Investigation of the processes of electrochemical perfluoroalkylation and fluorosulphation

167

.I. Electroanal. Chem., 325 (1992) 167-184 Elsevier Sequoia S.A., Lausanne

JEC 01894

Investigation of the processes of electrochemical perfluoroalkylation and fluorosulphation Part 1. Electrode processes and the electrochemical perfluoroalkylation mechanism V.A. Grinberg and Yu.B. Vassiliev A.N. Frumkin Institute of Electrochemistry, USSR Academy of Sciences, Moscow (USSR)

(Received 23 May 1991; in revised form 18 October 1991)

Abstract

The characteristics of the adsorption and electro-oxidation of perfluorocarboxylic acids and organic compounds, which are radical acceptors, on a platinum electrode have been investigated over a wide range of potential (O-3 V). The binding of radicals produced during the electrolysis of perfluorocarboxylates by perfluoro-olefins was studied using the electron paramagnetic resonance CEPR) technique. It was shown that, depending on the radical structure and the nature of the solvent, perfluoroalkyl radicals participate in the reactions involved in the perfluoroalkylation of organic compounds adsorbed on the electrode surface or dissolved in the solution bulk.

INTRODUCTION

Despite many technical problems, the electrochemical fluorination of organic compounds (the Simons process) has been implemented in industry and has been applied successfully to the synthesis of a large number of important organofluorine compounds. However, the application of electrochemistry to the synthesis of organofluorine compounds is not confined to fluorination methods. In recent years electrochemical generation of perfluorinated and polyfhrorinated radicals has been applied successfully in organofluorine synthesis. Strictly speaking, application of both the Kolbe reaction and its variant, the Brown-Walker reaction, to organofluorine acids are examples of electrochemical perfluoro-alkylation [l-51. The application of the Kolbe reaction to halogen-containing acids is generally accompanied by side-reactions associated with the reduc0022-0728/92/$05.00

0 1992 - Elsevier Sequoia S.A. All rights reserved

168

tion of the carbon-halogen bond, dehydrohalogenation etc. In the case of polyfluorinated acids the reduction of the carbon-fluorine bond does not take place and the perfluoroalkyl group participates in the reaction as a unit. The nature of the perfluoroalkyl group thus suggests that electrochemical alkylation could provide a convenient method for the synthesis of organofluorine compounds. However, the efficacy of electrochemical synthesis can be increased substantially by introducing into the electrochemical cell a substance capable of capturing free radicals, including the perfluoroalkyl radical. Lindsey and Peterson [6] were the first to.show in principle how to perform the trifluoromethylation of conjugated dienes under Kolbe reaction conditions. Later, activated olefins were included in this reaction [7]. Perfluoroalkylation reactions of lower olefins, fluoro-olefins and aromatic compounds have recently been performed successfully [8-161. Although the synthetic aspects of electrochemical alkylation are well known, it is worth considering separately the problems of electrochemical perfluoroalkylation which are directly related to the electrochemical behaviour of perfluorocarboxylic acids and organic acceptors. EXPERIMENTAL

The experimental procedure was the same as that described earlier 117,181.The surface coverage with adsorbate was determined from the decrease in hydrogen adsorption Q,/Qk upon application of a fast negative pulse [19], or by the decrease in oxygen adsorption Qo/Qg (prior to the onset of the oxidation of the adsorbed substance) upon application of a positive pulse 1201. The adsorption of perfluorocarboxylic acids and organic acceptors in the region of high positive potentials was determined as described by Vassiliev et al. [211. The kinetics of perfluorocarboxylate electro-oxidation was studied by means of voltammetry at scan rates ranging from 0.004 to 0.013 mV s-‘. Electrochemical activation of the electrode surface ensured reproducible results. The potentials were measured versus either a saturated aqueous calomel electrode (SCE) or a reversible hydrogen electrode (RHE) in the same solution. Investigations were carried out on stationary and rotating-disc platinum and glassy carbon electrodes, as well as on ring-disc electrodes. Electron paramagnetic resonance CEPR) spectra were recorded using a Varian E-12A radiospectrometer. The solutions were prepared from doubly-distilled water and chemically pure reagents. Preparative electrolysis and analysis were performed according to the techniques described in refs. 8-11. Acetonitrile ’ was purified as described by Miller and Hoffman [22] and trifluoroacetic acid was doubly distilled. Gas-liquid chromatography (GLC) analysis of the reaction products was performed in parallel with the electrochemical measurements.

