyclization better than the previous one. The dimerization process can now be thought of either as a Michael type addition if the doublet is mainly located on the fl carbon of the anion radical considered as a donor:

or as a radical addition, in the opposite case: x

The EHD of e-fl ethylenic carbonyl compounds is often considered as a separate topic presumably because an isolated carbonyl group is easier to reduce than other activating groups such as esters and nitriles. However, the reduction products are generally the same: hydrodimers, compounds resulting from the hydrogenation of the double bond and polymers. Also, the c~-fl ethylenic carbonyl compounds give rise to mixed coupling with compounds of the same class or with other activated olefins2°'43. In the following they will therefore be considered as belonging to the class of activated olefins. It is to be noted that a radical coupling mechanism is a more likely assumption for the reduction of c~-fl ethylenic carbonyl compounds than for the other activated olefins 44'45. Recently, the EHD mechanism of activated olefins has been analyzed using electrochemical kinetic techniques such as cyclic voltammetry and double-step potentiostatic chronoamperometry and chronocoulometry. These studies deal with solvents of low proton availability such as DMF and dimethylsulfoxide (DMSO) and with depolarizer concentrations in the millimolar range. As noted above, these conditions as regards the vicinity of the electrode are not too far from those featuring aqueous quaternary ammonium solutions. The study by Baizer and co-workers41 is concerned with the reduction of di-activated olefins in DMF using linear sweep voltammetry (LSV). Di-activated olefins have the advantage that the EHD process is often slower than in monoactivated olefins owing to the greater stability of the anion-radical leading thus to an easier kinetic analysis. From their data, the authors concluded that the coupling step is a radical -substrate one as mentioned above (Scheme (III)). Bard et al. 46 studied in detail the EHD of one particular di-activated olefin, diethyl fumarate, in DMF using mainly double-step potentiostatic chronoamperometry and chronocoulometry. In anhydrous DMF with tetrabutylammonium iodide as the supporting electrolyte the coulometric determinations indicate that polymerization is occurring besides EHD. The cyclic voltammetric behaviour has persu-

E. LAMY,L, NADJO,J. M. SAVI~ANT

194

aded the authors that the polymerization is not very fast so that it can be neglected in the potentiostatic analysis. Comparison of the formal kinetics of the various possible mechanisms: (i) first order decay of the anion radical, (ii) e.c.e, radicalsubstrate coupling (as in Scheme (III)), (iii) reaction of the di-anion on the substrate (Scheme (II)); (iv) radical coupling of the anion-radicals (Scheme (I)), with the experimental kinetics has shown that mechanism (iv) fits the experimental data and the others do not. Other mechanistic paths such as disproprotionation in the radical-substrate mechanism (Scheme (III)) should however be considered for a complete discussion of the problem as will be shown later. Having regard to the low proton availability of the medium and to the stability of the anion-radical of such a di-activated olefin versus protonation, the role of the neutral radicals AH. (Scheme (I)) was not taken into account. Furthermore, the authors have shown that the LSV results of Baizer et al. 4~ would fit this mechanism better than the radical-substrate one. Evans et al. 47 have studied the kinetics of the reduction of the two ~-fl ethylenic ketones: C6Hs-CH=CH-C-CH 3 and C6Hs-CH=CH-C-tC4H 9 II

II

0 0 in DMSO using cyclic voltammetry (CV). They have shown that the results of Baizer and co-workers on di-activated olefins4~ can be interpreted either by the radical coupling mechanism or by the radical-substrate coupling mechanism. They have also reported that their results fit the radical coupling mechanism well without however examining if they would not fit other possible mechanisms within the range of experimental error. Bowers and co-workers48 have examined the kinetics of the electro-chemical reduction of the ketone: tC,~H9-CH=CH420-tC4H 9 in hexamethylphosphorotriamide (HMPA) and DMF by cyclic voltammetry, e.p.r, and u.v. spectroscopies. Their conclusion is that the mechanism involves a radical coupling of two neutral radicals, the rate-determining step being the protonation of the anion-radical into the corresponding neutral radicals. These last three studies therefore lead to a reconsideration of the mechanism of the EHD of activated olefins and suggest that the coupling reaction might be a purely radical process (Scheme (I)) rather than an ionic (Scheme (II)) or a radical-ionic (Scheme (III)) attack of the substrate by the initially reduced species as assumed before. It is the main purpose of the present paper to analyze this problem on a number of activated olefins in solvents of low acidity such as acetonitrile (ACN) and DMF using mainly LSV as a tool for the kinetic study of the electrochemical reaction. EXPERIMENTAL Chemicals Activated olefins, p-Methylbenzylidenemalonitrile, ethyl cinnamate, dibenzo-

ylethylene-3-methyl-3-penten-2-one were obtained from Aldrich, acrylonitrile, cinnamonitrile, fumaronitrile, diethyl fumarate, chalcone, benzalacetone, 3-methyl-3buten-2-one, isophorone from Fluka and a-cyanoethyl cinnamate from Eastman.

ELECTRODIMERIZATION MECHANISM OF ACTIVATED OLEFINS

195

The mesityl oxide was a gift of Professor Wiemann (Universit6 de Paris VI--France). All the liquid compounds were re-distilled before use and the solids re-crystallized from ethanol. Solvents--supporting electrolytes. For polarography and LSV the ACN was a spectrograde product (Carlo Erba). For the preparative scale electrolysis the ACN was obtained from a commercial grade product distilled over sodium hydride. It exhibited the same polarographic behaviour as the Carlo Erba product. In both cases the water content, as measured by the Karl-Fischer technique, was about

0.05Vo. The DMF used in polarography and LSV was a spectrograde Carlo Erba product whereas for the preparative scale electrolysis the Merck (for synthesis) DMF was used. The water content was about 0.1%. Tetraethylammonium perchlorate (Carlo Erba, polarographic grade) was used as supporting electrolyte at a concentration of 0.1 mol 1-1. Some experiments were performed in alkaline buffered ethanol (tetrabutylammonium hydroxide and phenoltetrabutylammonium phenate). The experimental procedures were the same as previously described 53. HMPA was used for one polarographic and CV determination. Details concerning the electrochemical use of this solvent have been given elsewhere 58.

Reference electrodes For polarography, cyclic voltammetry, coulometry and preparative-scale electrolysis, an aqueous saturated calomel electrode was commonly used, in ACN, DMF and ethanol. In ACN, an Ag/0.01 M AgC10,, reference electrode was also often used for the polarographic determination and in every case for the LSV experiments.

Coulometry-- preparative-scale electrolysis The coulometric analyses were performed on relatively low concentrations of solutions (2 x 10 -3 tool 1-1) in conditions such that a 90~o consumption of the initial quantity could be reached in about 15 min. The working electrode was a mercury pool of about 15 cm 2 surface area, the volume of the solution being 25 cm 3. The counter electrode was a platinum wire separated from the cathodic compartment by a large fine porosity glass frit. The current was integrated by an electronic current integrator (Tacussel IG3-100). The accuracy in the coulometric determinations of the number of electrons exchanged was about 5~. Preparative-scale electrolyses were performed starting from 2-3 g of activated olefin with a solution volume of 250 cm 3 in a cell of the type already described by Moinet and Peltier 49. A 100 W potentiostat (Tacussel Asa 100-1) was used here as well as for coulometry. In both ACN and DMF the electrolyses were performed with a continuous addition of concentrated perchloric acid or acetic so as to prevent the appearance of an orange color that indicates the decomposition of the solvent due to the consumption of protons 5°'51. After electrolysis the ACN solution was evaporated under vacuum and then extracted with ether. In the case of DMF the electrolyzed solution was poured into water and the mixture carefully extracted with ether. The ethereal solution was then dried over magnesium sulfate and evaporated. The products were then recrystallized or separated by g.l.c.

