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Electrocarboxylation of styrene through homogeneous redox catalysis
J, Electroanal. Chem., 177 (1984) 303-309 Elsevier Sequoia S.A., Lausanne - Printed in The Netherlands
Preliminarv
note
ELECTROCARBOXYLATION OF STYRENE HOMOGENEOUS REDOX CATALYSIS
G. FILARDO, Istituto
303
S. GAMBINO
di Ingegneria
THROUGH
and G. SILVESTRI
Chimica dell’Universitir,
V.le delle Scienze,
90128 Palermo
(Italy)
A. GENNARO and E. VIANELLO Istituto
di Chimica Fisica dell’Universit6,
Via Loredan
2, 35131 Padova (Italy)
(Received 31st July 1984)
INTRODUCTION
The electrochemical carboxylation of olefinic substrates with CO2 , in solvents of low proton availability, has already been proposed as a useful procedure for the production of mono- and dicarboxylic acids. For activated olefins R, which are easier to reduce than CO,, the reaction mechanism has usually been formulated as a nucleophilic attack of the olefin anion radical RS, produced at the electrode, on COz acting as an electrophile, followed by further reduction of the resulting anion radical R-CO2 3 at the potential of formation [ 1,2] . However, the mechanism involving a homogeneous charge transfer from R’ to CO*, followed by coupling of the anion radicals R’ and COz’ has also been considered [ 31. More recently, the mechanism of electrocarboxylation has been discussed in terms of competition between the above two pathways [4] . Since, according to the Marcus theory, the rate of the outer sphere charge transfer between R’ and COz is expected to increase with the standard Gibbs energy of the reaction, the standard potential difference AE”=E”coJco, - - E”R/R~ is expected to provide useful information on the competition. The value of E’co,/co,~ in dry 0.1 M DMF in TEAP has been estimated to be 2.21 V vs. SCE [5]. It has been deduced, on grounds of kinetic evidence, that when AE” is lower than about -400 mV, the mechanism involving nucleophilic attack as the initial step should be predominant [ 41. Reactions of this last type have been studied in detail, both from the mechanistic and synthetic standpoints [ 2,4,6,7]. On the contrary, for olefins reducible in a very negative potential range (AE> - 400 mV), where the homogeneous charge transfer from R’ to CO* is expected to operate, analysis of the carboxylation mechanism is much less convincing. In the course of a systematic investigation on the behaviour of aryl-substituted olefins towards electrocarboxylation, we have obtained a series of results which could provide useful information on the competition between the two mechanisms mentioned above. In this preliminary com-
304
munication we report through homogeneous
the first results on the electrocarboxylation electrocatalysis promoted by benzonitrile.
of styrene
EXPERIMENTAL
Cyclic voltammetric experiments were performed with an E.G.&G. Princeton Applied Research Instrument, formed by a 173 Potentiostat equipped with a 179 Digital Coulometer, a 175 Function Generator and a 4203 Signal Averager. The working electrode was a Pt sphere. The reference electrode was non-aqueous Ag/AgI/I- whose potential was always measured vs. an aqueous SCE, to which all potentials are referred. DMF used in the voltammetric runs was vacuum-distilled and stored over neutral alumina (Merck, activity grade I) activated by heating at 400°C for 24 h. All chemicals used were reagent grade and further purified when necessary. Different CO? concentrations were obtained by bubbling the gas through DMF for suitable times. Preparative controlled potential electrolysis (CPE) was