J. Electroanal. Chem., 63 (1975) 429-433 © Elsevier Sequoia S.A., Lausanne - Printed in The Netherlands

429

SHORT COMMUNICATION

Electrochemical cyclization

III. Mechanism of the intramolecular pinaeolization of 1,3-dibenzoylpropane as studied by convolution potential sweep voltammetry

C. P. ANDRIEUX, J. M. SAVI~ANT and D. TESSIER

Laboratoire d'Electrochimie de l'Universit~ de Paris VII, 2, Place Jussieu, 75221 Paris c~dex 05 (France)

(Received 14th April 1975)

Preparative scale electrolysis of 1,3-dibenzoylpropane (DBP) leads almost quantitatively to the cis-cyclized pinacol in acetonitrile x (ACN). Similar results have also been obtained with the parahydroxy and methoxy derivatives in alkaline water and ethanol / . The mechanism of this reaction has been studied in the first solvent on the basis of the experimental variations of the linear sweep voltammetry (LSV) peak potential with sweep rate and initial concentration 1. The diagnosis was made on the basis of a previous formal kinetic analysis of reaction schemes involving an hydrocyclization process following a nernstian charge transfer step 3.

The analysis of the experimental peak potential shifts showed that four mechanisms were compatible with the data, involving: --either the cyclization of the anion radical:

e -c~ l -P-p or e -c-p-D-p

---or the cyclization of the di-anion:

e~t -c-P-p or e -d-C-p-p

(e: electrode-electron transfer; c: cyclizati0n reaction; d: solution electron transfer; p: protonation reaction1'3).

The first possibility was eliminated by an estimation of the respective rates of the solution electron transfer and of the protonation reaction and also by the observation that the kinetics do not vary upon a fifty-fold increase in the water concentration. To reject the second one, it was necessary to repeat the experiment in an ACN buffered solution and to check that the peak potential remained unaffected in such a medium by initial concentration variations. The conclusion was thus that the actual mechanism involves a radical-radical intramolecular coupling in the di-anion, preceded by a solution electron transfer from one initial radical anion to another and followed by protonation, the rate determining step being either the cyclization step or the protonation reaction. It was noted that the cyclization process is so fast that in, e.g. ACN+2% H20, the kinetic control by the charge transfer becomes apparent above 30 V s-1.

4 3 0 SHORT C O M M U N I C A T I O N

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SHORT COMMUNICATION 431

The aim of the present communication is to describe the application of convolution potential sweep voltammetry (CPSV) to the mechanism analysis of the same reaction. The fundamentals of CPSV, particularly the principles of its application to electrochemical reactions involving homogeneous follow-up chemical steps, have already been exposed 3,4 and experimentally tested 5. The method has also been applied to a kinetic analysis even more complex than the present one concerning the electrohydrodimerization of an activated olefin 6. The present study was undertaken not only to simply verify the previous mechanistic conclusion but to re-inforce it since more accuracy in diagnosis is to be expected from CPSV than from LSV 4-7.

Since the cyclization process is very fast, corresponding to completely irreversible patterns in cyclic voltammetry up to at least 1000 V s-1, no mixed diffusion-chemical reaction kinetic control is to be observed. In other words, the pure kinetic conditions are achieved in the whole accessible sweep rate range. According to the above remark on the progressive control by the charge transfer kinetics the sweep rates used here were rather low: 0.055, 0.19, 0.54 and 1.9 V s -1.

The various mechanistic possibilities as determined previously 3 are listed in Table 1 according to a previously defined nomenclature 3. The logarithmic analyses to be applied to the convoluted current-potential curves are of the form:

E = E k + V ( R T / F ) I n f ( I i - I , I, i)

i.e., at the temperature of experiment (22°C):

E = Ek+ 58.6 V In f ( I i - - / , / , i)

The 16 possible reaction schemes are distributed over only 3 forms of the logarithmic analysis (see Table 2) with equal or different values of the slope: 58.6 V. In Table 1, the number of the logarithmic analysis corresponding to each reaction scheme is given together with the characteristic value of the slope and the values of the LSV peak potential shifts with v and c ° are recalled.