’ Acetonitrile+water organic compounds

mixture was used [8-12,15,161.

as the solvent

in the electrochemical

perfluoroalkylation

of

169 RESULTS

AND DISCUSSION

Kinetics of oxidation and adsorption of perfluorocarboxylic platinum electrode

acids at a smooth

Since the electro-oxidation of perfluorocarboxylic acids (conventional Kolbe synthesis) is considered to be a convenient method of introducing the perfluoroalkyl group into various organic compounds, the characteristics of perfluorocarboxylate electrooxidation on a smooth platinum electrode were investigated. The differences in the electrochemical behaviour of perfluorocarboxylates and unsubstituted carboxylates, and the reasons why they behave differently in some conditions (for example, in aqueous solutions [23,24]) but in the same way in others (in solutions of corresponding acids and organic solvents [25,26]) are not well understood. The kinetics of perfluorocarboxylate electro-oxidation has not been studied properly, and the influence of the adsorption of the perfluorocarboxylate anion and the addition of organic adsorbers, such as acetonitrile, on the Kolbe synthesis and perfluoroalkylation reactions of organic compounds has not been established. As can be seen from Fig. 1, the anodic polarization curves for aqueous acetonitrile solutions of potassium trifluoroacetate and of anhydrous trifluo-

--

3

I

2

I

1

I

I

2

3 E/V

Fig. 1. Polarization curves for a smooth platinum electrode in 1 M CF,COOK {curve 1) and 1 M CH,COOK (curve 2) solutions in (a) water, (b) aqueous acetonitrile and (c) CF,COOH and CH,COOH. Scan rate, 0.013 V s-l; rotation rate, 5000 rev min-‘.

170

roacetic acid are completely analogous to those for the corresponding acetic solutions, while the polarization curves for aqueous solutions of acetate and trifluoroacetate show obvious differences. The latter curves do not exhibit the kink characteristic of the transition from the solvent oxidation process to carboxylate anion discharge and Kolbe synthesis. Analysis of the products of electrolysis performed at the potentials corresponding to different regions of the polarization curve has shown that, for aqueous solutions in the potential range 1.2-1.5 V, oxygen is the only reaction product formed as a result of water molecule discharge: H,O + OH’+ H++ e-

(1)

4 OH.+

(2)

0, + 2H,O

Between 1.5 and 2.0 V small amounts of CO,, CFsH, HF and CF,O-OCF, were observed in the electrolysis products in addition to oxygen (current efficiency, 80%-95%). As the positive potential increases from 2.0 to 2.2 V, the yield of CO,, CF,H and HF increases, but at 2.4 V traces of C,F, dimer are detected in the electrolysis products, the CF,O-OCF, content remains the same, the CF,H content drops and the HF content increases. The presence of CF,O-OCF, and HF in the electrolysis products is in good agreement with the data obtained by Kazmina et al. [27]. In aqueous acetonitrile solution the polarization curves for trifluoroacetate are similar to those for acetate and at the kink potential (1.4-1.5 V)CO,, C,F, and CF,H are detected in the products. As the potential is increased the C,F, current efficiency increases sharply and reaches 65%-80% at 2.5 V; the CF,H yield reaches 5%-8%. Thus at potentials above 1.6 V a discharge of trifluoroacetate anions occurs. It should be noted that, in the presence of acetonitrile, HF and CF,O-OCF, disappear from the system. Electrolysis of CF,COOK + CF,COOH solutions produces CO, and CF,H gases at the anode even at potentials of 1.4-1.5 V; the C,F, yield in this case is high, reaching 90%-96% at 3.2 V. Trace amounts of CF,H and CF, are detected in the electrolysis products. Figure 2 shows that in aqueous solutions at low positive potentials (up to 1.2 V) the oxidation currents increase with increasing CF,COOK concentration according to i = kc@

(3)

where p = 0.15-0.25. As in the case of CH,COOK 128,291, this region of the polarization curve is probably associated with the destructive oxidation of CFJOOinto the nucleus after its preliminary adsorption and dehalogenation [30] on a platinum electrode surface. However, in the case of perfluorocarboxylic acids this process is insignificant. In the potential range 1.3-2.2 V the polarization curve consists of two linear Tafel regions with slopes of 180-200 mV (at 1.3-1.7 V) and 420 mV (at 1.7-2.0 or 2.2 V). In this potential range the oxidation rate either does not depend on

171 log(j/mA

cm-‘) (b)

-2

6

-x

15 14

-0

T-3

13 12

-)c-cklI

- -2 I

I

I

-3

-2

I

I

-I

0

I

I

I

-I

1

0

I

I

log k/M1

Fig. 2. Effect of the concentration of potassium trifluoroacetate on (a) the rate of anodic processes in aqueous solutions and (b) the partial currents of perfluorocarboxylate discharge in aqueous acetonitrile (4 M CH,CN) solutions at various potentials (V/SCE): curve 1, 0.8; curve 2, 0.9; curve 3, 1.0; cuwe 4, 1.1; curve 5, 1.2; curve 6, 1.3; curve 7, 1.5; curve 8, 1.8; curve 9, 2.0; curve 10, 2.2; curve 11, 2.3; curve 12, 2.5; curve 13, 2.7; curve 14, 2.8; curve 15, 2.9.

