Langmuir 1996, 12, 3067-3074
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Radical vs Anionic Pathway in Mediated Electrochemical Reduction of Benzyl Bromide in a Bicontinuous Microemulsion De-Ling Zhou, Hermes Carrero, and James F. Rusling* Department of Chemistry, Box U-60, University of Connecticut, Storrs, Connecticut 06269-4060 Received December 7, 1995X Products derived from benzyl radicals or anions were obtained by reduction of benzyl bromide mediated by vitamin B12, Co(salen), and cobalt phthalocyaninetetrasulfonate in a bicontinuous microemulsion and homogeneous solvent. The reactions begin by electrochemical generation of a Co(I) complex (CoIL), followed by oxidative addition of benzyl bromide to give benzyl-CoIIIL. Reductive cleavage of the Co-C bond yields benzyl radicals or anions depending on the potential of reduction of benzyl-CoIIIL. Rate constants of Co-C bond formation (k1) are correlated with the formal potential E°′Co(II)/Co(I), indicative of control by the inherent activation free energy of the oxidative addition rather than by reactant distribution between phases in the microemulsion. Vitamin B12 gave a benzyl-CoIIIL intermediate reduced at -1.1 V vs SCE and yielded bibenzyl as the sole product of a radical pathway. The reduction potential of benzyl-CoIII(salen) is negative enough to reduce the benzyl radical to an anion, so reduction of benzyl bromide mediated by Co(salen) gave toluene in the microemulsion and a mixture of toluene and bibenzyl in DMF. Although Co(salen) reacts very rapidly with benzyl bromide, a slow rate of reductive cleavage of benzyl-CoIII(salen) creates a bottleneck in the catalytic pathway. The one-electron catalyst vitamin B12, with a smaller rate of oxidative addition, gives faster catalytic reduction for a given k1 relative to the two-electron catalyst Co(salen). Thus, a radical or anionic pathway can be chosen by controlling the potential of the benzyl-CoIII reduction. The facile formation of bibenzyl in the microemulsion suggests that it should be applicable to electro-organic syntheses featuring radical-based formation of carboncarbon bonds.
Introduction Environmental and health issues are driving a search for alternatives to organic solvents in synthesis. One such alternative is microemulsions, and we have been exploring their use in electrochemical catalysis.1,2 Microemulsions are clear, stable fluids made from water, oil, and surfactants. They are less toxic and less expensive than most organic solvents and have possible uses in many technological applications.3-5 Among structural types, bicontinuous microemulsions should be quite useful for electrochemical synthesis because of excellent mass transport properties, good solubilization of polar and nonpolar solutes, and high conductivity.1,5 Bicontinuous microemulsions feature intertwined microscopic networks of oil and water with surfactant at the interfaces. In electrochemical catalysis, electrons are shuttled between electrodes and reactants by a chemical mediator. The reaction between the active form of the catalyst and the reactant is often the rate-determining step (rds).1,6 We have identified situations in which microemulsion properties control the kinetics of the rds. For example, a preconcentration of nonpolar reactants in a coadsorbed layer of surfactant and catalyst on the electrode can X
Abstract published in Advance ACS Abstracts, May 1, 1996.
(1) Rusling, J. F. In Modern Aspects of Electrochemistry; Conway, B. E., Bockris, J. O’M., Eds.; Plenum Press: New York, 1994; No. 26, pp 49-104. (2) (a) Owlia, A.; Wang, Z.; Rusling, J. F. J. Am. Chem. Soc. 1989, 111, 5091-5098. (b) Iwunze, M. O.; Sucheta, A.; Rusling, J. F. Anal. Chem. 1990, 62, 644-649. (c) Kamau, G. N.; Hu, N.; Rusling, J. F. Langmuir 1992, 8, 1042-1044. (d) Iwunze, M. O.; Hu, N.; Rusling, J. F. J. Electroanal. Chem. 1992, 333, 353-361. (e) Zhang, S.; Rusling, J. F. Environ. Sci. Technol. 1993, 27, 1375-1380. (f) Schweizer, S.; Rusling, J. F.; Huang, Q. Chemosphere 1994, 28, 961-970. (g) Huang, Q.; Rusling, J. F. Environ. Sci. Technol. 1995, 29, 98-103. (h) Zhang, S.; Rusling, J. F. Environ. Sci. Technol. 1995, 29, 1195-1199. (3) Bourrel, M.; Schechter, R. S. Microemulsions and Related Systems; Marcel Dekker: New York, 1988. (4) Attwood, D.; Florence, A. T. Surfactant Systems; Chapman and Hall: London, 1983. (5) Mackay, R. A., Texter, J., Eds. Electrochemistry in Colloids and Dispersions; VCH Publishers: New York, 1992. (6) Rusling, J. F. Acc. Chem. Res. 1991, 24, 75-81.