E. LAMY. L. NADJO, J. M. SAVI~ANT

196

Polavography--cyclic voltammetry-- LSV The cells, electrodes and electronic devices have been described previously 52-55. The cell was thermostatted at a temperature of 25':C. Repeated experiments showed that the accuracy on peak potential determination was about +_3 inV. The effect of the ohmic drop on the peak potential was reduced by using a purely ohmic positive feedback compensator 55 and by reducing the working electrode surface area as the sweep rate increases. A long dropping-time capillary (50-100 s) was used and an electronic synchronization device imposed the following operation sequence: - - during an adjustable delay-time the working electrode potential was maintained at such a value that no appreciable current flowed. at the end of the delay-time the linearly variable potential was applied. at the end of this potential application a small hammer dislodged the mercury drop, the potential was reset at its initial value and the operation sequence repeated. The delay-time was adjusted according to the natural dropping time and to the scan-duration in such a way that surface variation was negligible during the scan. As the sweep rate increased, the scan duration became shorter so that the delay-time could be shortened, the electrode surface area remaining constant within the same degree of approximation. The operating conditions were kept such that the peak-current was approximately constant throughout t h e sweep rate range, by decreasing the delay-time according to the increase of the sweep rate. Under these conditions, the ohmic drop still increases with the sweep rate but markedly less than if the working electrode area were held constant. For initial concentrations of the order of 3 mmol 1-~ the ohmic drop could be neglected, up to a sweep rate of 50 V s-~ (for lower concentrations the upper limit of the sweep rate is larger). For higher values of the sweep rate the remaining uncompansated resistance, which is purely ohmic and depends on apparatus construction55, was mathematically corrected for by comparison with the behaviour of the peak potential of a reversible wave (fluorenone in the same solvent) and by application of linearized correction relationships 56'57 -

-

-

-

POLAROGRAPHY-~COULOMETRY--CYCL1C VOLTAMMETRY Changes of the polarograms with the acidity of the medium is shown in Figs. 1-6 for most of the compounds studied. The increase in acidity was obtained through successive additions of water and, in some cases, of phenol, For this reason, there is a slight decrease of the wave heights that must be taken into account when analyzing the development of the waves. For a few ~-/~ ethylenic ketones that are not hydrolyzed in alkaline medium, two ethanolic buffers, tetrabutylammonium hydroxide and a phenol buffer, were also used. The general trends are as follows: - - in the "anhydrous" solvent two waves are observed. The second wave is smaller than the first one, or even absent or replaced by a small dip. - - Upon addition of the proton donor the second wave develops and tends to reach the same height as the first. At the same time the two waves tend to merge, either through an increase of the height of the first one at the expense of

ELECTRODIMERIZAT1ON MECHANISM O F ACTIVATED O L E F I N S

197

the second, or by anodic shifting of the second wave, or by both processes. The tendency of the two waves to merge is shown semi-qualitatively in Table 1 (last column). In the case of the aliphatic esters and ketones no second waves are observed in the less acidic conditions. This may well be due to the fact that the first wave being very negative, the second one is obscured by the discharge of the supporting electrolyte. This situation is referred to by the notation "X" in the sixth column of Table 1. Not all the polarographic results obtained are shown in Figs. 1-6. Some were omitted when the change in solvent or compound did not involve significant variations in polarographic behaviour. These are however reported in Table l and denoted with an asterisk in the s~cond column. In the fourth column the height of the first wave in the less acidic condition is compared to that of fluorenone in i,~-sA

(A)

CHz=CFICN

1.g rnmot t "~

4.

ACN

0.2 V,

2,1

2.1

2.1

2.1 -E/V(Ag/Ag +)

1. j] o.,v. :I/ - T /...... 1.6

1,6

1.6

1.6

C6H5 CH= CHCN

2.0 !//4~

2.9 mmot l "1

~ (D)

2,;,0

CNCH=CHCN

-E/V(SCE) ACN

2.0 -E/V(Ag/Ag'f) 3.0 rn mot I-~

ACN

/ b 1.3

r

__ _jr 1.3

.

. 1.3

.

. 1.3 -E/V(Ag/Ag "~)

Fig. 1. Nitriles--variation of polarographic waves on addition of water. Numbers on each curve indicate water volume ~,~.

198

E. LAMY, L, NADJO, J. M. SAVI~ANT

©

0

£ %

L~

~.-~ O

B

=

o£

o

if8

a

~a

c~

~x w

~

}~

,-4 ~o

Z

,,,,,,1

£ oZ

z

~

~

~2

8o¢

¢

G

c

,< -

,,,,,,,I

'=

0

©

8

~ ~

'5 "n

~

o~

199

ELECTRODIMERIZATION MECHANISM OF ACTIVATED OLEFINS

~

o

o

.o

~

444

444

V,~ V,~

V? VI

~4

4~

~

~

~

~u~

" ~ "t:J

0

I II

I tl

I II

I II

0 ~D

oo

oo

I

oo

I

I I ---

o',,q dr-,i

o,'w rqr',i

o~

}

-

o ~,i

--

~!.

--

r,i

~

,"q

oo¢' ~,~i

~-"

=

z ~=~

z

< ~

0) 0

E 0

0 m

~u~

,~

o

ct

©

=

d

k;

200

E. LAMY, L. NADJO, J. M. SAVEANT

~uA (A)

p-CH~C6H4CH=C(CN) z

H~O '/. : 0,05

1.0

1.1 rnmoL [-1

ACN

3.0

5.0

:

, 1.2

~uA (B)

1.2

1,2

C6HsCH=C'COOCg.H5 I CN

2.Smmo[ 1-1

C&HsCH=C ,COOC~H5 I CN C(,HsOH(mmoL['~)

oo

1.2 -E/V (Ag/Ag÷)

1.9mmol t"t

/

ACN

ACN

f

8 °

0.9

2'-

0.9

,/

0.9

-E/V(Ag/Ag +)

Fig. 2. Nitriles---variation of polarographic waves on addition of water or phenol. Numbers on each curve indicate (A, B) water volume %, (C) phenol concn., in mol 1 ~.

conditions where it features a one-electron reversible exchange as checked by CV. The 5th and 6th columns give an estimation of the number of electrons exchanged on each of the two waves according to the change of the polarograms with the acidity of the medium. The coulometric results concerning the first wave are shown in Table 2. They generally confirm the polarographic estimations. In the case of cinnamonitrile however, the coulometric number of electrons is slightly smaller than the polarographic one. This can be accounted for by the effect of the basic or nucleophilic species generated by electrolysis (through, e.9. decomposition or polymerization of the starting material) which is more likely to occur in coulometry than in polarography due to longer electrolysis time and higher concentration. The results of cyclic voltammetry are shown in Table 3. Values of sweep rate for which the CV pattern becomes reversible are reported. From these, the values corresponding to a millimolar solution in the less acidic conditions have been estimated (last column).