CFJOOcontent or may even drop with the increase in perfluorocarboxylate concentration: i=kc-*

(4)

where (Y= 0.2-0.3. It follows from the analysis of the electrolysis products of CH,COOK solutions in the potential range 1.3-2.2 V that oxidation of the solvent (water) occurs and CF,COO- does not participate in this process. The results obtained indicate that at high positive potentials CF,COOadsorption is weak (compared with CH,COO-) and CF,COO- anions cannot replace the water molecules on the platinum electrode surface. However, in solutions with a high trifluoroacetate content, its adsorption is greater and increases with the positive potential. Therefore the rate of water oxidation drops with increasing trifluoroacetate bulk concentration, and with increasing potential this decrease begins at a lower bulk concentration (Fig. 2(a)). Figure 3 shows the effect of potential on the adsorption of CF,COO- anions. It is evident that the surface coverage by CF,COOanions increases rapidly at potentials more positive than 1.7 V, reaching a maximum at E = 2.3 V after which it drops. This dependence is analogous to that for the adsorption of CH,COO-

172

L

I

1.2

I

I

2.0

I

I

2.8 E,/V

Fig. 3. Coverage of platinum electrode surface versus (a) potential in 0.5 M H,SO, +2 M CF&OOH (curve 1) and 0.5 M H,SO, +2 M CH,COOH (curve 2) solutions, and (b) oxygen reduction pulse curves after the adsorption at 2.3 V (T.~~= 2 min) in 0.5 M K,SO, (curve 3), 0.5 M K,SO, +O.OS M CF,COOK (curve 4) and 0.5 M K,SO, + 0.05 M CHJOOH (curve 5) solutions.

anions (Fig. 3, curve 2) with the maximum of CF,COO- adsorption shifted to more positive potentials. However, quantitative comparison of CF,COOand CH,COOadsorbability in 0.5 M H,SO, background solution is not possible because of the large difference between the CF,COOH and CH,COOH dissociation constants (K = 0.588 1311 and K = 1.76 x lo-’ [32] respectively). The bulk concentration of CF,COO- anions is significantly higher than that of CH,COO-. Measurements of the CF,COO- and CH,COO- anion adsorption in comparable conditions (0.5 M K,SO, solution containing 0.05 M CF,COOK or 0.05 M CH,COOK respectively) show that the surface coverage with CH,COOat the potential of maximum adsorption is approximately twice as great as that for trifluoroacetate (Fig. 3(b)). Unlike CH,COO-, the CF,COOanion, which is weakly adsorbed on the surface, is not capable of supressing the solvent oxidation process, and at potentials more positive than that required for CF,COOdischarge water oxidation occurs on the electrode. Therefore the currents of CF,COO- discharge contribute very little to the total current and the yield of the Kolbe dimer C,F, is very low. Analysis of the gaseous products of the process suggests that the appearance of the CF; radical is the result of trifluoroacetate discharge at potentials exceeding 1.5 V. However, in aqueous solutions they are more likely to interact with OH: radicals: CFi + OH*+ [CFsOH] + HF + COFZ x

3HF + CO,

(5) The HF yield reaches 50%. It is possible that reaction (5) might proceed via CFC formed by the electro-oxidation of the CF, radical, which then reacts with the nucleophile OH-.

173 log 1j/mA 3 t

cmv2) (0)

(b)

(c)

h ’ I

I

I

2

3

E/V

Fig. 4. Effects of the addition of acetonitrile to aqueous solutions of (a) 1 M CHsCOOK and (b) 1 M CFsCOOK and of the addition of water to (c) acetonitrile solutions of 1 M CFsCOOK on the anodic polarization curve of a smooth platinum electrode. Concentration of acetonitrile (water): curve 1, 0.2 M; curve 2, 1O-3 M; curve 3, 10m2 M; curve 4, 0.5 M; curve 5, 1.0 M; curve 6, 2.0 M; curve 7, 5.0 M. Scan rate, 0.013 V s-‘; rotation rate, 5000 rev min-‘.