carried out on DMF and styrene, vacuum-distilled and dried over 4A molecular sieves before each experiment. Bu,NBr was crystallized from ethyl acetate. Anodes of 99.9% aluminium were mechanically polished, pickled by immersion in dilute HCl, washed with distilled water and dried before use. Graphite cathodes were thermally treated to remove adsorbed organic species. A diaphragmless tank cell of internal volume 100 cm3 was employed, in which parallel and alternate sacrificial Al anodes and graphite cathodes were assembled (overall cathodic area 30 cm2 ). Stirring and saturation of the solution were obtained by continuously bubbling CO2 between the electrode gaps throughout the electrolysis. The electrolysed solution was distilled off at reduced pressure, and the residue formed by the Al salts of the carboxylation products was hydrolysed with dilute HCl at room temperature and extracted with ether. Removal of ether after drying gave a mixture of the acids as a white solid. The crude was then treated at room temperature with benzene, which solubilizes only the monocarboxylic acid. The latter was recovered by removal of benzene; the insoluble residue is the dicarboxylic acid. Identification of the products was performed by melting point and NMR spectra. The purity of both acids, evaluated by titration with 0.1 M NaOH solutions, was always over 95%. Oxalic acid was recovered by 24 h continuous liquid-liquid extraction with ether from the hydrolysed residue. RESULTS
AND DISCUSSION
The electrode reduction process of COZ is markedly dependent upon the nature of the electrode material and the nature of the reaction medium [8,9]. Under our experimental conditions an irreversible cathodic peak, attributable to a slow one-electron transfer to COZ , is observed at -2.8 V, for a sweep rate u = 0.1 V s-l , i.e. at a potential far more negative than EO~o,/coZ~, estimated as 2.21 V, under very similar conditions. The resulting anion radical COZs is known to undergo a fast second-order decay, mainly by dimerization to oxalate, with an estimated rate constant of 10’ M-’ 1-l [ 51.
305
The reduction of styrene (S) in DMF has been reported to yield a twoelectron irreversible wave [lo], through the usual ECE protonation mechanism: S+e-
2
S7
S’ + DH + SH’ + e-
E”s/s = - 2.58 v
SH’ + D-
-+ SH-
SH-+DH
(1) (2)
and/or
SH’ + Ss -+ SH- + S
-+ SHZ
(3) (4)
where DH represents proton donors, in particular residual water. In fact, we have observed that the voltammetric pattern of S is markedly dependent upon the water content of the solvent. Even in very carefully dried DMF, only an irreversible cathodic peak is observed at low IJvalues. However, at u = 20 v s-l ) a cathodic/anodic peak system is obtained, corresponding to a reversible one-electron process, with E’,cJs= = -2.58 V. The maximum value of the rate constant of the protonation reaction (2) can be evaluated from the kinetic parameter x = (RT/F)(hc/u), which must not exceed the value lo-’ in order to have full reversibility of the peak [ 111. If we assume that, under our conditions, the residual water concentration c is at maximum, 10m3M, kG104 M-’ 6’ . The voltammetric results also indicate that the dimerization of S7, if any, is slower than protonation. As to benzonitrile (BN), the voltammetric pattern is very simple: a single cathodic/anodic peak system is observed even at very low u values, corresponding to a reversible one-electron process yielding the relatively stable BW anion radical [ 121, with E’BN~N~ = -2.28 V. On addition of increasing amounts of COZ to a DMF solution of BN, the cathodic peak of the latter increases several times in height and the corresponding anodic peak disappears (see Fig. 1). This behaviour clearly indicates that catalytic reduction of COZ occurs at the reduction potential of BN: BN+e-