201

ELECTRODIMERIZATION MECHANISM O F ACTIVATED O L E F I N S Calls CH=CH COOCHS

(A)

H~O '/. 4-

2.0

~A

1.9

(E~)

Iq.-,O7, .

~

3,7retool t-~

2.0

20.0

1.7

C6H5CH= GHCOOCgHs 0.05

DM F

-

1.7 (C)

1;8-E/V(Ag/Ag÷)

1.8

C&HsCH = CHCOOCH3

1.7

HzO Z

ACN

0.05

J

2.

1.8mmoi t-1

2.0

1.7 -E/V(SCE) 3.4 mmo[ -1

ACN

5.0

40 20 1.9

1.9

1,8

1.8 -E/V(Ag/Ag+)

Fig. 3. Esters--variation of polarographic waves on addition of water. Numbers on each curve indicate water volume %.

DISCUSSION--SELECTION OF

THE

ACTIVATED O L E F I N S

SUITABLE FOR

AN

LSV

K I N E T I C ANALYSIS

For the three cases, acrylonitrile, fumaronitrile and diethyl fumarate, the apparent number of electrons exchanged on the first wave is definitely less than one. These observations are in agreement with recently published results on diethyl fumarate 46. This may be related to polymerization or oligomerization reactions initiated by the dimer di-anion produced along this wave 18's9'6°. The rate determining step of the polymerization process has been postulated to be the dimerization reaction sg. However this is not sufficiently well established as to make this situation the basis of a kinetic analysis of the dimerization mechanism. The apparent number of electrons increases upon addition of water but at the same time the two waves merge into a single two-electron wave in the case of acrylonitrile. This is why the dimerization kinetics of this compound are not studied in the present work. The study of diethylfumarate was similarly precluded although the merging of the two

202

E. LAMY, L. NADJO, J. M. SAVI~ANT i//aA

(A)

CH3CH = CHCOOC2H 5

C~H5OH(mo I t -I)

0.O

3,Tremor r "1

DMF

/

4°

2.0

[/pAl

(B)

1.5mmot 1-1 ACN

C2HsOOCCH= CHCOOC2N 5

o oo5

o/o

4o

44

1,O

1.1

1.1

1J

-E/V (Ag/A9÷)

Fig. 4. Esters--variation of polarographic waves on addition of water or phenol. Numbers on each curve indicate (B) water volume %, (A) phenol concn, in tool 1-1. TABLE 2 COULOMETRIC RESULTS

Compound

Solvent

CH2=CH~2N

ACN

C6Hs-CH=CH-CN

HMPA ACN

NC-CH=CH-CN

DMF ACN

p-CH3-C6H4-CH=C( CN)2 C6Hs-'CH=C(CN)CO2C2H 5 C6Hs~CH=CH~CO2CH3 CHs C H = C H q 2 0 2 C H 3 CzHsO2C-CH=CH-CO2C2H 5

(CHa)2C=CH~O-CH 3

CN3/

ACN ACN DMF DMF ACN

ACN

O

" Water in volume % unless otherwise stated.

Protondonor content"

0.05 10 < 0.05 0.05 0.1 10 0.05 10 0.05 0.05 0.1 0.1 0.O5 1 2.8 0.05

Concn./ mmol 1-~

No. of electrons per molecule

2.3 3.O 2.0 6.0

0.3 1.9 0.1 0.8 0.8 1.1 0.6 0.94 O.94 0.93

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0.98

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204

E. LAMY, L. NADJO, J, M. SAVEANT

waves is somewhat less rapid. The study of fumaronitrile however is feasible in ACN containing 10",o water. In all other cases the height of the first wave corresponds to one electron per mole, even without addition of water. No drastic effort has been made here to reduce the residual water content of the solvents. It follows that a number of the compounds considered may well behave as acrylonitrile, fumaronitrile and diethyl fumarate in more highly anhydrous solvents. TABLE 3 REVERSIBILITY O F THE FIRST WAVE IN CV

Soh,ent

Compound

Water content/

%

CH2=CH-CN

C~H s-CH=CH-~CN NC-&'H=CH-CN p-CH3-C6Hs~CH=C(CN)2

C~,Hs-CH=C(CN)CO2C 2Hs

C6Hs-CH=CH-CO2CH3

(nearly the same values for C ~ H s ~ C H = C H -COzC2Hs) CH3~CH=CH-COzCH3 C2HsO2C~CH=CH-CO2CzH5 C6Hs~CH=CH-CO-C~,H5

C6Hs~CH=CH~OCH3 C6Hs~CO-CH=CH~CO~6H 5 (CH3)2C=CH~CO~CH3 /

CH2=C(CH J~CO-CH 3 (nearly the same values for C H 3-qS'H=C(C H 3) - C O C H 3)

CH3~):= CHj

0

ACN 0.05 DMF 0.1 H M P A <0.05 ACN 0.05 1.0 DMF 0.1 ACN 0.05 ACN 0.05 0.1 DMF <50 ACN 0.05 5.0 DMF 1.0 ACN 0.05 1.0 2.0 DMF 0.l DMF 0.1 ACN 0.05 1.0 ACN 0.05 DMF 0.1 EtOH NBu4OH phenol buffer ACN 0.05 DMF 0.1 DMF 0.1 ACN 0.05 DMF 0.1 ACN 0.05 DMF ACN DMF

0.1 0.05 0.1

Cohen.~ mmo[ l- ~

Reversibility /V s-

Reversibility in "anhydrous" conditions jbr I mmol l-t (estd.)/V s- t

1.0 1.0 1.0 4.9

> 1500 > 1500 >1500 0.5

2.5 5.3 1.0 2.75 3.9 2.0

> 1500 > 1500 > 1500 2.6 9 <0.1 > 1500 > 1500 ~ 1500 ~1500 40 40 <0.1 5 10 20 <0.1 100 1.6 1.0 13 5 0.5 250 7 19 <0.1 18 36 1500

