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Investigation of the feasibility of electrochemical allylic bromination in dichloromethane
J. Electroanal. Chem., 158 (1983) 369-373 Elsevier Sequoia S.A., Lausanne - Printed in The Netherlands
369
Short communication
INVESTIGATION OF THE FEASIBILITY OF ELECTROCHEMICAL ALLYLIC BROMINATION IN DICHLOROMETHANE
MARINA MASTRAGOSTINO, SERGIO VALCHER and MARIA BISERNI lstituto Chimico "'Ciamician'" dell'Universita" di Bologna, Via Selmi 2, 40126 Bologna (ltaly) lstituto di Polarografia ed Elettrochimica Preparativa del C.N.R., Corso Stati Uniti 4, 35100 Padua (Italy) (Received 24th January 1983; in revised form 1 lth April 1983)
INTRODUCTION
The electrochemical halogenation of olefinic compounds in nucleophilic solvents has been the subject of numerous papers [1-8] and some results have also been published for unreactive solvents [9,10]. In this paper we report the results of a study on the feasibility of electrochemically preparing allylic bromoderivative compounds, at a Pt electrode in CH2C1 z, which seems a suitable solvent to hinder double-bond saturation reactions. N-bromosuccinimide (NBS) in unreactive solvents is the usual chemical reagent for the preparation of allylic bromides [11]. The efficiency of this synthesis, which proceeds through a radical chain mechanism [12], is due to the capability of the reagent NBS to maintain the Br 2 concentration low and constant. Since Br" and Br 2 are electrochemically obtainable at controlled bromine concentrations and since an investigation on the electrode kinetics of the system Br-/Br2 [13] gave evidence for the possibility that Br" is present in CH2C12 solutions, we may expect the electrosynthesis of the allylic bromoderivatives to be possible by operating on Br-/olefin systems atthe Br- oxidation potential. On the other hand, the investigation of Faita et al. [14] in acetonitrile suggests the possibility of also obtaining this type of compounds by operating at higher potentials on Br-/olefin systems through the oxidation of the organic substrate. Indeed the use of CH2C12 could improve the yields, the nucleophilic attack of the solvent to carbocation being excluded. The two routes, differing in electrolysis potential, are considered here. EXPERIMENTAL
The C H 2 C 1 2 (RPE, C. Erba) was purified, depending on its use, by: (1) distillation over PzOs, after storage over CaC12 (2 h); (2) double distillation under anhydrous N 2 and dehydration for at least 6 days by contact with activated molecular sieves (4A Pellets, 4 mm, C. Erba) in the dark and under N 2. 0022-0728/83/$03.00
© 1983 Elsevier Sequoia S.A.
370
The molecular sieves were activated by heating at 673 K for 24 h. The residual water after procedures (1) and (2) was = 0.002 and < 0.0001 M respectively; in the latter case, with tetrabuthylammonium perchlorate (TBAP), potentials of about 2.4 V can be attained at a Pt electrode, an upper limit 0.6 V more positive than that reported in the literature [15]. All the chemicals were purified before use. The electrochemical experiments were carried out at 273 K with TBAP 0.2 M. All the potentials are referred to the aqueous SCE. The electrolysed solutions, after having been concentrated, were analysed, for the identification of the products, by gas chromatography (GC)-mass spectrometry (Dani 6800-Finnigan 112S) and, for quantitative determinations, by GC (10% silicon oil, 550 column). RESULTS A N D DISCUSSION
Voltammetric measurements with periodical renewal of the diffusion layer (PRDL) were carried out at different Br- concentrations, in the presence of cyclohexene (CH) which is oxidized at potentials more positive than Br-. After the addition of the olefin, when B r - concentration is low and the formation of Br 3 is not favoured, the limiting current of the single wave displayed by Br- is enhanced, while at higher concentrations only the second wave of the couple shown by the bromide ion increases. The trend of the phenomenon can thus be summarized: (a) the increment is enhanced by increasing the [CH]/[Br-] ratio and when this ratio is = 1, reaches a limit which corresponds to an apparent electron number ( n v ) < 2; this feature is made apparent in Fig. 1, in which the results obtained at [Br-] = 0.003 M are plotted; (b) the value of n v is increased by lowering the B r - concentration at constant [CH]/[Br-] ratio.
nv 1.4
"'I 1"Oo
[0] [,;] 1
2
5
1. Plot of n V = ( B r - second-wave total [cyc]ohe×ene]/[Br-], at [ B r - ] = 3 × 10 -3 M . Fig.
"
9 current)/~
Br-
first-wave
current)
vs.