2
BW
BW + COZ 2 2 coz=
+
czo;-
BN + COZs
E” = -2.28
V
(5) (6) (7)
Since EOBNB~ is slightly more negative than that reported for the redox couple COI/CO;, the homogeneous charge transfer (6) ought to be thermodynamically favoured (K6 -20) and CO; should be continuously displaced from the equilibrium by the fast dimerization (7). Under these conditions the rate of the charge transfer process is expected to be relatively high. This is in agreement with the formation, at low COZ concentrations, of a prepeak, which eventually merges with the main peak, increasing the CO2 content (see Fig. 1). In fact, the prepeak is expected to appear, for an equimolecular ratio of substrate to catalyst, when the kinetic parameter x > lo3 [13]. This means that, under our conditions, the rate constant of the charge transfer process (6) must be higher than lo6 M-l k’ . The reaction scheme reported above is also in agreement with the results of CPE, showing that oxalic acid is produced in a relative high yield and that no carboxylated derivatives of BN are formed (see Table 1). At the end of WE, BN is recovered almost quantitatively, indicating that it merely acts
306 as a catalyst for COz reduction to oxalate. The latter is formed in a yield comparable with that obtained by direct electrode reduction of CO2 [ 91, but at a substantially more positive potential. Addition of CO* to a DMF solution of S also gives rise to a prepeak, which increases in height, increasing the CO, content, and eventually merges with the main peak. As long as the prepeak is present, no reduction peak of CO2 is observed (see Fig. 2). This behaviour suggests also in this case a fast followup reaction involving ST, formed at the electrode by reaction (1) and COz . r
i\
4 I/,1
IAL 1oc
2ocI-
5c)-
o-
--J 1
- 2.0 -2.0
I
-2.5
I
-3.0 E/1
-2.5
Fig. 1. Cyclic voltammetry of benzonitrile in the presence of CO,. (a) CO, alone; (b) 5~ lo-” M BN; (c-f) effect of increasing amounts of CO,. 0.1 M Bu,NClO, , u = 0.1 V 6. Fig. 2. Cyclic volbammetry of styrene in the presence of CO,. (a) 10eZ M S;(b-e) increasing amounts of CO,, . 0.1 M Bu, NClO, , u = 0.1V 0.
Since E$,s = is significantly more negative than E”co,~co, such a follow-up reaction is likely to be the homogeneous ss + co2
2
s+coz5
effect of
7 (AE”=370 mV), charge transfer (8)
a down-hill process (Kg =2X lo6 ), which according to the Marcus theory is expected to be much faster than the analogous reaction (6). It should be noticed, however, that in this case the height of the main peak of S does not increase very much on addition of CO*, as would be expected if catalytic reduction of COz to oxalate was occurring. In fact, by CPE of the S/CO2 couple at the reduction potentials of S, oxalate is formed only in small amounts and the main products are the mono- and dicarboxylic derivatives of S (see Table 1).
301 TABLE
1
Results of CPE Moles of @-CH=CH,
@-CN
Charge passed/ mol electrons
0 2.5~ 1O-2 1.9x 10-z
2.3~ lo-$ 0 3.4x 10-a
6.9x lo-’ 5.2~ 10-l 4.0x 10-a
Cathodic potential /V
-2.15 -2.45 -2.15
to 2.20 to 2.50 to 2.20
Current density /mA cm-’
Styrene conversion * 1%
Product yields b Mono C 1%
Di c I%
20 to 10 24 to 12 24 to 4
60 85
29 22
70 d 60 52
a Referred to charge passed. b Referred to +CH=CH, converted. c Mono = 3-phenylpropionic acid; di = phenylsuccinic acid. d Oxalic acid; the yield is referred to the charge passed.
