Theoretical and Experimental Chemistry, Vol. 38, No. 5, 2002

ELECTROCHEMICALLY ACTIVATED REACTION OF BENZOYL HALIDES WITH CARBON DIOXIDE V. E. Titov, V. A. Lopushanskaya, and V. G. Koshechko

UDC 541.138.3+542.951+547.484

The electrochemical activation and carboxylation of benzoyl halides (benzoyl bromide, chloride, and fluoride) were studied. It was found that the yield of phenylglyoxylic acid increases from zero to 88% in the transition from benzoyl fluoride to the chloride and bromide. The effect of the nature of the halogen atom in the benzoyl halide and also the nature of the supporting electrolyte and the electrode material on the electrochemical reduction and carboxylation of benzoyl halides was studied. Key words: electrochemical carboxylation, benzoyl halides, carbon dioxide, phenylglyoxylic acid.

Among the various approaches to the utilization of carbon dioxide and its use as a cheap raw material attention has been attracted to methods involving the electrochemically activated insertion of CO2 into various organic substrates, which makes it possible to obtain valuable organic products under mild conditions in environmentally acceptable processes [1-5]. Not so long ago we found [6-8] that the electrochemical carboxylation of acyl chlorides made it possible to obtain α-oxocarboxylic acids, which have found extensive practical application as drugs, agricultural chemicals, perfumes, and other important products. It seemed of interest to study the possibility of using not only acyl chlorides but also other acyl halides, particularly benzoyl bromide (BB) and benzoyl fluoride (BF), in such processes. On the other hand, as we demonstrated [7, 8], the key stage in the electrochemically activated insertion of carbon dioxide into acyl halides is electrochemical reduction of the acyl halides, and the factors affecting it also affect the carboxylation reaction and the yield of the respective oxo acid. At the present time the range of papers devoted to the electrochemical behavior of BB and BF [9, 10] is extremely limited. In the light of the foregoing in the present work we studied the effect of the nature of the halogen atom in the benzoyl halide on the kinetics and mechanism of electrochemical reduction and carboxylation of aromatic acyl halides in dimethylformamide (DMF).

EXPERIMENTAL The reduction of BB, BC, and BF was investigated by cyclic voltammetry in a three-electrode glass cell at disk cathodes of Pt, Ni, Fe, Cu, Ti, W, and glassy carbon (GC) in DMF with Bu4NBF4, Et4NBF4, Me4NBF4, LiBF4, and LiBF4 as supporting electrolytes (s.e.) (CBB,BC,BF = 5·10–3 M, Cs.e = 0.1 M). The reference electrode was a silver chloride electrode, and the auxiliary electrode was a platinum plate. The investigations were conducted with a computerized electrochemical unit based on an EP 20A potentiostat and a Pentium PC at potential sweep rates in the range of v = 0.1-10 V/s. The rate constants of heterogeneous electron transfer (ks), the standard potential (E0), and the number of participating electrons (n) and transfer coefficient (α) were calculated in the course of optimization of the indicated parameters with comparison of the experimental and theoretically calculated voltammograms using software that we developed. Theoretical modelling of the cyclic ___________________________________________________________________________________________________ L. V. Pisarzhevskii Institute of Physical Chemistry, National Academy of Sciences of Ukraine, 31 Prospekt Nauky, 03039 Kyiv, Ukraine. E-mail: [email protected]. Translated from Teoreticheskaya i Éksperimental’naya Khimiya, Vol. 38, No. 5, pp. 283-287, September-October, 2002. Original article submitted July 16, 2002. 0040-5760/02/3805-0289$27.00 ©2002 Plenum Publishing Corporation

289

Fig. 1. Cyclic voltammogram of benzoyl bromide, benzoyl chloride, and benzoyl fluoride (Pt, 0.1 M Bu4NBF4, DMF). voltammograms corresponding to the given values of the parameters (α, n, ks, E0, and D the diffusion coefficient) was realized according to the approach proposed in [11] using the method of finite differences (FIED) [12]. Preparative electrolysis for the study of the products of electrochemical reduction of BB, BC, and BF was conducted in a stream of argon in an unseparated glass cell, fitted with a platinum cathode and a soluble zinc anode, in a 0.1 M solution of Bu4NBF4 in DMF with CBB,BC,BF = 3.3⋅10–2 M. In the preparative experiments on chemical carboxylation the solution was saturated with carbon dioxide, and carbon dioxide was passed throughout the synthesis. The cathodes were of Pt, Ni, W, and GC. The phenylglyoxylic acid was isolated and identified as described in [7]. For identification of the other products we used NMR and field-ionization mass spectrometry.