A different phenomenon is observed in aqueous acetonitrile solutions or in trifluoroacetic acid, where CF,COO- polarization curves are similar to those of CH,COO-. First, the oxidation of the solvent by traces of water (tri-fluoroacetic acid was not subjected to special purification procedures) occurs, but as the current density increases CF,COO- discharge begins, i.e. the phenomenon characteristic of Kolbe electrosynthesis in various solvents is observed [33,34]. Addition of water to the acetonitrile solution (Figs. 4 and 5) leads to an increase in the solvent oxidation rate and a water oxidation peak appears on the polarization curve. Its height increases with the water concentration according to the power law given by eqn. (3) (with /3 = 0.7 at water concentrations greater than 2 M). This indicates the participation of the water molecules adsorbed on the uniformly heterogeneous electrode surface in the rate-determining step of process (1). The addition of small. amounts of water to the solution has almost no effect on the partial current of CF&OO- discharge and hence on the C,F, yield. If acetonitrile is added to CF,COO- aqueous solution, then, as seen from Fig. 4, the addition of even small quantities of acetonitrile significantly inhibits the discharge of water molecules, leads to the appearance of a tri-fluoroacetate anion discharge region on the polarization curve, decreases the threshold current for

174 log(j/mA

cms2)

(b)

(0)

6 5 4 3

-I t I -I

I 0

I

I

I -3

I

I

-2

-I

I 0

I

I

logkC,,,CN /M ) Fig. 5. Effect of the addition of (a) water to acetonitrile solutions of 1 M CFsCOOK and (b, c) acetonitrile to aqueous solutions of 1 M CF,COOK on the water molecule discharge current (curves l-3) and the partial current of potassium trifluoroacetate anion discharge (curves 4-6) at various potentials: curve 1, 1.6 V, curve 2, 1.7 V; curve 3, 1.8 V; curve 4, 1.9 V, curve 5, 2.1 V; curve 6, 2.3 V.

CF,COO- anion oxidation and shifts the discharge potential threshold to a less positive value. The maximum value of the water oxidation current decreases with the acetonitrile concentration according to the power law given by eqn. (4) with (Y= 0.5. At acetonitrile concentrations of 2-3 M, these currents are stabilized and reach values approximately equal to the solvent oxidation currents in pure acetonitrile. It should be noted that the addition of acetonitrile to an aqueous solution of acetate which is well adsorbed on platinum has almost no effect on the anodic polarization curve (Fig. 4(a)). This effect of acetonitrile, as seen from Fig. 6, is associated with its strong adsorption on platinum electrodes over a wide potential range, i.e. on bare platinum electrodes and the platinum oxide (PtO,) layer formed on the surface at high positive potentials [21]. This is shown clearly by the pulse measurements (Fig. 6(b)). The adsorption of acetonitrile in the range of high positive potentials has no effect on the quantity of the first type of adsorbed oxygen, the formation of which is completed at 1.7 V, but does have an effect on the quantity of the second form of adsorbed oxygen. The nature of acetonitrile adsorption in CF,COOK aqueous solutions is completely analogous to that in H,SO, solutions. Since acetonitrile is adsorbed onto the surface of the platinum oxide electrode, it inhibits the process of water molecule discharge which leads to an increase in the. partial CF,COO- anion discharge current and to the formation of the Kolbe dimer C,F,. According to Mirkind [351, the modification of the

175

0.6 -

0

0.6

1.6

2.4

E,/V

Fig. 6. Dependence of surface coverage by acetonitrile on (a) the potential of the platinum electrode in 0.5 M H,SO, +5X 10e4 M CH,CN (curve 1) and 0.5 M H,SO, +5x10m3 M CH,CN (curve 2) solutions; (b) i-E curves of oxygen reduction at 1.7 V (curve 3) and 2.1 V (curve 4) in the background solutipn and at 2.1 V (curve 5) in the presence of 5 X 10m3 M acetonitrile.