S0743-7463(95)01517-4 CCC: $12.00
enhance the rate of the second-order rds, which occurs in this adsorbed layer on the electrode.2c When reactants and catalyst are not adsorbed onto the electrode, the rds occurs in the bulk of the microemulsion. Provided that the dynamics of reactant partition are not rate-limiting, the reaction rate for a given reactant in this situation is controlled primarily by the free energy of the electrode reaction which produces the active form of the catalyst.7 The latter mode of reactivity control was demonstrated for the catalytic reduction of trans-1,2-dibromocyclohexane and for SN2 reactions of n-alkyl bromides with electrochemically generated CoI complexes.7 The complexes resided in the water phase of a bicontinuous microemulsion, while the organobromides resided in the oil phase. For a given alkyl halide, a linear relation between the logarithm of the second-order chemical rate constant and E°′Co(II)/Co(I) was found for reactions in both DMF and the microemulsion. Data for both media fell on the same line. This relation showed that kinetic differences were controlled by activation free energies dependent on the reduction potential of Co(II)/Co(I) rather than by the distribution of reactants between phases. Reaction of alkyl halides with CoIL complexes leads to an organometallic reagent R-CoIII.8,9 Homolysis of the Co-C bond generates a carbon-centered radical which can be used in situ to form carbon-carbon bonds,10 either (7) Zhou, D.-L.; Gao, J.; Rusling, J. F. J. Am. Chem. Soc. 1995, 117, 1127-1134. (8) (a) Dolphin, D., Ed. B12; J. Wiley: New York, 1982; Vols. 1 and 2. (b) Toscano, P. J.; Marzilli, L. G. Prog. Inorg. Chem. 1984, 31, 105204. (c) Guilard, R.; Lecomte, C.; Kadish, K. M. Struct. Bonding 1987, 64, 205-268. (9) (a) Zhou, D.-L.; Walder, P.; Scheffold, R.; Walder, L. Helv. Chim. Acta 1992, 75, 995-1011. (b) Zhou, D.-L.; Yang, D.-H.; Walder, L. Unpublished results. (10) (a) Scheffold, R. Chimia 1985, 39, 203-211. (b) Scheffold, R.; Abrecht, S.; Orlinski, R.; Ruf, H.-R.; Stamouli, P.; Tinembart, O.; Walder, L.; Weymuth, C. Pure Appl. Chem. 1987, 59, 363-372. (c) Scheffold, R.; Rytz, G.; Walder, L.; Orlinski, R.; Chilmonczyk, Z. Pure Appl. Chem. 1983, 55, 1791-1797. (d) Scheffold, R.; Rytz, G.; Walder, L. In Modern Synthetic Methods; Scheffold, R., Ed.; J. Wiley: London, 1983; Vol. 3, pp 355-440.