RESULTS AND DISCUSSION On the cyclic voltammogram (CVA) of benzoyl bromide (Fig. 1) there is a peak with potential Ep = –0.48 V (Table 1), corresponding to the reduction of BB, and also a broad wave in the region of E = –1.10-1.60 V. The cyclic voltammogram for the reduction of benzoyl chloride has similar form. However, its reduction potential lies in the more negative region (Fig. 1, Table 1). The electrochemical reduction of benzoyl fluoride requires even more significant negative potentials (Ep = –1.78 V) than benzoyl bromide and benzoyl chloride. As follows from the data presented in Table 1, the processes of electrochemical reduction of benzoyl bromide and benzoyl chloride under the employed conditions are quasireversible reactions, and the rate of the heterogeneous stage of the reduction of benzoyl fluoride may indicate that this process lies at the boundary between quasireversible and irreversible reactions. The investigated processes are one-electron processes, although the value of n in benzoyl bromide is somewhat higher than in benzoyl chloride and benzoyl fluoride. In the transition from benzoyl bromide and benzoyl chloride to benzoyl fluoride the rate of the heterogeneous electron transfer stage is substantially reduced, and their standard reduction potential is shifted toward more negative potentials. The linear relations between E0 and ln ks on the one hand and the energy of the C—Hal bond on the other (E0 = 0.615 – 0.016EC—Hal, r = 0.99; ln ks = –1.637 – 0.084EC—Hal, r = 0.98) indicates that the energy of the C—Hal bond makes a determining contribution to the activation energy of the electrochemical reduction processes in the investigated benzoyl halides. We carried out some preparative investigations in order to study the probable paths of the electrochemical reduction processes of benzoyl halides in DMF. The cathodic reduction of benzoyl chloride was realized under potentiostatic conditions at E = –1.1 V. After a decrease in the current density (from 3 mA/cm2 to 0.1 mA/cm2) the electrolysis was completed. Mass-spectrometric analysis of the products showed the presence of the following masses: 106 m/z, which can be assigned to benzaldehyde; 210 m/z, benzil; 316 m/z, stilbenol benzoate; 420 m/z, stilbenediol dibenzoate. This agrees with data on the electrochemical reduction of benzoyl chloride in acetonitrile [10] and acetone [14] and also with the results of an investigation 290

TABLE 1. The Parameters of the Heterogeneous Stage of the Electrochemical Reduction of Benzoyl Halides (0.1 M Bu4NBF4, Pt) Benzoyl halide Benzoyl bromide Benzoyl chloride Benzoyl fluoride

Ep, V (v = 0.2 V/s)

E0, V

ks, cm/s

α

n

EC—Hal, kJ/mol [13]

–0.48

–0.34

1.5⋅10–3

0.38

1.35

54

–0.41

–4

0.37

0.96

71

–6

0.41

1.00

~117

–0.56 –1.78

7.5⋅10

–1.30

9.1⋅10

of this process by cyclic voltammetry. The reduction potential of benzil was E = –1.12 V. Stilbenol benzoate and stilbenediol dibenzoate are reduced in a more negative region [10], while benzaldehyde is reduced at –1.80 V. These processes may correspond to the second wave (E = –1.10 to –1.90 V) in the cyclic voltammogram of benzoyl chloride (Fig. 1). On the other hand it is impossible to exclude the possibility of a contribution to the current of this wave from the electrochemical reduction of the benzoyl radical, i.e., the product from the reduction of benzoyl chloride, the standard potential of which, determined in [15], amounts to E0 = –1.14 ± 0.15 V with reference to a saturated calomel electrode. Our model of the cyclic voltammogram of the cathodic reduction of the benzoyl radical from the standard values of the parameters of this process (α = 0.5; n = 1; ks < 10–2 cm/s; D = 1⋅10–5 cm2/s) indicates that the peak potential Ep may lie in the range of potentials of this wave. Possible paths for the formation of the discovered products can be represented by the following probable scheme: Ph

C

Ph

C

O

Ph

Hal

.

O + SH

.

2Ph

e

C

Ph

O

O

Ph

C

.

O

Ph

C _

O + Ph

Ph O

C

C

e

Ph

C

C

-

-Hal

Hal

Ph

C

C

C

C _

-.

O

.

Ph

C

O

.