2.1 2.03 2.63

350 21 25

1.0 1.0 1.2 1.2 1.2 1.2 1.2 1.2 5 1 1 1.2 1.2 2.8 0.9

<0,1 > 1500 > 1500 ~ 1500 30 <0.1 1

<0.1 100 0.15 10 2 0.5 25O 3 3.5 <0.1 7 9 75O 170 11 9

ELECTRODIMERIZATIONMECHANISMOF ACTIVATEDOLEFINS The tendency of the two waves to merge, as shown qualitatively in Table 1, can be accounted for by a thermodynamic analysis of the system in terms of E-pH diagrams similar to that previously made in the case of aromatic carbonyl compounds 5z. Indeed, the restrictive assumptions made concerning standard potentials and pK's also hold in the present case for analogous structural reasons. Similarly the E-pH diagram analysis is able to account for the trend in the competition between dimerization and hydrogenation in macroscale electrolysis when the acidity of the medium increases. In both cases however only conclusions concerning trends can be expected, since the E-pH analysis ignores kinetic factors that may play an important role in the separation or fusion of waves and also in the distribution of products. In the case where the acidity of the medium is such that the only existing form of the dimer is DH~ and of the hydrogenation product AHz, the condition for a separation of the waves at 25°C can be derived from previous resultsSZ: (E~ +0.06 pK~') - ( E , +0.06 pK~) < 0.06 (pA-pc) i.e. the extent of dimerization is dependent on the excess of reducibility of AH" into AHz v e r s u s A into AH'. The fact that the AH radical is generally more reducible than A itself, as frequently assumed in organic chemistry, cannot therefore be considered as a convincing argument against the participation of AH" in a dimerization process as is frequently done. In fact two factors must be taken into account when evaluating the trend toward a direct two-electron hydrogenation v e r s u s the trend toward a first oneelectron dimerization: the extent of dimerization itself, either intrinsically or through a concentration increase and the excess of reducibility of AH" into AH2 over that of A into AH'. At higher pH another possibility is the formation of the dimer anion DH-. Even when the conditions required for the existence of DHz are not fulfilled the dimer D H - can be formed in a limited pH-range. This shows that a decrease in the acidity of the medium favors dimer formation. This would be even truer if the dimerization of two anion radicals A- leading to D 2- had been considered. This possibility has not however been taken into account owing to the strong coulombic repulsion in the dipinacolate or, what amounts to the same thing, owing to the fact that the second acidity of the pinacolate is far less than the first: pK,]>>pK o (note the typographical error in p. 423 of ref. 52; one should read ...~pK]', pK d and not ...,~ pK';< pK~ since no comparison of the basicity of A z- and D zis made). In the present case, and especially with nitriles, the negative charges can be located mainly on the two activating groups in the dimer at a relatively large distance from one another, each being strongly solvated. The tendency to form a dimer di-anion is thus more likely in the present case than with the aromatic ketones. This is why a non-acidic medium favors dimerization v e r s u s hydrogenation as is the case for the conversion of AN into ADN and why therefore, the introduction of a quaternary ammonium salt into the aqueous solution is of key importance in obtaining a high yield of ADN. It has been shown that in the series of the aromatic carbonyl compounds the separation of waves roughly parallels the case of dimerization as can be predicted

E. LAMY,L. NADJO, J. M. SAVI~ANT

206

on the basis of steric repulsion 49. This is not the case in the present series of activated olefins, where the difference of reducibility of AH" over A clearly has also to be taken into account. This appears in Table 1 in the series of the aliphatic e-/~ ethylenic ketones where the compounds that are the most sterically crowded in the /~ position exhibit the lowest tendency in the merging of the waves. Even more striking is the comparison of acrylonitrile with the other nitriles. The steric hindrance to dimerization is small in the whole series and the predominant factor becomes the difference in reducibility of AH" and A. This is largest for a mono-activated aliphatic olefin such as acrylonitrile. An abnormally low height or even the absence of the second wave in extreme anhydrous conditions may reflect various phenomena: (i) The same dimerization reaction may occur in the potential region of the second wave as in that of the first, resulting in a lowering of the plateau of the second polarographic wave. This phenomenon cannot be accounted for by a direct "competition" between the dimerization of the anion-radical and its further reduction at the electrode 52"61'62. Consider the following reaction scheme: A+e~A'A-" ~ products A ~ + e ~ A zOn the plateau of the first wave, and for the more negative potentials, the concentration of A at the electrode surface is zero. The very existence of a second plateau implies that the concentration of A-" at the electrode surface is also zero. Thus, in this potential region, the concentration of A ~ is zero everywhere in the solution. It follows that its flux at the electrode is also zero and therefore that a two-electron reduction of A into A 2- is obtained. The height of the second wave is thus the same as the first and corresponds to one electron per molecule whatever the rate of the chemical reaction, e.9. a dimerization that de-activates A-'. This rate interferes in the location of the half-wave potential of the second wave but not in its height. Such a reaction scheme, involving a competition between a chemical deactivation of A ~ and its reduction at the electrode is therefore inconsistent with the lowering of the wave height observed experimentally. This can be explained however by considering the interference in the mechanism of solution electron transfer reactions: A + e ~:~ A-" A'-- + e ~::~A21

A z - + A ~:~ 2 2 A:- (solution electron transfer) A "- ---* product (chemical de-activation)

(1)

At equilibrium, the solution electron transfer reaction can be assumed to be largely in favor of the right-hand side since for organic compounds it is generally considered that the standard potential of the A'-/A z- couple is much more negative than the standard potential of the couple A/A ~. It follows then, that if step 1 in the solution electron transfer is fast enough as well as the chemical de-activation, e.9.

ELECTRODIMERIZATION MECHANISM OF ACTIVATED OLEFINS

207

dimerization, of A-', as soon a s A 2 - is produced at the electrode a part of A is chemically de-activated and becomes unavailable for reduction at the electrode. This results then in the lowering of the plateau of the second wave which may even disappear if the sequence of the chemical reaction is fast enough. Such phenomena have been evidenced in the two-step anodic oxidation of ene-diamines 63, and could be of significance in the present case. This may represent a serious shortcoming in selectivity as a function of electrolysis potential in electrochemical synthesis. Indeed, the formation of the de-activation product, e.9. the dimer, in the potential region corresponding to the second reduction, resulting for example in hydrogenation, rises when the electrolysis time is increased. It follows that even if in polarography the second wave is not significantly lowered below the value corresponding to 1 e/ molecule it may well be that in long-time electrolysis the formation of the de-activation product is predominant over that of the successive reduction product. In the present case when a proton donor is added the di-anion A 2- is rapidly converted into A H - or even into AH2. Conditions are then less favorable for the solution electron transfer reaction: A + A H - ~ A "- + AH (solution electron transfer)

(2)

I

A-" ~ p r o d u c t s (chemical de-activation) AH

Indeed, even if the overall thermodynamic trend is in favor of the deactivation product (for a dimerization this would mean that the conditions are those of the diagrams 3-7 of Fig. 2 in ref. 52, in the appropriate pH range), the solution electron transfer equilibrium is now in favor of the left-hand side so that step 2 may be of small efficiency from the kinetic point of view. This is consistent with the experimental observation that the second wave increases upon addition of a proton donor. In other words, the di-anion is now chemically de-activated by the proton donor and this may be more efficient than the chemical de-activation of the anion-radical. (ii) In the potential region of the second wave, the de-activation leading to the dimer may occur in a different way as with the first wave, i.e. by a direct attack of the di-anion on the substrate leading to the dimer di-anion: A + A 2- _~ D 2 T h i s species can then be the initiator of a polymerization process as in the region of the first wave. If these reactions are fast enough no second wave will be observed. In these conditions, the effect of proton donors can be explained in a similar way as in the preceding case. (iii) As regards the first wave, polymerization has been assumed to occur not only by addition of the dimer di-anion D 2 - on the substrate but also through a proton transfer reaction between D 2- and the substrate leading to an anion that can initiate another kind of chain growth sg. If this is true, it follows that in the region of the second wave A 2- can initiate the same chain growth through a proton transfer which seems even more likely with A 2 - than with D 2-. Polymerization may thus be more efficient at the second wave than at the first. This may be an interpretation