ratio
371
The results depend neither on the type of the solvent dehydration procedure nor on the illumination conditions. Cyclic voltammetry [0 to 1.5 V, v = 0.2 V s-~, solvent purified as in (2)] has shown that by addition of CH to Br-, both the cathodic peaks of the reverse scan (Br 2 --, Br3, Br 3 ~ Br-) decrease to disappear at high Br- concentrations; on the other hand, if the sweep is reversed before reaching the second anodic peak, no influence of the CH presence is noticed. All the results reported above point out that: (c) Br 2 reacts with olefin, but the reaction may be incomplete; (d) Br 2 undergoes (at least) two parallel reactions, one of which leads to the increment of the electron number with respect to the oxidation of Br- to Br2; these reactions have, after evidence (b), a different order in respect of bromine, but after evidence (a), the same order in respect of the olefin. Exhaustive electrolyses coupled with coulometric determinations were carried out at the controlled potential (1.25 V) corresponding to the plateau of the second oxidation wave of Br- ([Br-] > 0.001 M ) on solutions containing CH in stoichiometric amount with respect to bromide. The results of electrolyses carried out at [Br-] = [CH] = 0.0045 M, in CH2C12 treated according to procedure (1), are shown in Table 1, where the average of results of five reproducible experiments are reported. No influence of illumination conditions on those experiments was observed. By lowering the water content of the solvent the formation of product II is impeded with a corresponding increment of IV. The presence of water is therefore irrelevant to the purpose of the present work. From the results of Table 1 (with allowance for GC precision) it appears that, accounting for their different bromine content, all the brominated products have been identified. Furthermore, if the charge required for the various products
TABLE 1 Results of preparative electrolyses at 1.25 V nv
n E"
Products
Relative yields/ mol %
1.4
1.3
3-bromocyclohexene (I) 2-bromocyclohexanol (II) 1-bromo-2-chlorocyclohexane (III) b 1,2-dibromocyclohexane (IV)
20 5 5 70
In electrolysed solutions:
[I]+[Ill]+[III]+[IV]
=
0.0025 M
a n E = (exp. expended charge)/(th, charge for Br one-electron oxidation). b Originated by CI- produced at the cathode, even if 5% anhydrous acetic acid in CH2CI 2 was used in that compartment.
372 formati,~n is taken into account, the value of r/E is not in contrast with the amounts of the detected compounds. On the other hand the values of n E and n v are not in disagreement. As a result, the deductions inferred from the voltammetric results can be clarified by the electrolyses results by assuming the electrochemical path leading to I and IV to be similar to the chemical one of NBS, with allowance for a non-radical addition reaction in the solution bulk. The voltammetric indications are in favour of low reactant concentrations, in order to improve the ratio between substitution and addition products in the electrolysis, but this method does not accord with practical considerations. Even if the electrochemical yield of 3-bromocyclohexene is not very high, the results obtained appear to be promising and the investigation was extended to other olefins. In the case of the presence of either cyclopentene or cyclooctene the voltammetric behaviour of Br- is quite similar to that observed with CH, and n v is respectively 1.2 and 1.7 at [Br-] -- 0.0035 M. Therefore, with those substrates also, the electrochemical preparation of allylic bromoderivatives appears to be feasible. In order to investigate the possibility of obtaining allylic bromoderivatives via the substrate oxidation, the solvent purified by procedure (2) was used because of the high oxidation potential of CH. In fact, by PRDL voltammetry a single bielectronic diffusion-limited oxidation wave (E~/2 = 2.2 V) is displayed by that olefin. Cyclic voltammetry (0.4 V s - l ) indicated the irreversibility of the electrode process by showing, on the reverse sweep, only a peak attributable to H ÷ reduction. This feature prevents the use of the technique to investigate the possible occurrence of a reaction between cyclohexenyl carbonium ion and Br-. On the other hand, PRDL voltammetry cannot give information on reactions following the irreversible carbonium ion generation if such reactions do not modify the number of the exchanged electrons. The preparative aspect of the problem was then examined. Exhaustive electrolyses coupled with coulometric determinations were carried out at controlled potential (2.3 V) on B r - / C H solutions with both the components at concentrations - 0.002 M (in this case 5% acetic acid was also present in the cathodic compartment). The charge consumed in the process was somewhat higher than 2 electrons per molecule of CH and the following products have been identified: I (40%)and IV (60%), but the total yield of the detected products was only 15% with respect to oxidized CH. After the electrolyses H + is the only species detectable by voltammetry and its amount is in accord with a complete oxidation of CH to cyclohexenyl carbonium ion. The only acceptable hypothesis on the reason for the product losses is that bromination takes place on organic fragments produced by the carbonium ion decomposition. In fact, only light compounds may elude GC detection by overlapping of the solvent tail. These results show that the second method did not prove as efficient as expected, probably because of the high instability of cyclohexenyl carbonium ion in an insufficiently polar solvent as dichloromethane.