© 1996 American Chemical Society
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by dimeric coupling or by reaction with another reagent. Such cobalt-mediated bond formation has been used for the preparation of a variety of natural products.10,11 Benzyl bromide forms organocobalt complexes with Co(I) macrocycles.12 Direct electrochemical reduction of benzyl bromide at a mercury electrode gave toluene and dibenzylmercury, along with a minor amount of bibenzyl.13,14 The product distribution depends on electrode potential,15 with more negative potentials favoring the production of toluene. Electroreduction in protic as well as aprotic solvents such as dimethylformamide (DMF), acetonitrile, and methanol affords more toluene than bibenzyl.13 Benzyl anion has been proposed as an intermediate in toluene formation.14,16 Bibenzyl would be expected from a radical pathway involving C-C bond formation.15 Indirect electrochemical reduction of benzyl bromide mediated by transition metal complexes has been reported.17 Ratios of toluene to bibenzyl were highly dependent on the mediator and experimental conditions. Fry and co-workers examined the double electrochemical catalytic reduction of benzal chloride (ArCHCl2) using Co(salen), in which the SN2 coupling and organic radical formation occur at the same potential.18 Again, the ratio of toluene to coupling products depended strongly on potential. In general, potentials at which benzyl radical is reduced seem to favor production of toluene. As mentioned above, carbon-carbon bond formation can be achieved by radical chemistry promoted by transition metal complexes.19,20 An aim of the present work was to examine the ability of a microemulsion to sustain such radical reactions. This led to an examination of the conditions which govern the radical vs anionic pathways. There is also a possibility for radical stability control with a microemulsion.6,21,22 In this paper, we explore the catalytic reduction of benzyl bromide with macrocyclic cobalt complexes, in which the initial formation and cleavage of organocobalt complexes (11) (a) Auer, L.; Weymuth, C.; Scheffold, R. Helv. Chim. Acta 1993, 76, 810-818. (b) Busato, S.; Tinembart, O.; Zhang, Z.; Scheffold, R. Tetrahedron 1990, 46, 3155-3166. (c) Scheffold, R. In Electroorganic Synthesis [Manuel M. Baizer Memorial Symposium]; Little, R. D., Weinberg, N. L., Eds.; Dekker: New York, 1991; pp 317-322. (d) Hemamalini, S.; Scheffold, R. Helv. Chim. Acta 1995, 78, 447-451. (12) Schrauzer, G. N.; Grate, J. H. J. Am. Chem. Soc. 1981, 103, 541-546. (13) Hawley, M. D. Acyclic Aliphatic Halides. In Encyclopedia of Electrochemistry of the Elements, Organic Section; Bard, A. J., Lund, H., Eds.; Marcel Dekker: New York, 1980; Vol. 14, pp 83-103. (14) Baizer, M. M.; Chruma, J. L. J. Org. Chem. 1972, 37, 19511960. (15) Brown, O. R.; Thirsk, H. R.; Thornton, B. Electrochim. Acta 1971, 16, 495-503. (16) (a) Andrieux, C. P.; Gorande, A. L.; Saveant, J.-M. J. Am. Chem. Soc. 1992, 114, 6892-6904. (b) Wawzonek, S.; Duty, R. C.; Wagenknecht, J. H. J. Electrochem. Soc. 1964, 111, 74-78. (17) (a) Shi, S.; Bakac, A.; Espenson, J. H. Inorg. Chem. 1991, 30, 3410-3414. (b) Sadler, N.; Scott, S. L.; Bakac, A.; Espenson, J. H.; Ram, M. S. Inorg. Chem. 1989, 28, 3951-3954. (c) Komura, T.; Tetsuo, T.; Takahashi, K. Denki Kagaku oyobi Kogyo Butsuri Kagaku 1991, 59, 780-785. (d) Jennings, P. W.; Pillsbury, D. G.; Hall, J. L.; Brice, V. T. J. Org. Chem. 1976, 41, 719-722. (e) Wellmann, J.; Steckhan, E. Synthesis 1978, 901-902. (18) (a) Fry, A. J.; Singh, A. H. J. Org. Chem. 1994, 59, 8172-8177. (b) Fry, A. J.; Fry, P. F. J. Org. Chem. 1993, 58, 3496-3501. (c) Fry, A. J.; Sirisoma, U. N.; Lee, A. S. Tetrahedron Lett. 1993, 34, 809-812. (19) Iqbal, J.; Bhatia, B.; Nayyar, N. K. Chem. Rev. 1994, 94, 519564. (20) (a) Curren, D. P. Synlett 1991, 63-72 and references therein. (b) Curren, D. P. Synthesis 1988, 417-439, 489-513. (c) Giese, B. Radicals in Organic Synthesis: Formation of Carbon-Carbon Bonds; Pergamon: Oxford, 1986. (d) Hart, D. J. Science 1984, 223, 883-887. (e) Ramaiah, M. Tetrahedron 1987, 43, 3541-3676. (f) Beckwith, A. L. J. Tetrahedron 1981, 37, 3073-3100. (21) Rusling, J. F. In Electroanalytical Chemistry; Bard, A. J., Ed.; Marcel Dekker: New York, 1994; Vol. 19, pp 1-88. (22) Iwunze, M. O.; Rusling, J. F. J. Electroanal. Chem. 1991, 303, 267-270.