208

E. LAMY, L. NADJO, J. M. SAVI~ANT

of the presence of a dip on the plateau of the first wave, in the place of the second wave, as observed in the present work and even more clearly by other authors 41'46 depending presumably on the residual water content of the solvent. The mechanistic analysis of the dimerization process occurring at the first wave was made using LSV as the main tool. However, some preliminary remarks can be made on the basis of the above polarographic results, concerning the ionic mechanism (Scheme II). In the case where, upon sufficient addition of a proton donor, two successive one-electron waves are obtained, there is little doubt that the second wave is concerned with the addition of an electron to the anion-radical A T or to the neutral radical AH', leading to the di-anion A 2- or to the anion A H - . If the mechanism were an ionic one at the first wave, A 2 - or A H - would be formed in its potential region taking into account the positive shift in potential due to the coupling reactions (A + A 2 - or A + A H - ) and then no second wave corresponding to an electrohydrogenation would be observed. When no proton donor is added, a dip in the plateau of the first wave is often observed which is also related to a second-electron uptake leading to the d i - a n i o n A 2 - as confirmed by the fact that a second wave develops in the same potential region upon addition of a proton donor. This shows again that neither A 2- or A H - is formed in the potential region of the first wave. It follows from this that the ionic mechanism of dimerization is very unlikely to operate at the first wave. In the potential region of the dip, however, it may well be that dimerization results from this ionic process as shown above. In the LSV study the formal kinetics corresponding to this reaction scheme will nevertheless be compared to the experimental kinetics, in order to obtain further support for this view. It has been seen in the case of acrylonitrile that the variations in acidity of the medium offer too narrow a gap between hydrogenation and polymerization to allow a realistic study of the dimerization mechanism in the millimolar range. This is also true for diethyl fumarate. The LSV study of dimerization of dibenzoylethylene was precluded in ACN by the insufficient separation of the waves. In DMF, separation is better, but the first wave is reversible even at the lowest sweep rate used. In some cases the LSV analysis could be performed better in ACN than in D M F (cinnamonitrile, e-cyanoethyl cinnamate, methyl and ethyl cinnamate) since with a concentration of 1 mmol 1-~ the first wave is reversible at all sweep rates when no proton donor is added and the addition of a proton donor would lead too rapidly to the merging of the wave. In some cases (methyl and ethyl cinnamate) the study in D M F has nevertheless been possible by using larger concentrations ( ~ 5 mmol 1-1) since the reversibility then decreases and the waves are better separated so that more proton donor can be added to the solution which, in turn, decreases the reversibility. A search of the literature (ref. 2 and refs. cited therein) shows that the hydrodimers are effectively the main products in the preparative scale electrolysis of the e-// ethylenic nitriles, esters and ketones studied, or of closely similar compounds in comparable media. In the case of the ketones, the initial e-di-ketone is often converted partially into the/J-cetol and ethylenic ketones by successive cetolization and crotonization

ELECTRODIMERIZATION MECHANISM O F ACTIVATED OLEFINS

209

reactions. We have performed the macro-scale electrolysis of all the e-/~ ethylenic ketones considered in this paper according to the procedures given above. Our results are in good agreement with those of the literature as to the structure of the dimers and the yields. LSV ANALYSIS OF THE KINETICS AT THE FIRST WAVE

LSV analysis of the dimerization mechanism has been performed by measuring the variation of the first peak potential Ev as a function of the sweep rate v, the initial concentration c o and the proton donor content [TH]. This was done for all the series of activated olefins with the three exceptions mentioned above. For a given initial concentration and proton donor content, all the compounds exhibit an oblique linear dependence of Ev vs. log v at slow sweep rates and an independence of Ep of [~ at high sweep rates as shown schematically in Fig. 7. The first behaviour corresponds to a completely irreversible CV pattern and the second to a completely reversible one. The reversible behaviour features the fast conversion of the activated olefin into the corresponding anion radical which is then stable in the time-range considered. In some cases, the dimerization was too fast. to reach complete reversibility in the range of sweep rate values used, so that only the oblique linear portion of the Ep-log v relation was then obtained.

log v

Fig. 7. Peak-potential vs. sweep rate (schematic). (ab) shift with an increase in initial concn.. (AB) shift with increase in acidity.

Increasing the initial concentration shifted the oblique linear portion anodically, without any change in the horizontal portion when this was observable. On increasing the proton donor content both the oblique and the horizontal portions either remain the same or shift in the anodic direction, not necessarily to the same extent. The shift of the horizontal portions may represent both a change in the junction potential between the solution and the reference electrode and a variation in the solvation of the anion-radical. The interference of this last factor is shown by the fact that the shift of the horizontal part on proton donor addition depends upon the'particular olefin studied.

210

E. LAMY, L. NADJO, J. M. SAVI~ANT

TABLE 4 NITRILES

H20/°.o (or C~HsOH)

c°,/ -[~'Ep/~' log v] /mmol l 1 /mV

Cinnamonitrile C6Hsq2H=CH-CN, ACN 0.05 4.94 18 2.01 19

vjVs-1

[?Ep/? log c °] /mV

10 -3 k (media) [?Ep/? Io0 (H20 ,,'tool I -l s -1 (C6HsOH)J/mV

2.6 0.8

22

15

9 0.9

21

58

12 1

4.94 0.494

20 19

p-methylbenzylidenemalonitrile p-CHa-C6H4-CH=C(CN)2, ACN 0,05 5.5 20 > 1000 2.25 18 > 1000 1.0 18 > 1000 21 0.23 18 > 1000 1 3

1,0 1.0

19 20

> 1000 > 1000

~-cyanoethyl cinnamate CeHs-CH=C( COOEt ACN 0.05 3.39 21 CN 90 1.20 19 40 0.339 19 11

21

900

Negl. 5 1.20 C6HsOH 50 mmol 1-1 1.89

20 18

Fumaronitrile NC~CH=CH-CN, ACN 10 3.68 21 (polym.) 0.616 20 0.308 19

44 57

> 1000 > 1000 > 1000

1100 1000

21

Tables 4-7 give the slopes of the oblique portion of the Er-log v diagram obtained with the various olefins for each solvent (ACN, DMF, EtOH), for various initial concentrations and acidity of the medium. The acidity is given by the amount of water or phenol added in ACN and DMF or by the definition of the buffer solution in EtOH. The variation of peak potential with initial concentration is in the Tables under the form dEp/dlog c °. In cases where it was possible to reach the horizontal portion of the diagram, the value of the sweep rate vl corresponding to the intersection of this horizontal line with the oblique portion is also noted in the Tables. The variations of peak potential with the acidity of the medium in the Tables (SEp/# log(H20 ) o r 63Epic3 log(C6HsOH)) represent the influence of the acidity on the de-activating chemical reaction only. They represent the difference between the shift of the oblique portions and the shifts of the horizontal portions. In other words they are corrected for the influence of the acidity on the junction potentials and on the solvation of the anion radical. It follows that this evaluation has been possible only in those cases where reversibility, i.e. the horizontal part of the

211

ELECTRODIMERIZATION MECHANISM OF ACTIVATED OLEFINS TABLE 5 ESTERS H20/°/~

c° /mmol l 1

- [ ~ E p / ~ log v] /mV

vl/V s - t

Methyl cinnamate C6HsvCH=CH-COOCH3, ACN 0.05 5.04 22 4 5.04

[~Ep/~ lo(t c °) /mV

10 -3 k (media) [~Ep/~ log (1t20)] / m o l l -1 s -1 /mV

--

25

10 3,7 0.3

21

63

0.18

23 22 19

5.04

23

21

--

130

----

5 14 20

1.98

20 2

Methyl cinnarnate C~Hs-CH=CH-COOCH3, DMF <5 4.7 Rev, 5 4.7 18 0.7 10 4.7 20 2 20 4.7 22 3 Ethyl cinnamate C6Hs~SH=CH-COOC2Hs, ACN 0.05 4.61 20 3.5 4.61 3.78 0.378