373 REFERENCES 1 2 3 4 5 6 7 8 9 I0 11 12 13 14 15
M. Verniette, Ch. Deramon and J. Simonet, Electrochim. Acta, 23 (1978) 929. M, Mastragostino, G. Casalbore, S. Valcher and CI Zucchi, Ann. Claim. (Rome), 69 (1979) 307. P. Pouillen, R. Minko, M. Verniette and P. Martinet, Electrochim. Acta, 24 (1979) 1189. A.P. Korotkof, L.N. Nekrasov and V.M. Milman, Elektrokhimiya, 15 (9) (1979) 1407. P. Pouillen, R. Minko, M. Verniette and P. Martinet, Electrochim. Acta, 25 (1980) 711. Sigery Torii et al., Org. Chem., 46 (1981) 3312. Sigery Torii et al., Tetrahedron Lett., 22 (33) (1981) 3193. M. Verniette, P. Pouillen and P. Martinet, Bull. Soc. Chim. Fr., I (1981) 343. W. Schmidt and E. Steckhan, J. Electroanal. Chem., 10! (1979) 129. M. Novak, Cs. Visy and K. Bodor, Electrochim. Acta, 27 (1982) 1293. C. Djerassi, Chem. Rev., 43 (1948) 271. J. Adam, P.A. Gosselain and P. Goldfinger, Nature, 171 (1953). 704. M. Mastragostino, S. Valcher and P. Lazzari, J. Electroanal. Chem., 126 (1981) 189. G. Faita, M. Fleischmann and D. Pletcher, J. Electroanal. Chem., 25 (1970) 455. H. Lund and P. Iversen in M.M. Baizer (Ed.), Organic Electrochemistry, Marcel Dekker, New York, 1973, p. 211.
369
Short communication
INVESTIGATION OF THE FEASIBILITY OF ELECTROCHEMICAL ALLYLIC BROMINATION IN DICHLOROMETHANE
MARINA MASTRAGOSTINO, SERGIO VALCHER and MARIA BISERNI lstituto Chimico "'Ciamician'" dell'Universita" di Bologna, Via Selmi 2, 40126 Bologna (ltaly) lstituto di Polarografia ed Elettrochimica Preparativa del C.N.R., Corso Stati Uniti 4, 35100 Padua (Italy) (Received 24th January 1983; in revised form 1 lth April 1983)
INTRODUCTION
The electrochemical halogenation of olefinic compounds in nucleophilic solvents has been the subject of numerous papers [1-8] and some results have also been published for unreactive solvents [9,10]. In this paper we report the results of a study on the feasibility of electrochemically preparing allylic bromoderivative compounds, at a Pt electrode in CH2C1 z, which seems a suitable solvent to hinder double-bond saturation reactions. N-bromosuccinimide (NBS) in unreactive solvents is the usual chemical reagent for the preparation of allylic bromides [11]. The efficiency of this synthesis, which proceeds through a radical chain mechanism [12], is due to the capability of the reagent NBS to maintain the Br 2 concentration low and constant. Since Br" and Br 2 are electrochemically obtainable at controlled bromine concentrations and since an investigation on the electrode kinetics of the system Br-/Br2 [13] gave evidence for the possibility that Br" is present in CH2C12 solutions, we may expect the electrosynthesis of the allylic bromoderivatives to be possible by operating on Br-/olefin systems atthe Br- oxidation potential. On the other hand, the investigation of Faita et al. [14] in acetonitrile suggests the possibility of also obtaining this type of compounds by operating at higher potentials on Br-/olefin systems through the oxidation of the organic substrate. Indeed the use of CH2C12 could improve the yields, the nucleophilic attack of the solvent to carbocation being excluded. The two routes, differing in electrolysis potential, are considered here. EXPERIMENTAL
The C H 2 C 1 2 (RPE, C. Erba) was purified, depending on its use, by: (1) distillation over PzOs, after storage over CaC12 (2 h); (2) double distillation under anhydrous N 2 and dehydration for at least 6 days by contact with activated molecular sieves (4A Pellets, 4 mm, C. Erba) in the dark and under N 2. 0022-0728/83/$03.00
© 1983 Elsevier Sequoia S.A.