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occur at well-separated potentials. Reactions are compared in a bicontinuous microemulsion and in two homogeneous solvents. The rate of the initial coupling of Co(I)L with benzyl bromide depends largely on the activation free energy governed by the standard potential of the Co(II)/Co(I) couple. Vitamin B12r, with a relatively positive redox potential and smaller rate of initial coupling, facilitated a fast radical pathway yielding bibenzyl. Co(salen), with a more negative redox potential and very fast coupling rate, facilitated a less efficient anionic pathway yielding toluene. The microemulsion gave exclusively bibenzyl or toluene under the proper conditions. Experimental Section Chemicals and Solutions. Benzyl bromide (>98%) from Fluka and bibenzyl (99%) from Aldrich were used as received. Baker Analyzed toluene was distilled before use. The cobalt complexes, DDAB microemulsion, solvents, and all other chemicals were the same as described previously.7 The bromide of tetrabutylammonium (TBABr) was used as supporting electrolyte to avoid halide exchange reaction.16b Solutions were prepared immediately before use. Instruments and Procedures. The cyclic voltammetric apparatus and procedures were described previously.7 PARC 273 and Bioanalytical Systems BAS-100B electrochemical analyzers were employed at 25 ( 1 °C. The reference was an aqueous saturated calomel electrode (SCE) separated from the solution by a salt bridge ending in porous Vycor (BAS). All potentials were referred to the SCE. The ohmic drop of the cell was compensated electronically. Controlled-Potential Electrolysis. Solutions of benzyl bromide (10 mM) and a cobalt complex (1 mM) were prepared in 0.1 M TBABr-DMF or in the microemulsion. The gas-tight, separated electrolysis cell was equipped with a stirrer and N2 flow system. Before electrolysis, solutions (20 mL) were flushed with N2 for 30 min then blanketed by N2 with stirring during electrolyses. The working electrode was 2 × 1 × 0.5 cm3 WDF carbon felt (Union Carbide), and the reference was an SCE. The counter electrode, which was separated from the reaction chamber by a glass frit, was a spectroscopic carbon rod (Ultracarbon Co.). Electrolyses were done at 25 °C using a PARC Model 273 potentiostat. Electrolyzed mixtures were analyzed by HPLC and GC/MS. The HPLC had a Spectra Physics SP8810 isocratic pump combined with an SP8450 UV-vis detector at 254 nm. The column was an ULTREMEX 3 C18 (4.6 mm diameter × 100 mm long) used with a mobile phase of 55:45 (v/v) THF-water at flow rate of 0.3 mL/min. For microemulsions, 10 µL of saturated KCl was added to 1 mL of reaction mixture to form two phases and precipitate DDAB. The organic phase was analyzed by chromatography. HPLC analyses were calibrated with toluene, bibenzyl, and benzyl bromide standards taken through the identical procedures. GC-MS was done with a Hewlett-Packard 5890 gas chromatograph equipped with a 5970 quadrupole mass spectrometer and a 12.5 m methyl silicone capillary column. Extraction of the microemulsion by methylene chloride, or by cyclohexane in the case of DMF, gave solutions suitable for GC-MS. Standards were treated under identical conditions. Peaks for electrolysis samples gave MS fragmentation patterns identical with those of authentic toluene and bibenzyl.