21 18 19

6.8 5.0 O.5

20

24

20

45

Ethyl cinnamate C6Hs-CH=CH~COOC2H 5, DMF <5 5.66 Rev. 5 5.66 21 0.6

3.4

10

7

20 5.66

21

1.2

Ethyl crotonate CHs-CH=CH~COOC2Hs, ACN 0.05 2,28 22 > 500 1.48 22 > 500 0.2l 21 > 500

23

Ethyl crotonate CH3-CH=CH-COOC2H 5, DMF 0. l 4.4 19 400

d i a g r a m , w a s a c c e s s i b l e in the s w e e p r a t e r a n g e a v a i l a b l e . It f o l l o w s f r o m the results in the T a b l e s t h a t : (i) in e v e r y c a s e the r a t e o f v a r i a t i o n o f p e a k p o t e n t i a l w i t h the s w e e p r a t e is c l o s e to - 2 0 m V p e r d e c a d e ; (ii) the r a t e o f v a r i a t i o n o f E v w i t h initial c o n c e n t r a t i o n o f olefin is also close to 20 m V ; (iii) in s o m e cases the v a r i a t i o n o f Ep w i t h the a c i d i t y o f the m e d i u m is significant ( a r o u n d 20 m V ) . In s o m e o t h e r cases it is less t h a n the e x p e r i m e n t a l e r r o r . W h e n a v a r i a t i o n is o b s e r v e d it often a p p e a r s as small for the first a d d i t i o n s

212

E. LAMY, L. NADJO, J. M, SAVI~ANT

TABLE 6 AROMATIC KETONES

H20/0,0 (or ethanolic buffer)

c°

-[¢'.Ep/,; log v]

~retool I 1

/mV

UIIiVS -1

in1 V

10- 3 k (media) [?Ev/? log ( HzO)] ~tool l -~ s -~ ~mY

18

320

[?Ep/? Ioy c °]

Chalcone C~H 5 C H = C H - C O - C 6 H s, ACN 0.05 1.15 18 0.115 18

13 1.4

1

1.15

19

22

610

5

1.15

21

126

3500

Chalcone C6Hs~CI-I=CH-CO-C6H5, D M F 0.1 2.8 20

5

58

1

2.8

20

5.7

64

10

2.8 0.68

20 21

10 3.5

20

2.8~

19

18

6 23

Negl. 6 18

115 17

Benzalacetone C 6 H s - C H = C H - C O - C H 3 , ACN 0.05 4.86 21 2.47 20 0.247 20 0.5 4.86 21 2.42 19 0.242 19 5 4.86 18

11 7 0.9 90 65 6.5 700

Benzalacetone C ~ H s - C H = C H - C O - C H 3 , D M F 0.1 5.32 21 1 5.32 . 24 5 5.32 23 10 5.32 21

22 19 24 20

Benzalacetone C e H s - C H = C H - C O - C H > EtOH Buffer 1.33 19 Bu4NOH 0.1 tool 1-* 0.24 17

210

20

90

19

730

17

4500

135 115 144 120

Negl.

17.4 20

510

4.5

of water (5-6 mV) and then takes more significant values (,-~ 20 mV) on increasing the water content. This suggests that this variation would be negligible for water contents less than that corresponding to our "anhydrous" conditions which are indeed not very "anhydrous". DISCUSSION O F THE D I M E R I Z A T I O N M E C H A N I S M

In every case where reversibility can be reached by raising the sweep rate,

ELECTRODIMERIZATION

MECHANISM

OF ACTIVATED

213

OLEF1NS

TABLE 7 ALlPHATIC

H20/°;,

KETONES

co ~retool l- l

- [ F E p / F lo9 v] /m V

Mesityl-oxide ( C H 3 ) , C = C H - C O - C H 3 , 0.05

2.75 0.275

20 19

(?Ee/i ~ toy c °] /m V

10 -3 I; (media) [gEp/( lo9 ( H . O ) ] /mol 1- l s- ' /m V

17

250

36

--

300

38

--

310

> 1000 > 200

24

--

--

> 1000 > 300

17

--

--

21 2.1

20

345

--

vi/V s 1

ACN 18 2.5

Mesityl-oxide ( C H 3 ) 2 C = C H - C O ~ Z H 0.1 3.9 21

--

3, D M F

Negl. 1

3.9

20

H 2 C = C ( C H 3 ) ~ C ' O 4 Z T H 3, A C N 0.05

3.07 0.307

22 22

CHs-CH=C(CHs)-CO~CH 0.05

1.33 0.4

3, A C N

20 20 CHa

lsophorone

O,

ACN

CH3 0.05

2.03 1.85

22 19

cHa lsophorone

OH3 ~ - - ~ j/~= O ,

DMF

CH3 0. l

2.63

21

25

--

300

l

2.63

20

32

--

390

Negl.

the electron transfer from the electrode to the activated olefin to give the anion radical appears as a fast and reversible reaction. On the other hand irreversibility is obtained by lowering the sweep rate and not by raising it. Furthermore, the slope of the Ep-log v diagram obtained when the wave is completely irreversible is close to 20 mV, the variation with the initial concentration being 20 mV per decade. These observations thus exclude the initial electron transfer as being the ratedetermining step. It follows.that the cause of the observed irreversibility must be related to a chemical reaction following a fast and reversible electron transfer at the electrode. An increase of the rate of this chemical reaction does not render the electron transfer reaction rate-determining in the range of concentrations and acidities used here. The observed variations of Ep with log v and log c o indicate that the chemical reaction is a bimolecular reaction ~'4-67 and not a first order one. This

214

E. LAMY, L. NADJO, J. M. SAVI~ANT

last case would ultimately cover a rate-determining protonation of the anion radical. If the proton donor content is markedly larger than the depolarizer concentration, such a reaction scheme would lead to a 30 mV variation of Eo with log v and no variation with the initial concentration 64' 6s. If not, it would lead to a variation of Ep in the cathodic direction when increasing c °, i.e. opposite to the direction observed. It follows that the irreversibility of the first wave must be related to the coupling reaction or to a sequence of reactions involving the coupling reaction. Let us first consider the case where there is no appreciable variation of the peak potential with the acidity of the medium. Among the olefins and conditions of media studied here, this is unambiguously the case for: c~-cyanoethyl cinnamate in ACN (up to 5~o H20 or to 50 mmol 1-1 q~OH) chalcone in DMF (up to 1% H20) benzalacetone in DMF (up to 20~ H20) mesityl oxide in DMF (up to 1~o H20) isophorone in DMF (up to 1~o H20) and most probably in the "anhydrous" solvents for: methyl cinnamate in ACN ethyl cinnamate in ACN chalcone in ACN, i.e. in 8 cases of the 12 where the effect of addition of a proton donor was tested. As noted previously, three types of coupling mechanisms, radical-radical, radicalsubstrate and ion-substrate, must be considered. Tables 8-10 show detailed reaction schemes which are possible in each case according with the fact that the protonation reactions do not influence the kinetics observed. For each possible reaction scheme the expected rates of variation of peak potential with sweep rate and initial concentration at the experimental temperature (25°C) are given. For the purely radical mechanism (Table 8), protonation involves the dimer di-anion D 2- and the dimer mono-anion D H - , the coupling step of two anion radicals A-" being rate-determining. The coupling step itself is most probably reversible owing to the charge repulsion between the two anion radicals. The function of the protonation reactions is thus to render this step irreversible. It is considered that the neutral radical AH" is not involved in the coupling process,