These results are by no means in contrast with reaction (8), provided the latter is followed by a fast reaction involving the couple S/F, such as the radicalradical coupling s7 + cozs
+
s-co’;
(9)
as suggested for the reduction of ethyl acrylate in the presence of CO* [3] and/or a radical addition followed by reduction of the resulting anion radical, as proposed for the carboxylation of unactivated olefins which are more difficult to reduce than CO2 itself [2] : s+co,7 S-C02’
--f + e-
s-co2s
(19)
-+
(11)
S-CO’;
The dianion S-CO’,would then, depending on the proton availability of the medium, combine with a proton or a CO? molecule, giving the mono- and dicarboxylic derivatives, respectively: S-CO;-
+ DH
-+
SH-C02-
s-Co:-
+ co*
+
s--(coz-
+ D-
(12)
)*
(13)
This reaction scheme, which leads to an overall two-electron process, would explain the moderate increase of the main peak of S caused by CO? addition. In fact, in the presence of COZ , the reaction sequence (8)-(13) occurs in place of the relatively slow ECE pathway (2)-(4), which is also a two-electron reaction. As regards the nature of the monocarboxylation product, the formation of 3-phenylpropionic acid is well accounted for by the radical-radical coupling (9), being the spin density mainly localized on the C atom of COP? [ 141 and on the atom in position 3 of F [ 15,161. Also, a radical attack of CO,? on the double bond of S would lead to the 3-isomer rather than the 2-isomer [ 171. It is worth noting, however, that although the charge transfer (8) is thermodynamically much favoured and consequently very fast, concurrence of the nucleophilic addition ST + co2
+
s-co;
(14)
308
followed by reactions (ll), (12) and (13), which also lead to an overall twoelectron process, cannot be ruled out on the basis of our kinetic results. An interesting situation arises when both BN and S are present in the DMF solution. In the absence of COZ , the voltammetric patterns of these compounds remain practically unchanged and, in paticular, the reversibility of the BN peak is preserved. In fact, the charge transfer BW+S
2
BN+L?
(15)
is an up-hill reaction (AE” --300 mV; Ki5 r10-5 ) which the relatively slow protonation (3) is unable to displace towards the right. However, by CO* addition, the BN peak undergoes a catalytic enhancement, losing its reversibility; at the same time, the styrene peak diminishes and eventually disappears (see Fig. 3). This voltammetric behaviour is paralleled by the results of CPE showing that, by macro-scale reduction of the three-component system BN + S + CO? at the BN reduction potential, 3-phenylpropionic acid and phenylsuccinic acid are produced in a good yield. Also, in this case, BN is recovered at the end of CPE almost completely, consistent with its catalytic role. Although the kinetics of the three-component system are presumably rather complex, the results of cyclic voltammetry and CPE reported above may be qualitatively rationalized taking into account reactions (5) and (6) as the initial steps of electrocarboxylation, as suggested by the catalytic character of the BN peak. The resulting CO; would interact with S rather than dimerize to oxalate. Such interaction could be either a radical attack on the double bond of S (reactions 10 and 11)
” ii
ii i i
i i
Fig. 3. Cyclic voltammetry of benzonitrile and styrene in the presence of CO,. (a) 3.7~ low3 M BN + 2.9x 10m3 M S; (b-d) effect of increasing amounts of CO,. 0.1 M Bu,NClO,, u = 0.1 v s-l.
309
or a radical-radical coupling with ST. The latter might result from the charge transfer (8) which, although thermodynamically unfavoured, could be continuously displaced towards the right by the fast reaction (9). Further work is in progress to provide a sounder kinetic analysis of the reaction mechanism. However, stress should be laid on the synthetic results, indicating that electrodimerization of CO2 to oxalate and electrocarboxylation of S can be carried out, through a catalytic pathway, at the reduction potential of BN. ACKNOWLEDGEMENT
Financial support by the C.N.R., through Fine e Secondaria”, is acknowledged.