surface layer produced by adsorption of acetonitrile molecules in. perfluorocarboxylate solutions [36] weakens the metal-adsorbate (RCOO; R’) bond and ensures that the adsorbed radical is removed in the form of Kolbe dimer products. It should be noted that acetonitrile adsorbed on the electrode surface does not participate in the trifluoroacetate anion discharge process. Therefore, at a constant potential, the maximum current of this process decreases according to eqn. (4) (with (Y= 0.6), i.e. an increase in trifluoroacetate discharge potential is observed although it has almost no effect on the yield of electrolysis products. The kinetics of electrode processes in aqueous acetonitrile solutions of trifluoroacetate were investigated in detail because these solutions are used in the preparative electrosynthesis. Investigations showed that the rate of the anion discharge process increased with increasing trifluoroacetate bulk concentration according to the power law given by eqn. (3) (with p = 0.59, reaching a maximum value at 1 M trifluoroacetate and then decreasing (Fig. 2(b)). This indicates that the adsorbed trifluoroacetate anion participates in the rate-determining step of the process. The marked increase in the Tafel slope for partial trifluoroacetate discharge polarization curves (b = 0.3-0.4 V) indicates that the surface coverage by adsorbed trifluoroacetate anions decreases at potentials more positive than 2.3 V. Figure 7 presents the polarization curves for a smooth platinum electrode in aqueous acetonitrile solutions of various perfluorocarboxylic acids (CF,COOH, (CF,),CHCOOH, C,F,COOH and C,F,,COOH). The polarization curves for all these acids show identical behaviour. It follows from Fig. 7 that the potential of the transition to the perfluorocarboxylic acid anion discharge process shifts to. more positive values with increasing fluorocarbon chain length. The Tafel slope has the

176 log (j /mA

cm-‘1

I-

O-

-I

I I

1

1

2

3

-

I

I 2.2

1

I

,

2.6

3.0

E/V

Fig. 7. Effect of various perfluorocarboxylic acids on (a) the anodic polarization curve of smooth platinum and (b) the partial pertluorocarboxylate discharge current in the aqueous acetonitrile (8: 1) solution: curve 1, 1 M (CF,), CHCOOK+ 1 M (CF,),CHCOOH; curve 2, 1 M CF,COOK+l M CF,COOH; curve 3, 1 M C,F,COOK+l M C,F,COOH; curve 4, 1 M C,F,sCOOK+l M C,F,,COOH. Scan rate, 0.013 V s-‘; rotation rate, 5ooO rev min-‘.

same value (0.4 V) for all the acids except perfluorocaprilic acid, which exhibits a sharp increase in the slope value probably associated with the inhibition of the process by high molecular weight products of anodic condensation. The polarization curves in the region of the solvent discharge show similar characteristics, as they are determined not by the adsorption of perfluorocarboxylic acid anions but by acetonitrile adsorption. Thus the most important characteristic of the electro-oxidation of perfluorocarboxylates - a source of perfluoroalkyl radicals - is that the adsorption of perfluorocarboxylate (specifically CF,COO-) on platinum is much lower than that of unsubstituted carboxylates and they do not replace the solvent molecules on the electrode surface, thus explaining the low yields of Kolbe dimers during the electrolysis of aqueous solutions. If a poorly oxidizable solvent is used or the process of water oxidation is suppressed by non-oxidizable additives which are good adsorbers (e.g. acetonitrilel, then perfluorocarboxylate electrolysis produces a high yield of Kolbe dimers. The role of adsorption of organofluorine acceptors in perjluoro-alkylation processes

The addition of radicals generated electrochemically during perfluorocarboxylic acid electrolysis to different acceptors, including fluoro-olefins, provides a method

177 log tj/mA cm-‘)

2

(a)

1

I

1

I

1

I

I

2

3

2.4

2.6

2.8

,

3.0

3.2

E/V

Fig. 8. Effect of various organic acceptors on (a) the anodic polarization curve of platinum and (b) the partial potassium trifluoroacetate discharge in aqueous acetonitrile solutions of 1 M CFsCOOK+ 1 M CF,COOH: curve 1, without acceptor; curve 2, saturated with C,F,H,; curve 3,saturated with CzH,; curve 4, saturated with CsF,. Scan rate, 0.013 V s-r; rotation rate, 5000 rev min-‘.

of synthesizing a variety of organofluorine compounds. The large product yield resulting from the perfluoroalkylation of various organic acceptors attests to the presence of a high concentration of olefins on the electrode surface or in its immediate vicinity. In many cases the possibility of perfluoroalkylation is defined by the adsorption of the reagents on platinum at high positive potentials [16]. Figure 8 presents polarization curves for the discharge of potassium trifluoroacetate on a platinum electrode in aqueous acetonitrile solution in the presence of ethylene, 3,3,3_trifluoropropene and hexafluoropropene (the solutions were saturated with the acceptors at atmospheric pressure). The polarization curves show similar behaviour in the range 1.0-2.3 V. In the range 2.4-3.2 V the rate of oxidation of the CF,COO- anion increases in the presence of ethylene, decreases significantly in the presence of hexafluoropropene and decreases slightly in the presence of 3,3,3_trifluoropropene. The character of the polarization curves is defined by acetonitrile adsorption in the solvent discharge region and by the adsorption of organic acceptors in the CF,COO- discharge region. In contrast with the behaviour in aqueous acetonitrile mixtures, in aqueous solutions ethylene decreases the rate of anodic processes both in the first region of the polarization curve and in the CF,COO- discharge region. The increase in the anodic process rate in the region of high positive potentials is assumed to be due to the. rapid desorption of the products of CFj radical addition to the acceptor adsorbed on the electrode surface [81. In aqueous acetonitrile solution the rate of the anodic