TABLE 8 RADICAL-RADICAL C O U P L I N G

Reaction scheme

A+e-~A ~ 2 A T ~ D 2 - rate-det.---, D 2- + TH--*DH- + T DH- +TH~DH 2+TIf: A- + T H ~ A H + T A- + AH --* D H -

[~,Ep/c~ Io0 v] /mV at 25°C

[dEp/d log c °] /mV at 25°C

-

19.7

19.7

interference, then Ep varies with the TH content

ELECTRODIMERIZATION MECHANISM OF ACTIVATED OLEFINS

215

TABLE 9 RADICAL-SUBSTRATE COUPLING Reaction scheme

A+e-~A ~ A~+A~D Either: e.c.e. DT + e - ,~_D 2D 2- +TH'--,DH- +T DH + T H ~ D H 2 + T - Disproportionation DT+AT ~ D 2 - + A D 2- + T H ~ D H - + TDH- + T H ~ D H 2 + T Or: -e.c.e. D~ + T H ~ D H ' + T DH'+e- ~DHDH + T H ~ D H 2 + T - Disproportionation D r +TH ~ D H ' + T D H ' + A - ~ D H - +A DH- +TH~DH2+T

{ {

[ ?Ep/? log v] /mV at 25°C

[ ?Ep/c~ log c°] /mV at 25°C

- 29.6

29.6

-29.6 --19.7

29.6 39.4

(1)

(1): rate-det. ---, (2)(1): rate-det. ~ (2): rate-det.

(3) (l): rate-det. ~ -29.6 29.6 If (3) rate-det. Ep then varies with the TH content (3) (1): rate-det. ~ -29.6 29.6 (4) If either (3) or (4) rate-det, then Ep varies with the TH content

TABLE 10 ION-SUBSTRATE COUPLING Reaction scheme

A+2e-~A 2 A+A2-~D 2 D2- + T H ~ D H - + T DH- + T H ~ D H 2 + T If: A2 +TH~AH-+T } A+AH-~DH-

rate-det. --*

[~3Ep/c~ 1o9 v] /mV at 25°C

[OEp/~ log c°] /mV at 25°C

- 14.8

14.8

interfere, then E v varies with the TH content

o t h e r w i s e this w o u l d r e q u i r e a p r e - p r o t o n a t i o n step a n d t h u s a v a r i a t i o n o f E v with the a d d i t i o n of a p r o t o n d o n o r . I n these c o n d i t i o n s the f o r m a l k i n e t i c s are the s a m e as a l r e a d y d e r i v e d 64' 66.67 in the a b s e n c e o f o t h e r k i n e t i c c o m p l i c a t i o n s such as p r o t o n a t i o n . F r o m this derive the rates of v a r i a t i o n s of Ep w i t h v a n d c o in the T a b l e . F o r the r a d i c a l - s u b s t r a t e c o u p l i n g ( T a b l e 9) it is c o n s i d e r e d s i m i l a r l y that the n e u t r a l r a d i c a l is n o t i n v o l v e d in the c o u p l i n g step, o t h e r w i s e the p r o t o n a t i o n step l e a d i n g to its f o r m a t i o n w o u l d i n f l u e n c e the p e a k p o t e n t i a l . T h e c o u p l i n g step is t h u s a n a t t a c k of the a n i o n r a d i c a l o n the substrate, either a r a d i c a l a t t a c k o r a n ionic, M i c h a e l - t y p e , one. At this p o i n t the d i m e r a n i o n r a d i c a l c a n be either

216

E. LAMY, L. NADJO, J. M. SAVEANT

electronated and then protonated or protonated and then electronated. The first situation is the most likely one since the basicity of the dimer anion radical D T is not much different from that of the monomer anion radical A" that we have considered as being unprotonated. In this first situation, the dimer anion radical D : can be electronated either by the electrode or through a solution electron transfer from A 7. The first eventuality gives rise to an e.c.e.-type reaction scheme, whereas the second one looks like a disproportionation reaction scheme with reference to simpler cases where the chemical reaction interposed between the first and second electron transfers does not involve the substrate as here, but a substance, such as proton, extraneous to the reducing system itself6852. In both cases it can be formally considered that two particles of the same degree of oxidation are transformed into one particle of a lower degree of oxidation. It has been shown 68 that whenever an e.c.e.-type mechanism is considered, the occurrence of a disproportionation reaction must also be taken into account and vice versa. If an e.c.e.-type mechanism is considered in the present case, it is necessary to assume that the reduction potential of D " taking into account the succeeding protonation reactions is anodic to the reduction potential of A in order to justify the first electron character of the wave. If so, it follows that the solution-electron transfer equilibrium between A" and B Y (reaction (2)) is in favor of the left-hand side taking also into account the succeeding proton transfers. This leads to a consideration of a mechanism involving this electron transfer as well as the e.c.e.-type mechanism. An additional reason for this is that the coupling reaction (1) may well be, at equilibrium, in favor of the left-hand side this having been shown to be a favorable situation for the occurrence of a disproportionation mechanism 68' 52 The formal kinetics of the e.c.e.-type radical-substrate dimerization mechanism have already been established 67 ("DIM2" in ref. 67) and lead to the values given in Table 9. The possibility of a disproportionation-type mechanism has been recently outlined 67"*v but the formal kinetics were not derived in the papers cited. Two limiting cases must be considered where, respectively: (i) th~ coupling step (1) is rate-determining (this is the analog of the "DISPI" mechanism in ref. 52); (ii) the electron transfer step (2) is rate-determining (1) being a mobile pre-equilibrium (this is the analog of "DISPT' in rcf. 52). The analysis of the formal kinetics leads to the value of gEp/( ~ log v and gEp/( log c o in Table 9. The detailed treatment of these formal kinetics is given in a succeeding paper of the series 69. In every case either the coupling reaction (1) or the solution-electron transfer reaction (2) has to be the rate-determining step in order to account for the lack of influence of proton donor addition upon E r. If now, the dimer anion radical is first protonated and then electronated, this last step can occur by means of the electrode or the anion radical A T. Both an e.c.e.-type mechanism and a disproportionation type mechanism have thus to be considered here. However, the coupling step (1) must be the rate-determining one in both cases for the protonation of D T not to influence the peak potential. From this, follow the only possible values of ~?Er,/~ log v and ?,Ep/~ log c o given in Table 11. Although ion-substrate coupling has been shown to be unlikely on the basis of the above polarographic analysis, it has also been considered here relative to the variations of E v with v and c o (Table I0). Reasoning as to the possible place of