the “Progetto
Finalizzato
Chimica
REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17
D.A. Tyssee, J.H. Wagenknecht, M.M. Baizer and J.L. Chruma, Tetrahedron Lett., 47 (1972) 4809. D.A. Tyssee and M.M. Baizer, J. Org. Chem., 39 (1974) 2823. H. Lund and J. Simonet, J. Electroanal. Chem., 65 (1975) 205. E. Lamy, L. Nadjo and J.M. Saveant, Nouv. J. Chim., 3 (1979) 21. E. Lamy, L. Nadjo and J.M. Saveant, J. Electroanal. Chem., 78 (1977) 403. Lun-Shu R. Yeh and A.J. Bard, J. Electrochem. Sot., 124 (1977) 355. S. Gamhino, G. Filardo and G. Silvestri, J. Appl. Electrochem., 12 (1982) 549. B.R. Eggins and J. McNeill, J. Electroanal. Chem., 148 (1983) 17. J.C. Gressin, D. Michelet, L. Nadjo and J.M. Saveant, Nouv. J. Chim., 3 (1979) 545. P.G. Grodzka and P.J. Elving, J. Electrochem. Sot., 110 (1963) 231. R.S. Nicholson and I. Shain, Anal. Chem., 35 (1964) 706. A.M. Romanin, A. Gennaro and E. Vianello, J. Electroanal. Chem., 88 (1978) 175. C.P. Andrieux, C. Blocman, J.M. Dumas-Bouchiat, F. M’Halla and J.M. Saveant, J. Electroanai. Chem., 113 (1980) 19. D.W. Ovenall and D.H. Whiffen, Mol. Phys., 4 (1961) 135. H.D. Roth and M.L. Manion Shilling, J. Am. Chem. Sot., 101 (1979) 1898. H.D. Roth and M.L. Manion Shilling, J. Am, Chem. Sot., 102 (1980) 4303. G.H. Whitham in J.F. Stoddart (Ed.), Comprehensive Organic Chemistry, Vol. 1, Pergamon Press, Oxford, 1979, pp. 155-157.
Preliminarv
note
ELECTROCARBOXYLATION OF STYRENE HOMOGENEOUS REDOX CATALYSIS
G. FILARDO, Istituto
303
S. GAMBINO
di Ingegneria
THROUGH
and G. SILVESTRI
Chimica dell’Universitir,
V.le delle Scienze,
90128 Palermo
(Italy)
A. GENNARO and E. VIANELLO Istituto
di Chimica Fisica dell’Universit6,
Via Loredan
2, 35131 Padova (Italy)
(Received 31st July 1984)
INTRODUCTION
The electrochemical carboxylation of olefinic substrates with CO2 , in solvents of low proton availability, has already been proposed as a useful procedure for the production of mono- and dicarboxylic acids. For activated olefins R, which are easier to reduce than CO,, the reaction mechanism has usually been formulated as a nucleophilic attack of the olefin anion radical RS, produced at the electrode, on COz acting as an electrophile, followed by further reduction of the resulting anion radical R-CO2 3 at the potential of formation [ 1,2] . However, the mechanism involving a homogeneous charge transfer from R’ to CO*, followed by coupling of the anion radicals R’ and COz’ has also been considered [ 31. More recently, the mechanism of electrocarboxylation has been discussed in terms of competition between the above two pathways [4] . Since, according to the Marcus theory, the rate of the outer sphere charge transfer between R’ and COz is expected to increase with the standard Gibbs energy of the reaction, the standard potential difference AE”=E”coJco, - - E”R/R~ is expected to provide useful information on the competition. The value of E’co,/co,~ in dry 0.1 M DMF in TEAP has been estimated to be 2.21 V vs. SCE [5]. It has been deduced, on grounds of kinetic evidence, that when AE” is lower than about -400 mV, the mechanism involving nucleophilic attack as the initial step should be predominant [ 41. Reactions of this last type have been studied in detail, both from the mechanistic and synthetic standpoints [ 2,4,6,7]. On the contrary, for olefins reducible in a very negative potential range (AE> - 400 mV), where the homogeneous charge transfer from R’ to CO* is expected to operate, analysis of the carboxylation mechanism is much less convincing. In the course of a systematic investigation on the behaviour of aryl-substituted olefins towards electrocarboxylation, we have obtained a series of results which could provide useful information on the competition between the two mechanisms mentioned above. In this preliminary com-
304
munication we report through homogeneous
the first results on the electrocarboxylation electrocatalysis promoted by benzonitrile.