178

0.6

I

0

I-

I

0.6

I

1.6

I

1

2.4

EJV

Fig. 9. Platinum electrode surface coverage vs. potential in 0.5 M H,SO, + 5 X 10m4 M CH,OOCCH=CHCOOCH, (curve l), 0.5 M H,SO, +CF,CH=CH, saturated at 1 atm (curve 2) and 0.5 M H,SO, +CF,CF=CF, saturated at 1 atm (curve 3). Adsorption time, 2 min. process increases with increasing ethylene concentration (electrolysis under a pressure of 60 atm). The rate of CF,COO- discharge increases by a factor of 1.7 in the presence of ethylene under 60 atm pressure at E = 2.7 V/SCE. Thus the radical acceptors do not participate in the anodic process, but they do have an effect on the rate of perfluorocarboxylate oxidation through their adsorption on the electrode. We have also investigated the adsorption of some acceptors used in the perfluoroalkylation reaction [lO,ll]: the dimethyl ether of maleic acid (DMEMA), trifluoropropene (C,F,H,) and hexafluoropropene (C,F,). The effect of adsorption on the perfluoroalkylation reaction was also investigated. Figure 9 shows that the dependence of adsorption on the electrode potential is similar for the three olefins. As with other organic compounds 137-391, two adsorption regions are distinguished. In the first, chemisorption of the acceptor molecule occurs accompanied by the rupture of the c--C and C-H bonds and the formation of new C-Pt and H-Pt bonds. The increase in the number of fluorine atoms in the fluoro-olefin molecule results in a shift in the adsorption maximum towards less positive potentials and to a decrease in the adsorbability of the organic compound (however, it should be remembered that the solubility of the olefins in water increases in the order C,F, < C,F,H, < DMEMA). In the second region (on the platinum oxide layer) the adsorption maximum for all three acceptors occurs at the same potential (2.3 VI. As the investigations showed, the adsorption in this region could be regarded as chemisorption accompanied by complete or partial c--C bond rupture and the formation of two new C-O bonds. In this potential range adsorption has an effect only on the second form of the adsorbed oxygen as in the case of acetonitrile. The introduction of fluorine atoms into the molecule produces a substantial increase in adsorbability (particularly if the sharp decrease in solubility is taken into account) in contrast with the behaviour at low positive potentials. However, the increase in the adsorbability of completely fluorinated compounds does not result in an increased yield of perfluoroalkylated products 111-l.The low rate constant of the interaction between the CFj radical and hexafluoropropene compared with the rate constant of radical dimer-

179

ization accounts for this phenomenon [40]. As will be shown below, the localization of perfluoroalkyl radicals and the acceptor interaction depends strongly on their structure and the properties of the solvent. Effect of the structure of the electrochemically generated radicals and the properties of the solvent on perfluoroalkylation reactions of organic compounds

Anodic oxidation of carboxylate anions is widely used for generating radicals suitable for both dimerization reactions (Kolbe synthesis) [l-5,41] and the formation of products of radical addition and substitution in the presence of suitable substrates [6-161. However, despite the publication of numerous papers describing the reactions in question, a number of problems associated with the mechanism of the process still remain. One of the major problems is the subsequent behaviour of electrochemically generated radicals: are they desorbed into the solution bulk [41-441 or do they participate in the reactions taking place in the region where they are generated, i.e. on the electrode surface [35,45,46]. In the latter case, the necessary condition for a radical to interact with an organic acceptor is that it is adsorbed on the electrode surface [39,47]. If the radicals can exist only on the electrode surface then on anodic oxidation of carboxylate anions in the presence of the acceptor, the product yield of the interaction between the acceptor and the radicals is expected to increase with increasing surface concentration of the acceptor adsorbed on the anode. In order to confirm this prediction, we have studied the electrolysis of fluoroaliphatic carboxylic acids in the presence of an acceptor (ethylene) on anodes with different adsorption capacities (platinum [48] and glassy carbon [491X The major product of the electrolysis of perfluoro-cY-propoxypropionic acid (I) and its sodium salt in aqueous acetonitrile ([MeCN] : [Hz01 = 8 : 1) in the presence of ethylene at atmospheric pressure on a glassy carbon anode at a current density of 30 mA cm-* is the Kolbe dimer perfluoro-5,6-dimethyl-4,7-dioxaoctane (III). The yield of 6,6,7,7-tetrahydroperfluoro-5,8-dimethyl-4,9-dioxadodecane (IVa) resulting from the addition of perfluoro-cu-propoxyethyl radicals (II) to ethylene does not exceed 17% and the formation of small amounts (1.4%) of dioxatetradecane (IVb) is also observed: C,F,OCFCOO-