Results Electrochemistry of Mediators. The bicontinuous microemulsion was made from didodecyldimethylammonium bromide (DDAB), water, and dodecane (21/39/40 wt %). It has been characterized as bicontinuous2b,23 and was used in our previous work on catalytic and SN2 reactions.7 Voltammetric data in bicontinuous microemulsions can be analyzed using the theory for homogeneous media.2b The electrochemical properties of the Co(II)/Co(I) redox couples for Co(salen) (A), cobalt phthalocyaninetetrasul-
Electrochemical Reduction of Benzyl Bromide
Langmuir, Vol. 12, No. 12, 1996 3069 Table 1. Electrochemical Parameters for CoIIL/CoIL Redox Couplesa complex Co(salen) vitamin B12 CoPcTS
mediumb
-E°′, V vs SCE
106D, cm2 s-1
103k°′, cm s-1
DMFc DDAB µEc DMF-H2O DMFc DDAB µEc DMF-H2O DMFc DDAB µEc DMF-H2O
1.225 1.085 1.230 0.710 0.815 0.785 0.324 0.250 0.330
5.2 0.55 3.26 1.8 0.28 0.6 1.5 d 1.2
8.2 4.5 5.4 3.0 1.2 1.8 2 d 1.7
a Glassy carbon electrode; 25 ( 1 °C. b Composition of microemulsion (abbreviated as µE): DDAB/H2O/dodecane ) 21/39/40 (wt %). DMF: 0.1 M TBABr/DMF. DMF-H2O: 0.1 M TBABr/(4:1 v) DMF-H2O. c Data from ref 7. d Not estimated owing to adsorption of reactant on electrode.
fonate (B, CoIIPcTS), and vitamin B12r (CoIIB12) (C) on glassy carbon electrodes in the microemulsion and in DMF were reported recently.7 The electrochemical parameters of the complexes found by cyclic voltammetry (CV) in DMF-H2O were similar to those in DMF (Table 1). All CoII complexes in all media gave diffusion-controlled, reversible cyclic voltammograms at relatively low scan rates, except for CoPcTS in the microemulsion.7 Oxidative Addition of Benzyl Bromide to CoIL. The CoIIL complexes undergo a cobalt-centered oneelectron reduction, as in eq 1 (Scheme 1), giving rise to a reversible pair of reduction/oxidation peaks in CV (Figure 1a). Direct reduction of benzyl bromide is irreversible
Scheme 1 CoIIL + e- h CoIL (at electrode) E°′1
(1)
CoIL + PhCH2Br f PhCH2CoIIIL + Br- k1 (2) PhCH2CoIIIL + e- f [PhCH2CoIIIL]•- or [PhCH2CoIIL] (3) [PhCH2CoIIL] f PhCH2• + CoIL
(4)
[PhCH2CoIIL] f PhCH2- + CoIIL
(5)
PhCH2• + e- f PhCH2- (at electrode)
(6)
PhCH2• + CoIL h PhCH2- + CoIIL
(7)
PhCH2• + HS f PhCH3 + S• (HS ) solvent) (8) PhCH2- + H+ f PhCH3
(9)
PhCH2• + PhCH2• f PhCH2CH2Ph
(10)
PhCH2- + PhCH2Br f PhCH2CH2Ph + Br- (11)
Figure 1. Cyclic voltammograms at 0.02 V s-1 on glassy carbon electrodes in the DDAB microemulsion: (a) 0.6 mM vitamin B12r alone; (b) 1.2 mM benzyl bromide alone; (c) 0.6 mM vitamin B12r + 1.2 mM benzyl bromide.