TABLE2

FORMS OF THE LOGARITHMIC ANALYSIS

E=Ek + y(RT/F) In fill--I, 1, i)

Conventional f ( I t - I, I, i) Theoretical slopes Experimental slopes numbering /m V (22 ° C) /m V (22 ° C)

1 (1,-I)/i 29.5 36.5 39.0 58.6

2 (I 1- l)/i ~ 58.6 42.1 3 (I~- I)/I ~ i ~ 58.6 33.8

The experiments were performed in ACN + 0.1 M Et4NCIO4 with 2~o water added. The working electrode was a mercury droplet hanging from an amalgamated gold disk and the reference electrode was the Ag/AgC104 0.01 M couple. The cell, the controlling and measuring instrument and the procedures for digital acquisition,

432 SHORT COMMUNICATION

convolution, correction for iR drop and sphericity and logarithmic analysis were the same as already described 4-v.

- 4

~-2

D ~ L

~L

f I I I I I f I I I - 2 . 0 - 2 . 15 - 2 . 20 E/V

Fig. 1. Logarithmic analysis of the convoluted i-E curves, c °= 1 mmol 1 -a, v=0.055, 0.19, 0.54 and 1.89 V s - j . Sweep rate: ( 0 ) 1.89, ( I ) 0.54, (A) 0.19, (A) 0.055 V s -~.

With an initial concentration of 1 mmol 1-1 the logarithmic analysis 1 obtained for the 4 sweep rates is shown in Fig. 1. It gives a slope of 36.5 mV. The slopes obtained with the other logarithmic analysis are given in Table 2. For c °= 0.4 and 2 mmol 1-1 the results are essentially the same.

It is seen, by reference to Table 1, that the only possibility compatible with the experimental data corresponds to the logarithmic analysis 1 with a slope of 39 mV. The experimental value is slightly different from the theoretical one but the agreement is anyway far better that for any other possibility.

Inspection of Table 1 shows that the only mechanisms matching the experimental data are:

either e-c--d-P-p

or e-d--c-P-p and e -d -C-p-p .

The fourth possibility found in LSV, i.e., e-c-p-D-p, is excluded here by the results of the logarithmic analysis without the necessity of having recourse to a complementary experiment in b/lffered medium. However in order to eliminate the first possibility one has to resort to the same discussion as already developed in the LSV study.

Our conclusion is thus that the CPSV study confirms the fact that the cyclization reaction proceeds at the level of the di-anion and not of the initial anion-radical and that the di-anion is produced through a solution electron transfer, the rate determining step being either the cyclization step or the next protonation.

Acknowledgement The work was supported in part by the C.N.R.S. (Equipe de Recherche

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Associ6e No. 309: E lec t roch imie Organ ique ) . Prof. M. Hulin, Universi t~ de Par i s VI, is t hanked for the pe rmiss ion to use the 1130 I B M c o m p u t e r of the " G r o u p e de Phys ique des Sol ides de l 'Ecole N o r m a l e Sup6rieure, Paris" .

REFERENCES

1 F. Amrnar, C. P. Andrieux and J. M. Say,ant, J. Electroanal. Chem., 53 (1974) 407. 2 R. N. Gourley and J. Grimshaw, J. Chem. Soc. (C), (1968) 2388. 3 C. P. Andrieux and J. M, Sav6ant, J. Electroanal. Chem., 53 (1974) 165. 4 J. C. Imbeaux and J. M. Sav6ant, J. Electroanal. Chem., 44 (1973) 169. 5 J. M. Sav6ant and D. Tessier, J. Electroanal. Chem., 61 (1975) 251. 6 L. Nadjo, J. M. Sav6ant and D. Tessier, J. Electroanal. Chem., in press. 7 L. Nadjo, J. M. Say,ant and D. Tessier, J. Electroanal. Chem., 52 (1974) 403.