-e

-

C,F,OCF-CFOC,F, I

I

CF,

CF,

(1)

(11)

C,F,OCF(CH,CH,),CFOC,F, CF,

CF, IVIVa,n=l;IVb,n=2

CF,

(III)

180

If this reaction is carried out under identical conditions on a platinum anode, the yield of the reaction products changes: the amount of IVa and IVb produced almost doubles, reaching 30%. Increasing the current density up to 100 mA/cm2 produces a further change in the yield of reaction products: when the reaction takes place on a glassy carbon electrode the yield of IVa is 1.25%, but when a platinum electrode is used the yield of IVa is 8.7% and almost no IVb is produced. The data obtained are in good agreement with the concept of surface localization of electrochemically generated radicals. However, an attempt to apply the reaction described above to a-C/3-fluorosulphoniltetrafluoroethoxy) tetrafluoropropionic acid (V), the electro-oxidation of which leads to the formation of a-(p-fluorosulphoniltetrafluoroethoxyjtetrafluoroethyl radical (VI) which is an electronic and steric analogue of II has led to unexpected results. Electrolysis of V in the presence of ethylene on glassy carbon and platinum anodes (at a current density of 30 mA cmP2) produces identical mixtures of reaction products, with 70% of the compounds containing one or two acceptor molecules (VIIIa and VIIIb) and the Kolbe dimer (VII) contributing 30%: FSO,CF,CF,OCFCOO -

s

[ Fso2~~p20~;;]

CF3

V

VI I

/

FSO,CF,CF,OCF-CFOCF,CF,SO,F I I CF, CF, VII

CzH,

FS02CF,CF20CF(CH,CH2)n~OCF,CF,S0,F I CF3

CF3 VIII

VIIIa, n = 1; VIIIb, n = 2; VIIIc, n = 3,4

When the ethylene pressure is increased to 60 atm, the yield of oligomer products resulting from the addition of radical VI to ethylene increases to 91.2% (the mixture contains 15% disulphofluoride NIIIa), 68% VIIIb and 17% higher homologues) and the dimer (VII) content drops to 8.8%. The identical reaction product compositions obtained on anodes with different adsorption capacities, the high yield of the products of the reaction between radical VI and ethylene, and the dependence of both their yield and the ratio of the adduct VIIIa to the products of radical dimerization and ethylene oligomerization (VIIIa and VIIIb) on ethylene pressure l, which is characteristic of free-radical processes, supports unambiguously the fact that, in the case in question, electro’ Karapetyan et al. [47] have shown that, when the Kolbe electrolysis is performed in the presence of ethylene, the yield of the products of the interaction of electrochemically generated radicals with ethylene, as well as the ratio of the products of the addition of the radicals to ethylene to the products of radical-ethylene dimerization, increase only slightly with increasing ethylene pressure from 8 to 65 atm. This behaviour is interpreted as being due to the localization of radicals near the electrode such that they can only react with the adsorbed ethylene molecules whose surface concentration is limited.

181

chemically generated radicals are desorbed from the surface into the solution and interact with the acceptor in the solution bulk. It has been shown earlier [ill that the behaviour of fluoro-olefins in trifluoroalkylation reactions does not show a strong dependence on the method of radical generation. Under the conditions of CF,COO- anodic oxidation, processes analogous to those observed in reactions in the gaseous and homogeneous liquid phases occur. This probably indicates the absence of strong adsorption of CFj radicals on the surface of the platinum anode. A similar result has been obtained in reactions involving H atom substitution by CFj in benzene derivatives under conditions when the adsorption of CFj was not observed [91. An analogous conclusion concerning cycloalkyl radicals has been drawn in an investigation of the stereochemistry of Kolbe synthesis with the participation of 4-tret.butylcyclohexanecarboxylic acid [43]. The different behaviour of radicals II and VI is probably connected with the greater solubility of VI in acetonitrile, owing to the presence of the polar fluorinesulphanil group in this radical. If this assumption is correct, the yield of perfluoroalkylation products on anodes with low adsorption capacities should increase with increasing solubility of radicals in the reaction medium. One could assume that the solubility of fluoroalkyl radicals with different substituents will vary in the same sequence as that for fluoroaliphatic compounds with analogous substituents. The solubility of III, VII and H(CF,),H (IX) in acetonitrile was determined by GLC measurements with an accuracy to 0.05%. It was found that III is almost insoluble, VII has a solubility of 0.4%-0.45% and (IX can be mixed with acetonitrile in any ratio. Consequently, upon Kolbe synthesis at a glassy carbon anode in the presence of ethylene, the yield of the products of the radical-ethylene interaction must increase in the order I < V < ohydroperfluorocarboxylic acid. The electrolysis of w-hydroperfluorovalerianic acid (Xl (current density, 35 mA -*; solution saturated with ethylene at atmospheric pressure) provides experizntal confirmation of the assumption. The total yield of hexahydrodecane (XIIa) and decahydrododecane (XIIb) exceeds 75%, i.e. more than the yield of their analogues upon electrolysis of both I and V: H(CF,),COO-