O

(b)

+S

H

(a)

O

(c)

Ph

O O

Ph -

Hal -Hal

O e, PhCOHal, HS . -Hal , -S Ph

O

C

Ph Ph

C

O

O

C

C

C

Ph C

O

OH Ph

(e)

O O

Ph O 2e, 2PhCOHal C C O Ph -2Hal Ph

(d) (1)

Ph

C C

O Ph

C Ph

(f)

O

In contrast to benzoyl chloride, we did not find appreciable quantities of benzaldehyde among the products from preparative electrolysis of a solution of benzoyl bromide, conducted under the same conditions as for benzoyl chloride. However, benzil, stilbenol benzoate, and stilbenediol dibenzoate were present. Analysis of the products from preparative reduction of benzoyl fluoride (potentiostatic conditions, E = –2.0 V) showed that the main product of the process was benzaldehyde, identified by mass spectrometry and 1H NMR, and there were no appreciable amounts of other compounds. The differences in the composition of the products from preparative electrolysis of benzoyl bromide, benzoyl chloride, and benzoyl 291

fluoride can probably be explained by the different stability of the intermediates formed as a result of one-electron reduction of the benzoyl halides. Thus, benzoyl bromide has relatively low energy in the C—Br bond (Table 1), which probably can be easily broken, and the cathodic reduction of benzoyl bromide (scheme 1a) can take place by a mechanism close to dissociative electron transfer. This can lead to a local increase in the concentration of the products from reductive dissociation of benzoyl bromide, i.e., benzoyl radicals, in the region adjacent to the electrode and to preferential realization of the process through their dimerization and further transformations of the benzil (scheme 1c, f, g). In contrast to this, the energy of the C—F bond is significantly higher than that of the C—Br bond (Table 1). As a result the radical-anion of benzoyl fluoride can have sufficient energy to escape from the layer adjacent to the electrode into the solution. This increases the probability of preferential reaction of the benzoyl radical formed as a result of dissociation of the benzoyl fluoride radical-anion with the solvent leading to the formation of benzaldehyde (scheme 1b). Among the investigated benzoyl halides benzoyl chloride occupies an intermediate position both in the energy of the C—Hal bond and in the composition of the products from electrochemical reduction. As we established, saturation of a solution of benzoyl halides with carbon dioxide has a significant effect both on the cyclic voltammograms of benzoyl bromide and benzoyl chloride and on the processes involved in their preparative reduction. Here the cyclic voltammogram shows a significant decrease of the current of the wave in the region of potentials E = –1.10 to –1.90 V. This may indicate partial suppression of the processes involved in the cathodic reduction of the products from the electrochemical reduction of benzoyl bromide and benzoyl chloride and the appearance of a new path for the consumption of the intermediates of this process in reaction with carbon dioxide, which may lead to the formation of the anion of phenylglyoxylic acid: O Ph

C

Hal

+2e, CO2 -

-Hal

Ph

C

O COO -

(2)

This suggestion is confirmed unambiguously by the results of our preparative investigations. Thus, the electrolysis of a solution of benzoyl bromide (0.1 M Bu4NBF4, Pt), saturated with carbon dioxide, under potentiostatic conditions (E = –1.1 V) led to the formation of phenylglyoxylic acid with a yield of 88%. Phenylglyoxylic acid is also formed during the electrochemical reduction of benzoyl chloride under analogous conditions but with a yield of 39%. The results from investigation of the electrochemical reduction of benzoyl fluoride in the presence of carbon dioxide differ substantially from those for benzoyl bromide and benzoyl chloride. The addition of CO2 leads not to a decrease, as in the case of benzoyl bromide and benzoyl chloride, but to an increase of the current of the peak with E = –1.78 V in the cyclic voltammogram of benzoyl fluoride and to the appearance of a strong wave in the region of E < –2.0 V. The observed changes may be due to the superimposition of the reduction wave of CO2 (Ep = –2.34 V) on the reduction peak of benzoyl fluoride itself. The electrolysis of a solution of benzoyl fluoride (E = –2.0 V) in the presence of carbon dioxide does not lead to the formation of appreciable amounts of phenylglyoxylic acid and gives mostly benzaldehyde. The effect of the nature of the halogen atom on the processes involved in the electrochemical reduction of benzoyl halides in the presence of carbon dioxide and on the products that are formed can be explained by the different stability of the radical-anions of the benzoyl halides. The radical-anions of benzoyl chloride and benzoyl fluoride more stable than the benzoyl bromide radical-anion can escape from the layer adjacent to the electrode into the solution, where cleavage of the C—Hal bond occurs and a benzoyl radical is formed in the bulk of the solution. This increases the probability of its reaction with the solvent with the formation of benzaldehyde. The formation of benzaldehyde is not observed during the cathodic reduction of benzoyl bromide, and the intermediates of the process are probably used up to a lesser degree in reaction with the solvent (scheme 1b). This leads to the high yield of phenylglyoxylic acid in the electrochemical carboxylation of benzoyl bromide (in contrast to benzoyl chloride and benzoyl fluoride). In spite of the considerable number of papers on the electrochemical carboxylation of halogen-containing organic compounds [1-3], due attention has not been paid to the question of the effect of the nature of the supporting electrolyte on these processes. Since we had shown that the electrochemical activation of benzoyl halides is a key stage in carboxylation, the effect of the supporting electrolyte both on the electrochemical reduction and the carboxylation processes was studied. It was found that in the transition from LiBF4 to the tetraalkylammonium salts the potential of the reduction peak of benzoyl chloride and benzoyl fluoride is shifted substantially toward the less negative region (Table 2), and the peak current is increased by 1.5-2 times. The observed specific effect of the tetraalkylammonium salts can be due to their ability to form associates with 292