370
The molecular sieves were activated by heating at 673 K for 24 h. The residual water after procedures (1) and (2) was = 0.002 and < 0.0001 M respectively; in the latter case, with tetrabuthylammonium perchlorate (TBAP), potentials of about 2.4 V can be attained at a Pt electrode, an upper limit 0.6 V more positive than that reported in the literature [15]. All the chemicals were purified before use. The electrochemical experiments were carried out at 273 K with TBAP 0.2 M. All the potentials are referred to the aqueous SCE. The electrolysed solutions, after having been concentrated, were analysed, for the identification of the products, by gas chromatography (GC)-mass spectrometry (Dani 6800-Finnigan 112S) and, for quantitative determinations, by GC (10% silicon oil, 550 column). RESULTS A N D DISCUSSION
Voltammetric measurements with periodical renewal of the diffusion layer (PRDL) were carried out at different Br- concentrations, in the presence of cyclohexene (CH) which is oxidized at potentials more positive than Br-. After the addition of the olefin, when B r - concentration is low and the formation of Br 3 is not favoured, the limiting current of the single wave displayed by Br- is enhanced, while at higher concentrations only the second wave of the couple shown by the bromide ion increases. The trend of the phenomenon can thus be summarized: (a) the increment is enhanced by increasing the [CH]/[Br-] ratio and when this ratio is = 1, reaches a limit which corresponds to an apparent electron number ( n v ) < 2; this feature is made apparent in Fig. 1, in which the results obtained at [Br-] = 0.003 M are plotted; (b) the value of n v is increased by lowering the B r - concentration at constant [CH]/[Br-] ratio.
nv 1.4
"'I 1"Oo
[0] [,;] 1
2
5
1. Plot of n V = ( B r - second-wave total [cyc]ohe×ene]/[Br-], at [ B r - ] = 3 × 10 -3 M . Fig.
"
9 current)/~
Br-
first-wave
current)
vs.
ratio
371
The results depend neither on the type of the solvent dehydration procedure nor on the illumination conditions. Cyclic voltammetry [0 to 1.5 V, v = 0.2 V s-~, solvent purified as in (2)] has shown that by addition of CH to Br-, both the cathodic peaks of the reverse scan (Br 2 --, Br3, Br 3 ~ Br-) decrease to disappear at high Br- concentrations; on the other hand, if the sweep is reversed before reaching the second anodic peak, no influence of the CH presence is noticed. All the results reported above point out that: (c) Br 2 reacts with olefin, but the reaction may be incomplete; (d) Br 2 undergoes (at least) two parallel reactions, one of which leads to the increment of the electron number with respect to the oxidation of Br- to Br2; these reactions have, after evidence (b), a different order in respect of bromine, but after evidence (a), the same order in respect of the olefin. Exhaustive electrolyses coupled with coulometric determinations were carried out at the controlled potential (1.25 V) corresponding to the plateau of the second oxidation wave of Br- ([Br-] > 0.001 M ) on solutions containing CH in stoichiometric amount with respect to bromide. The results of electrolyses carried out at [Br-] = [CH] = 0.0045 M, in CH2C12 treated according to procedure (1), are shown in Table 1, where the average of results of five reproducible experiments are reported. No influence of illumination conditions on those experiments was observed. By lowering the water content of the solvent the formation of product II is impeded with a corresponding increment of IV. The presence of water is therefore irrelevant to the purpose of the present work. From the results of Table 1 (with allowance for GC precision) it appears that, accounting for their different bromine content, all the brominated products have been identified. Furthermore, if the charge required for the various products
TABLE 1 Results of preparative electrolyses at 1.25 V nv
n E"
Products
Relative yields/ mol %
1.4
1.3
3-bromocyclohexene (I) 2-bromocyclohexanol (II) 1-bromo-2-chlorocyclohexane (III) b 1,2-dibromocyclohexane (IV)
20 5 5 70
In electrolysed solutions:
[I]+[Ill]+[III]+[IV]
=
0.0025 M
a n E = (exp. expended charge)/(th, charge for Br one-electron oxidation). b Originated by CI- produced at the cathode, even if 5% anhydrous acetic acid in CH2CI 2 was used in that compartment.