5

2

H(CF,),

-

H(CF,l,H IX

C2f.b

HKF,

l,(CH ,CH 2),(CF,), H

XII XIIa, n = 1; XIIb, n = 2 As seen from the data presented, in the absence of acceptor adsorption on the anode surface, the solubility of the radicals formed is the key factor governing their interaction with the acceptor. This predetermines the dependence of adduct yield not only on the radical structure, but also on the solvent properties. Indeed, in the electrolysis of I on a glassy carbon anode in the presence of ethylene, the

182

substitution of aqueous acetonitrile by the mixture I + MeCN + H,O in the ratio 9 : 1: 1 leads to an increase of the total yield of IVa + IVb from 18.4% to 34.5% at the same current density. The yield of these compounds increases to 60.5% when 2,2,3,3-tetrafluoropropanol-1 is used as the solvent. Thus the problem of localization of radicals generated in the electro-oxidation of carboxylate anions cannot be solved unambiguously. Depending on the radical solubility, defined by the structure of the initial carboxylic acid and the solvent chosen, the radicals are either adsorbed on the electrode surface or desorbed into the solution bulk and undergo chemical conversion. The concept that the solubility of electrochemically generated perfluoroalkyl radicals determines the character of their subsequent conversions has enabled us to establish some rules for the formation of cross-coupling products in Kolbe electrosynthesis [50]. EPR anal&is of radicals produced in the electro-oxidation of perjluorocarboxylate

Conventional Kolbe electrosynthesis is considered to involve the participation of free radicals [45]. However, they have not been detected by EPR techniques. Nevertheless, the possibility that the radicals generated electrochemically are desorbed into the solution bulk, as shown by preparative methods [501, required confirmation by EPR analysis. Performance of the perfluoroalkylation reaction in the presence of a stable radical trap allows the active intermediate particles to be recorded. It has been shown that the electro-oxidation of potassium trifluoroacetate in trifluoroacetic acid in the presence of the perfluoro-olefins [(CFJ,CF],C==CFCF, and CF,CF=C(C,F,)CF(CF,),, used as radical traps, leads to the formation of the stable radicals C:[CF(CF,),], and (CF,),CF(C,F,)~CF(CF,), as shown by the characteristic EPR spectra [51]. The accumulation of these radicals can be observed in an electrochemical cell placed inside the resonator of the EPR spectrometer as well as during the electrolysis outside the resonator. It has been shown that the spin-adduct concentration during electrolysis on a glassy carbon anode is 20%-30% higher than that obtained when a platinum anode is used. This indicates that the addition of CF; radicals to the acceptor occurs in the electrolyte bulk, as the adsorbability of the perfluoroalkyl radicals and acceptor on glassy carbon is significantly lower than that on platinum [49]. At the same time, the yield of perfluoroalkylated product does not exceed a few percent, as was observed earlier for CF; radical addition to hexafluoropropene and tetrafluoroethylene [ll]. The electrochemical behaviour and the properties of perfluorinated stable radicals are described elsewhere [52]. Thus the results of organic compound preparative perfluoroalkylation [g-16,50], the investigation of perfluorocarboxyhc acid electro-oxidation kinetics, the investigation of organic acceptor adsorption and the detection of stable radicals resulting from perfluoroalkylation processes allow us to draw conclusions about the mechanism of these reactions. In perfluoroalkylation reactions the acceptor does not participate in the electrochemical step of the process but competes with anions

183

and the solvent for the electrode surface. There are two parallel perfluoroalkylation reaction routes: interaction of electrochemically generated perfluoroalkyl radicals with acceptors in the electrode adsorption layer, and the reaction of electrochemically generated radicals in the electrolyte bulk. The latter depends on the radical structure and the properties of the solvent.

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