TABLE 2. The Peak Potentials Ep on the Cyclic Voltammograms for the Reduction of Benzoyl Halides Using Various Supporting Electrolytes (DMFA, Pt) Supporting electrolyte

LiBF4

Bu4NBF4

Et4NBF4

Me4NBF4

C6H5COCl

–0.82

–0.56

–0.48

–0.46

C6H5COF

–2.23

–1.78

–1.79

–1.63

halogen-containing organic compounds [16]. It is likely that such associates can be formed as a result of the partial negative charge of the halogen atom in the benzoyl halide molecule and the positive charge of the tetraalkylammonium cation and can possess stronger electron-accepting characteristics compared with the initial benzoyl halides. This can lead to a shift of their reduction potentials toward the less negative region. We observed such an effect in the case of benzoyl bromide, but the shift of the potential in this case was not so significant (~0.07 V), due probably to the lower electronegativity of the bromine atom and the smaller partial negative charge on it compared with benzoyl fluoride and benzoyl chloride. On the other hand, as known [17], the tetraalkylammonium salts are characterized by specific adsorption on metallic cathodes. As a result of this the molecules of the benzoyl halides can be “drawn” by the adsorbed tetraalkylammonium cations into the region adjacent to the electrode, leading to an increase in the peak current. We established that the nature of the supporting electrolyte can have a significant effect not only on the electrochemical activation of the benzoyl halides but also on their carboxylation. Thus, in particular, the use of LiBF4, Et4NBF4, and Me4NBF4 instead of Bu4NBF4 as supporting electrolytes in the electrochemical carboxylation of benzoyl chloride makes it possible to increase the yield of phenylglyoxylic acid from 39 to 60, 66, and 67% respectively. The observed increase in the yield of the acid can be due both to facilitated activation of benzoyl chloride in the transition from Bu4NBF4 to Et4NBF4 and Me4NBF4 and to the fact that the smaller-sized cations are capable with greater probability of forming ion pairs with the charged intermediates and the products of the process (radical-anion, carbanion, phenylglyoxylate anion, etc.), thereby stabilizing them. As we showed, the other important factor affecting the electrochemical activation and carboxylation processes in the benzoyl halides is the electrode material. As established as a result of the investigations, the potential of the reduction peak of benzoyl chloride and benzoyl bromide is shifted toward the more negative region in the following series of materials: Pt (–0.48 V;* –0.56 V†) > Ni (–0.64 V;* –0.82 V†) > Fe (–0.67 V;* –0.92 V†) > Cu (–0.80 V;* –1.02 V†) > Ti (–0.88 V;* –1.04 V†) > W (–1.14 V;* –1.22 V†) > GC (–1.48 V;* –1.36 V†) (*for benzoyl bromide, †for benzoyl chloride, supporting electrolyte Bu4NBF4). Such an order of variation of the potential in benzoyl bromide and benzoyl chloride may indicate a similar mechanism for their adsorption on the investigated cathodes. The potentials of the reduction peaks at the employed cathode materials lie in the region of E = –1.33 to –2.24 V and do not coincide with the order observed for benzoyl bromide and benzoyl chloride: Cu (–1.33 V) > Fe (–1.55 V) > Ti (–1.76 V) > Pt (–1.78 V) > GC (–1.96 V) > W (–2.18 V) > Ni (–2.24 V). In the electrochemical carboxylation of benzoyl chloride and benzoyl bromide the highest yields (67 and 88% respectively) can be obtained with a platinum electrode. The electrochemical carboxylation of benzoyl chloride and benzoyl bromide with the formation of phenylglyoxylic acid can also be realized at other cathode materials (GC, W, Ni) but with smaller yields (23-43%). Thus, it was shown as a result of the investigations that the electrochemical carboxylation of benzoyl bromide and benzoyl chloride leads to the formation of phenylglyoxylic acid with a yield amounting to 88% (benzoyl bromide, Pt cathode, Zn anode, 0.1 M Bu4NBF4, DMF). It was established that the nature of the halogen atom and the electrode material can have a significant effect on the electrochemical activation and carboxylation processes in benzoyl halides. In particular, it was shown that in the transition from benzoyl fluoride to benzoyl chloride and benzoyl bromide the standard potential for reduction of the benzoyl halide is shifted from –1.30 and –0.41 to –0.34 V, and the rate constant of heterogeneous electron transfer is increased 293