372 formati,~n is taken into account, the value of r/E is not in contrast with the amounts of the detected compounds. On the other hand the values of n E and n v are not in disagreement. As a result, the deductions inferred from the voltammetric results can be clarified by the electrolyses results by assuming the electrochemical path leading to I and IV to be similar to the chemical one of NBS, with allowance for a non-radical addition reaction in the solution bulk. The voltammetric indications are in favour of low reactant concentrations, in order to improve the ratio between substitution and addition products in the electrolysis, but this method does not accord with practical considerations. Even if the electrochemical yield of 3-bromocyclohexene is not very high, the results obtained appear to be promising and the investigation was extended to other olefins. In the case of the presence of either cyclopentene or cyclooctene the voltammetric behaviour of Br- is quite similar to that observed with CH, and n v is respectively 1.2 and 1.7 at [Br-] -- 0.0035 M. Therefore, with those substrates also, the electrochemical preparation of allylic bromoderivatives appears to be feasible. In order to investigate the possibility of obtaining allylic bromoderivatives via the substrate oxidation, the solvent purified by procedure (2) was used because of the high oxidation potential of CH. In fact, by PRDL voltammetry a single bielectronic diffusion-limited oxidation wave (E~/2 = 2.2 V) is displayed by that olefin. Cyclic voltammetry (0.4 V s - l ) indicated the irreversibility of the electrode process by showing, on the reverse sweep, only a peak attributable to H ÷ reduction. This feature prevents the use of the technique to investigate the possible occurrence of a reaction between cyclohexenyl carbonium ion and Br-. On the other hand, PRDL voltammetry cannot give information on reactions following the irreversible carbonium ion generation if such reactions do not modify the number of the exchanged electrons. The preparative aspect of the problem was then examined. Exhaustive electrolyses coupled with coulometric determinations were carried out at controlled potential (2.3 V) on B r - / C H solutions with both the components at concentrations - 0.002 M (in this case 5% acetic acid was also present in the cathodic compartment). The charge consumed in the process was somewhat higher than 2 electrons per molecule of CH and the following products have been identified: I (40%)and IV (60%), but the total yield of the detected products was only 15% with respect to oxidized CH. After the electrolyses H + is the only species detectable by voltammetry and its amount is in accord with a complete oxidation of CH to cyclohexenyl carbonium ion. The only acceptable hypothesis on the reason for the product losses is that bromination takes place on organic fragments produced by the carbonium ion decomposition. In fact, only light compounds may elude GC detection by overlapping of the solvent tail. These results show that the second method did not prove as efficient as expected, probably because of the high instability of cyclohexenyl carbonium ion in an insufficiently polar solvent as dichloromethane.
373 REFERENCES 1 2 3 4 5 6 7 8 9 I0 11 12 13 14 15
M. Verniette, Ch. Deramon and J. Simonet, Electrochim. Acta, 23 (1978) 929. M, Mastragostino, G. Casalbore, S. Valcher and CI Zucchi, Ann. Claim. (Rome), 69 (1979) 307. P. Pouillen, R. Minko, M. Verniette and P. Martinet, Electrochim. Acta, 24 (1979) 1189. A.P. Korotkof, L.N. Nekrasov and V.M. Milman, Elektrokhimiya, 15 (9) (1979) 1407. P. Pouillen, R. Minko, M. Verniette and P. Martinet, Electrochim. Acta, 25 (1980) 711. Sigery Torii et al., Org. Chem., 46 (1981) 3312. Sigery Torii et al., Tetrahedron Lett., 22 (33) (1981) 3193. M. Verniette, P. Pouillen and P. Martinet, Bull. Soc. Chim. Fr., I (1981) 343. W. Schmidt and E. Steckhan, J. Electroanal. Chem., 10! (1979) 129. M. Novak, Cs. Visy and K. Bodor, Electrochim. Acta, 27 (1982) 1293. C. Djerassi, Chem. Rev., 43 (1948) 271. J. Adam, P.A. Gosselain and P. Goldfinger, Nature, 171 (1953). 704. M. Mastragostino, S. Valcher and P. Lazzari, J. Electroanal. Chem., 126 (1981) 189. G. Faita, M. Fleischmann and D. Pletcher, J. Electroanal. Chem., 25 (1970) 455. H. Lund and P. Iversen in M.M. Baizer (Ed.), Organic Electrochemistry, Marcel Dekker, New York, 1973, p. 211.