of styrene
EXPERIMENTAL
Cyclic voltammetric experiments were performed with an E.G.&G. Princeton Applied Research Instrument, formed by a 173 Potentiostat equipped with a 179 Digital Coulometer, a 175 Function Generator and a 4203 Signal Averager. The working electrode was a Pt sphere. The reference electrode was non-aqueous Ag/AgI/I- whose potential was always measured vs. an aqueous SCE, to which all potentials are referred. DMF used in the voltammetric runs was vacuum-distilled and stored over neutral alumina (Merck, activity grade I) activated by heating at 400°C for 24 h. All chemicals used were reagent grade and further purified when necessary. Different CO? concentrations were obtained by bubbling the gas through DMF for suitable times. Preparative controlled potential electrolysis (CPE) was carried out on DMF and styrene, vacuum-distilled and dried over 4A molecular sieves before each experiment. Bu,NBr was crystallized from ethyl acetate. Anodes of 99.9% aluminium were mechanically polished, pickled by immersion in dilute HCl, washed with distilled water and dried before use. Graphite cathodes were thermally treated to remove adsorbed organic species. A diaphragmless tank cell of internal volume 100 cm3 was employed, in which parallel and alternate sacrificial Al anodes and graphite cathodes were assembled (overall cathodic area 30 cm2 ). Stirring and saturation of the solution were obtained by continuously bubbling CO2 between the electrode gaps throughout the electrolysis. The electrolysed solution was distilled off at reduced pressure, and the residue formed by the Al salts of the carboxylation products was hydrolysed with dilute HCl at room temperature and extracted with ether. Removal of ether after drying gave a mixture of the acids as a white solid. The crude was then treated at room temperature with benzene, which solubilizes only the monocarboxylic acid. The latter was recovered by removal of benzene; the insoluble residue is the dicarboxylic acid. Identification of the products was performed by melting point and NMR spectra. The purity of both acids, evaluated by titration with 0.1 M NaOH solutions, was always over 95%. Oxalic acid was recovered by 24 h continuous liquid-liquid extraction with ether from the hydrolysed residue. RESULTS
AND DISCUSSION
The electrode reduction process of COZ is markedly dependent upon the nature of the electrode material and the nature of the reaction medium [8,9]. Under our experimental conditions an irreversible cathodic peak, attributable to a slow one-electron transfer to COZ , is observed at -2.8 V, for a sweep rate u = 0.1 V s-l , i.e. at a potential far more negative than EO~o,/coZ~, estimated as 2.21 V, under very similar conditions. The resulting anion radical COZs is known to undergo a fast second-order decay, mainly by dimerization to oxalate, with an estimated rate constant of 10’ M-’ 1-l [ 51.
305
The reduction of styrene (S) in DMF has been reported to yield a twoelectron irreversible wave [lo], through the usual ECE protonation mechanism: S+e-
2
S7
S’ + DH + SH’ + e-
E”s/s = - 2.58 v
SH’ + D-
-+ SH-
SH-+DH
(1) (2)
and/or
SH’ + Ss -+ SH- + S
-+ SHZ
(3) (4)
where DH represents proton donors, in particular residual water. In fact, we have observed that the voltammetric pattern of S is markedly dependent upon the water content of the solvent. Even in very carefully dried DMF, only an irreversible cathodic peak is observed at low IJvalues. However, at u = 20 v s-l ) a cathodic/anodic peak system is obtained, corresponding to a reversible one-electron process, with E’,cJs= = -2.58 V. The maximum value of the rate constant of the protonation reaction (2) can be evaluated from the kinetic parameter x = (RT/F)(hc/u), which must not exceed the value lo-’ in order to have full reversibility of the peak [ 111. If we assume that, under our conditions, the residual water concentration c is at maximum, 10m3M, kG104 M-’ 6’ . The voltammetric results also indicate that the dimerization