substantially [9.1·10–6 cm/s (benzoyl fluoride); 7.5·10–4 cm/s (benzoyl chloride); 1.5·10–3 cm/s (benzoyl bromide)]. The yield of phenylglyoxylic acid increases with the benzoyl halides in the same order from zero to 67 and 88%. The authors express their sincere gratitude to Academician of the National Academy of Sciences of Ukraine V. D. Pokhodenko for valuable advice in setting up the investigations and also to Candidate of Physical and Mathematical Sciences V. G. Golovatyi and his colleagues for assistance afforded in the mass spectrometric investigations. The work was carried out with support from the State Fund of Fundamental Research of Ukraine (project No. 03.07/00113).

REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17.

294

J. Silvestri, S. Gambino, and G. Filardo, Enzymatic and Model Carboxylation and Reduction Reactions for Carbon Dioxide Utilization , M. Aresta (ed.), Kluwer Academic Publishers, Dordrecht (1990), pp. 101-127. J. Chaussard, J.-C. Folest, J.-Y. Nedelec, et al., Synthesis, No. 5, 369-381 (1990). A. P. Tomilov, Élektrokhimiya, 30, No. 6, 725-738 (1994). V. D. Pokhodenko and V. G. Koshechko, Free Radicals in Biology and Environment: NATO ASI Ser., F. Minisci (ed.), Vol. 27, Kluwer Academic Publishers, Netherlands (1997), pp. 145-159. V. G. Koshechko and V. D. Pokhodenko, Teor. Éksp. Khim., 33, No. 5, 268-283 (1997). V. D. Pokhodenko, V. G. Koshechko, V. E. Titov, and V. A. Lopushanskaya, Tetrahedron Lett., 36, No. 18, 3277-3278 (1995). V. G. Koshechko, V. E. Titov, V. A. Lopushanskaya, and V. D. Pokhodenko, Teor. Éksp. Khim., 34, No. 1, 1-5 (1998). V. G. Koshechko, V. E. Titov, and V. A. Lopushanskaya, Élektrokhimiya, 36, No. 2, 167-172 (2000). P. Arthur and H. Lyons, Anal. Chem., 24, No. 9, 1422-1425 (1952). G. T. Cheek and P. A. Horine, J. Electrochem. Soc., 131, No. 8, 1796-1801 (1984). C. P. Andrieux and J.-M. Saveant, Investigation of Rates and Mechanisms of Reactions, Techniques of Chemistry , C. F. Bernasconi (ed.), Vol. VI, Pt. II, Chap. VII, Wiley, New York (1986). M. Rudolph, D. P. Reddy, and S. W. Feldberg, Anal. Chem., 66, No. 10, 589-600 (1994). L. V. Gurvits, G. V. Karachevtsev, V. N. Kondrat’ev, et al., Dissociation Energy of Chemical Bonds. Ionization Potentials and Electron Affinity [in Russian], Nauka, Moscow (1974). A. Guirado, F. Barba, C. Manzanera, and M. D. Velasco, J. Org. Chem., 47, No. 1, 142-144 (1982). D. Occhiliani, K. Daasbjerg, and H. Lund, Acta Chem. Scand., 47, 1100-1106 (1993). S. G. Mairanovskii, Élektrokhimiya, 5, No. 6, 757-759 (1969). Yu. M. Loshkarev, V. P. Kuprin, and N. B. Grigor’ev, Élektrokhimiya, 4, No. 4, 84-87 (1973).