ELECTRODIMERIZATION MECHANISM OF ACTIVATED OLEFINS

217

proton transfers in the reaction sequence as concerns the noted independence of Ep upon addition of a proton donor is the same as in the previous cases. The values of ?Ep/(? log v and ?Ev/? log c ° in the Table then derive from a previous analysis performed in the case where no proton transfer complication 67 arises ("DIM3"). Comparison of the predicted values of ~Ep/?' log v and ?Ep/? log c o tbr each possible reaction scheme with experimental values shows that the purely radical mechanism fits the experimental data whereas the others do not. It can thus be concluded, in the case where the acidity of the medium does not influence the overall kinetics, that the mechanism of the EHD in the series of

activated olefins considered here incolves a rate-determining purely radical coupling of two anion radicals*. This does not exclude the possibility that the hydrodimerization process occurs along another mechanistic path, e.g. of the ionic type, in the potential region of the second wave when this is small or even unapparent. In this last case there would be a change of the mechanism along the plateau of an apparent single electron wave. According to this conclusion, the magnitude of the dimerization rate constant has been derived from the measurement of the value vi of the sweep rate at the intersection of the oblique and horizontal part of the Eo-log v diagrams using the equation valid in the case of a radical coupling mechanism 67, i.e.: k/mol-

1 1 s- 1

3 2 vi/V s - 1

- cO/tool ~-f (at 25°C)

in the case where Ep is not dependent u p o n the addition of proton donor as well as in the case where it is (the corresponding k values are given in Tables 4-7). As regards this last situation, the general trend is as follows: first the peak potential remains constant upon addition of the proton donor, then Epbegins to vary with increase of this factor. No reverse situation is observed. This shows that the interference of the proton donor does not involve the protonation of species such as D T, D 2-, D H - formed after the coupling step and must therefore be related to the protonation of A* leading to AH. In other words, since, at low proton donor content, the mechanism of the E H D involves the radical coupling of two A ~ there is no reason for the way in which A T interferes in the coupling process to change when increasing the addition of proton donor. The only possibility is then the interference of AH" in the coupling process. One must decide if AH" either undergoes a radical coupling with itself or with A ~ : AT +TH~AH'+T

(5)

2 AH" ~ D H 2 AH'+A T ~ DHor attacks the substrate giving a dimer neutral radical which leads to the final dimer through electronation by the electrode (e.c.e.) or by A T (disproportionation): * A similar conclusion has been reached 7° recently by Bard and co-workers in a study of cinnamonitrile, fumaronitrile and dimethyl fumarate by ring-disk techniques in DMF.

E. LAMY, L. NADJO, J. M. SAVl~ANT

218 AH'+A~DH"

(6)

D H ' + e -o D H DH'+A ~ ~ DH- +A

(7)

As shown in a forthcoming paper in the series 7~ that provides a detailed discussion of the kinetic role of protonation in E H D according to the various possible mechanisms, an important factor in the diagnostic analysis is to know whether the medium is buffered or not. In this connection, the results concerning benzalacetone in the tetra-alkylammonium hydroxide ethanolie buffer (Table 6) are of particular interest. A direct estimate of the variation of Ev with pH was unfortunately precluded in this case by the occurrence of a two-electron hydrogenation wave in the more acidic phenol buffer (it is noted incidentally that the mechanism of this hydrogenation involves a rate-determining disproportionation as shown by the fact that the Ep of the two-electron wave varies by about 20 mV per decade of v and c °, as in the case of the aromatic carbonyl compounds52). Nevertheless the dimerization rate constant in the ethanolic hydroxide buffer is markedly larger (about 5 times) than in D M F and "dry" ACN, indicating that interference with the proton transfer reactions is the cause of this enhancement of the overall dimerization rate. It is noted here again that the values of ~Ep/cqlog t, and ~Ep/? log c o are close to 20 mV. This fits with a purely radical coupling and not with other types of mechanism as far as a buffered medium is concerned 71. The behaviour of benzalacetone in the ethanolic hydroxide buffer thus shows the possibility of a purely radical E H D involving not only the anion radicals A T but also the neutral radical AH', either through an AH'/AH coupling or more probably an A-/AH" coupling since the acidity of the medium remains low as in the case of, for example, benzaldehyde 49. In ACN and DMF, despite the fact that the medium is not deliberately buffered, it is difficult to say whether, in the presence of acids such as water, tetra-alkylammonium cations and basic and acid impurities, difficultly avoidable in the millimolar range, the medium actually behaves as unbuffered or, if some kind of buffering occurs, through, for example, a fast decay of the O H - ion. As concerns formal kinetics, the 20 mV behaviour for both v and c °, can feature either as a purely radical rate-determining coupling of A" and AH" in a buffered medium or a rate-determining disproportionation (reaction 7) preceded by a mobile equilibrated attack of AH" on the substrate (reaction 6) and a mobile equilibrated protonation.of A T (5) in an unbuffered medium 71. It is therefore not possible to decide unequivocally on the mechanism on the basis of the kinetic data, An indirect argument in favor of the radical coupling is that if an attack of AH" on the double bond of A occurs the reduction should exhibit a trend toward polymerization which is not observed experimentally since, on the contrary, the addition of a proton donor tends to suppress the polymerization occurring at the first wave. Our general conclusion then is that the mechanism of EHD, at least in the millimolar concentration range, involves unambiguously the radical coupling of two of the anion-radicals formed initially and not any attack on the substrate in the case where the overall kinetics is independent of the addition of a proton donor. In the case where it is not, the same kind of mechanism involving the neutral

ELECTRODIMERIZATION MECHANISM OF ACTIVATED OLEFINS

219

radicals seems the most probable although some ambiguity still remains as to the interpretation of the kinetic data. Turning back to the arguments against radical coupling that derive from mixed coupling experiments, it can be said that these experiments are not in absolute opposition with the radical coupling mechanism. Indeed, one can argue that: (i) either the formation of an A-B type dimer occurs through the attack of A ~ on B while A ~ couples with itself to give the A-A type dimer or (ii) the formation of the A-B type dimer also occurs through a radical coupling, e.9.: A~+B~A+B ~

(8)

A T +B ~ ~ - A - B - A - B - +2 TH--* H A - B H + 2 TObviously reaction (8) is, at equilibrium, shifted toward the left-hand side since A is more reducible than B. However the shift is probably not so large as to block completely this pathway since mixed coupling occurs under conditions where the waves of A and B are not too far removed one from the other and in the presence of an excess of B. ACKNOWLEDGEMENT

The work was supported in part by the C.N.R.S. (E.R.A. No. 309: "Electrochimie organique"). SUMMARY

The mechanism of the reductive coupling of activated olefins in solvents of low acidity (acetonitrile, dimethylformamide,alkaline ethanol) is studied in a series of 13 compounds. The kinetics of the electrohydrodimerization occurring at the first reduction steps is derived from the variation of peak potential in linear sweep voltarnmetry with the sweep rate, the initial concentration and the amount of proton donor in the solution. In the cases where the protonation reactions do not influence the overall kinetics, the mechanism of the dimerization is shown to involve the purely radical coupling of two anion-radicals resulting from the initial electron transfer step. When the addition of a proton donor influences the apparent kinetics, the mechanism seems to remain of the radical coupling type, although some ambiguity still exists as to the interpretation of the kinetic data.

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221