of S7, if any, is slower than protonation. As to benzonitrile (BN), the voltammetric pattern is very simple: a single cathodic/anodic peak system is observed even at very low u values, corresponding to a reversible one-electron process yielding the relatively stable BW anion radical [ 121, with E’BN~N~ = -2.28 V. On addition of increasing amounts of COZ to a DMF solution of BN, the cathodic peak of the latter increases several times in height and the corresponding anodic peak disappears (see Fig. 1). This behaviour clearly indicates that catalytic reduction of COZ occurs at the reduction potential of BN: BN+e-
2
BW
BW + COZ 2 2 coz=
+
czo;-
BN + COZs
E” = -2.28
V
(5) (6) (7)
Since EOBNB~ is slightly more negative than that reported for the redox couple COI/CO;, the homogeneous charge transfer (6) ought to be thermodynamically favoured (K6 -20) and CO; should be continuously displaced from the equilibrium by the fast dimerization (7). Under these conditions the rate of the charge transfer process is expected to be relatively high. This is in agreement with the formation, at low COZ concentrations, of a prepeak, which eventually merges with the main peak, increasing the CO2 content (see Fig. 1). In fact, the prepeak is expected to appear, for an equimolecular ratio of substrate to catalyst, when the kinetic parameter x > lo3 [13]. This means that, under our conditions, the rate constant of the charge transfer process (6) must be higher than lo6 M-l k’ . The reaction scheme reported above is also in agreement with the results of CPE, showing that oxalic acid is produced in a relative high yield and that no carboxylated derivatives of BN are formed (see Table 1). At the end of WE, BN is recovered almost quantitatively, indicating that it merely acts
306 as a catalyst for COz reduction to oxalate. The latter is formed in a yield comparable with that obtained by direct electrode reduction of CO2 [ 91, but at a substantially more positive potential. Addition of CO* to a DMF solution of S also gives rise to a prepeak, which increases in height, increasing the CO, content, and eventually merges with the main peak. As long as the prepeak is present, no reduction peak of CO2 is observed (see Fig. 2). This behaviour suggests also in this case a fast followup reaction involving ST, formed at the electrode by reaction (1) and COz . r
i\
4 I/,1
IAL 1oc
2ocI-
5c)-
o-
--J 1
- 2.0 -2.0
I
-2.5
I
-3.0 E/1
-2.5
Fig. 1. Cyclic voltammetry of benzonitrile in the presence of CO,. (a) CO, alone; (b) 5~ lo-” M BN; (c-f) effect of increasing amounts of CO,. 0.1 M Bu,NClO, , u = 0.1 V 6. Fig. 2. Cyclic volbammetry of styrene in the presence of CO,. (a) 10eZ M S;(b-e) increasing amounts of CO,, . 0.1 M Bu, NClO, , u = 0.1V 0.
Since E$,s = is significantly more negative than E”co,~co, such a follow-up reaction is likely to be the homogeneous ss + co2
2
s+coz5
effect of
7 (AE”=370 mV), charge transfer (8)
a down-hill process (Kg =2X lo6 ), which according to the Marcus theory is expected to be much faster than the analogous reaction (6). It should be noticed, however, that in this case the height of the main peak of S does not increase very much on addition of CO*, as would be expected if catalytic reduction of COz to oxalate was occurring. In fact, by CPE of the S/CO2 couple at the reduction potentials of S, oxalate is formed only in small amounts and the main products are the mono- and dicarboxylic derivatives of S (see Table 1).
301 TABLE
1
Results of CPE Moles of @-CH=CH,
@-CN
Charge passed/ mol electrons
0 2.5~ 1O-2 1.9x 10-z
2.3~ lo-$ 0 3.4x 10-a
6.9x lo-’ 5.2~ 10-l 4.0x 10-a
Cathodic potential /V
-2.15 -2.45 -2.15
to 2.20 to 2.50 to 2.20
Current density /mA cm-’
Styrene conversion * 1%
Product yields b Mono C 1%
Di c I%
20 to 10 24 to 12 24 to 4
60 85
29 22
70 d 60 52
a Referred to charge passed. b Referred to +CH=CH, converted. c Mono = 3-phenylpropionic acid; di = phenylsuccinic acid. d Oxalic acid; the yield is referred to the charge passed.
These results are by no means in contrast with reaction (8), provided the latter is followed by a fast reaction involving the couple S/F, such as the radicalradical coupling s7 + cozs
+
s-co’;
(9)
as suggested for the reduction of ethyl acrylate in the presence of CO* [3] and/or a radical addition followed by reduction of the resulting anion radical, as proposed for the carboxylation of unactivated olefins which are more difficult to reduce than CO2 itself [2] : s+co,7 S-C02’
--f + e-
s-co2s
(19)
-+
(11)
S-CO’;
The dianion S-CO’,would then, depending on the proton availability of the medium, combine with a proton or a CO? molecule, giving the mono- and dicarboxylic derivatives, respectively: S-CO;-
+ DH
-+
SH-C02-
s-Co:-
+ co*
+
s--(coz-
+ D-
(12)
)*
(13)
This reaction scheme, which leads to an overall two-electron process, would explain the moderate increase of the main peak of S caused by CO? addition. In fact, in the presence of COZ , the reaction sequence (8)-(13) occurs in place of the relatively slow ECE pathway (2)-(4), which is also a two-electron reaction. As regards the nature of the monocarboxylation product, the formation of 3-phenylpropionic acid is well accounted for by the radical-radical coupling (9), being the spin density mainly localized on the C atom of COP? [ 141 and on the atom in position 3 of F [ 15,161. Also, a radical attack of CO,? on the double bond of S would lead to the 3-isomer rather than the 2-isomer [ 171. It is worth noting, however, that although the charge transfer (8) is thermodynamically much favoured and consequently very fast, concurrence of the nucleophilic addition ST + co2
+
s-co;
(14)
308
followed by reactions (ll), (12) and (13), which also lead to an overall twoelectron process, cannot be ruled out on the basis of our kinetic results. An interesting situation arises when both BN and S are present in the DMF solution. In the absence of COZ , the voltammetric patterns of these compounds remain practically unchanged and, in paticular, the reversibility of the BN peak is preserved. In fact, the charge transfer BW+S
2
BN+L?
(15)
is an up-hill reaction (AE” --300 mV; Ki5 r10-5 ) which the relatively slow protonation (3) is unable to displace towards the right. However, by CO* addition, the BN peak undergoes a catalytic enhancement, losing its reversibility; at the same time, the styrene peak diminishes and eventually disappears (see Fig. 3). This voltammetric behaviour is paralleled by the results of CPE showing that, by macro-scale reduction of the three-component system BN + S + CO? at the BN reduction potential, 3-phenylpropionic acid and phenylsuccinic acid are produced in a good yield. Also, in this case, BN is recovered at the end of CPE almost completely, consistent with its catalytic role. Although the kinetics of the three-component system are presumably rather complex, the results of cyclic voltammetry and CPE reported above may be qualitatively rationalized taking into account reactions (5) and (6) as the initial steps of electrocarboxylation, as suggested by the catalytic character of the BN peak. The resulting CO; would interact with S rather than dimerize to oxalate. Such interaction could be either a radical attack on the double bond of S (reactions 10 and 11)
” ii
ii i i
i i
Fig. 3. Cyclic voltammetry of benzonitrile and styrene in the presence of CO,. (a) 3.7~ low3 M BN + 2.9x 10m3 M S; (b-d) effect of increasing amounts of CO,. 0.1 M Bu,NClO,, u = 0.1 v s-l.
309
or a radical-radical coupling with ST. The latter might result from the charge transfer (8) which, although thermodynamically unfavoured, could be continuously displaced towards the right by the fast reaction (9). Further work is in progress to provide a sounder kinetic analysis of the reaction mechanism. However, stress should be laid on the synthetic results, indicating that electrodimerization of CO2 to oxalate and electrocarboxylation of S can be carried out, through a catalytic pathway, at the reduction potential of BN. ACKNOWLEDGEMENT
Financial support by the C.N.R., through Fine e Secondaria”, is acknowledged.
the “Progetto
Finalizzato
Chimica
REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17
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