and occurs at a much more negative potential (Figure 1b). When benzyl bromide24 and CoIIL are present together in the medium, a new peak appears at a potential negative of the Co(II) reduction peak but positive of the direct benzyl bromide reduction peak (Figure 1c). The Co(II)/Co(I) reduction wave remains about the same height (Figure 1c), becomes chemically irreversible, and is shifted to more positive potentials. Disappearance of the oxidation peak for Co(I) suggests reaction of CoIL with benzyl bromide to give benzyl-CoIIIL. The new peak is caused by catalytic reduction of benzyl-CoIIIL. Similar CV results for benzyl bromide reduction were obtained for all three Co complexes in all media, except for CoPcTS in the microemulsion, which was strongly adsorbed onto the electrode.7 Complete disappearance of the Co(I) oxidation peak was facilitated by high concentrations of benzyl bromide and low scan rates and was highly dependent on the nature of the cobalt complex. (23) (a) Chen, S. J.; Evans, D. F.; Ninham, B. W.; Mitchell, D. J.; Blum, F. D.; Pickup, S. J. Phys. Chem. 1986, 90, 842-847. (b) Evans, D. F.; Mitchell, D. J.; Ninham, B. W. J. Phys. Chem. 1986, 90, 28172825. (c) Blum, F. D.; Pickup, S.; Ninham, B. W.; Chen, S. J.; Evans, D. F. J. Phys. Chem. 1985, 89, 711-713. (d) Chen, S. J.; Evans, D. F.; Ninham, B. W. J. Phys. Chem. 1984, 88, 1631-1634. (e) Zemb, T. N.; Hyde, S. T.; Derian, P.-J.; Barnes, I. S.; Ninham, B. W. J. Phys. Chem. 1987, 91, 3814-3820. (24) (a) Benzyl bromide is slowly hydrolyzed by water.24b-d About 18% of the benzyl bromide (2 mM) in the microemulsion and 12% in DMF was hydrolyzed in 8 h. In 4:1 (v/v) DMF-H2O, half of the benzyl bromide decomposed in 8 h. The solubility of benzyl bromide in the oil microphase may protect it from water. (b) Al-Lohedan, H.; Bunton, C. A.; Mhala, M. M. J. Am. Chem. Soc. 1982, 104, 6654-6660. (c) Vitullo, V. P.; Sridharan, S.; Johnson, L. P. J. Am. Chem. Soc. 1979, 101, 23202322. (d) Priebat, M. K.; Chauffe, L. J. Org. Chem. 1976, 41, 39143916.
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Figure 2. Cyclic voltammograms at 0.035 V s-1 on glassy carbon electrodes for 0.6 mM Co(salen) in the DDAB microemulsion with various equivalent ratios (γ) of benzyl bromide.
Complete disappearance of the anodic peak for Co(salen) occurs at low γ ) [PhCH2Br]/[CoL] (e.g. e 1) even at scan rates up to 10 V s-1, while for CoPcTS this requires high γ (e.g. g 10) combined with scan rates e50 mV s-1. This is because the rate constant for coupling with benzyl bromide (eq 2) is very fast for CoI(salen) and much slower for CoIPcTS. The cathodic CV peak (at -1.2 V in Figure 1c) for reduction of the organometallic complex benzyl-CoIIIL (eq 3) was always more positive than the direct benzyl bromide reduction peak (Figure 1b). The half-peak potentials (Epc/2) vs SCE for the direct reduction of 1 mM benzyl bromide at 100 mV s-1 were -1.470 V in DMF, -1.435 V in the microemulsion, and -1.334 V in DMFwater. The chemically irreversible peak of a cobalt complex in the presence of benzyl bromide reflects the so-called ErCir (electron transfer, reversible; chemical step, irreversible) pathway,25 as in eqs 1 and 2. The second-order rate constant k1 for the reaction (eq 2) of CoIPcTS with benzyl bromide was obtained from the ratio of anodic peak currents in the absence (ipa°) and in the presence (ipa) of bromide, as described previously.7 Cyclic voltammograms at a series of concentrations of benzyl bromide with switching potential negative of the Co(II) reduction but positive of the second reduction peak were used. For the reaction of either Co(salen) or vitamin B12 with benzyl bromide, the peak ratios could not be used because the Co(II) reduction peak was too close to that of benzylCoIIIL, and the chemical reaction is so fast that ipa/ipa° is too small to be measured accurately (Figure 2). For reactions of CoI(salen) and CoIB12, k1 was estimated from peak shifts of Co(II) reduction in the presence of a large excess of the bromide according to25,26
Ep - E°′ ) -0.780(RT/nF) + (RT/2nF) ln(λ)
(12)
where λ ) kf(RT/nF)/ν, kf ) k1[RX] is the pseudo-firstorder rate constant for eq 2, RX is benzyl bromide, Ep is the peak potential of the Co(II) complex in the presence of RX, E°′ is the formal potential of the Co(II)/Co(I) couple, n is the number of electrons (here n ) 1), and ν is the potential scan rate. R is the gas constant, T is absolute temperature, and F is Faraday’s constant. The slope of a kf vs [RX] plot yields k1. The value of λ reflects the sensitivity of the electrode process to the chemical reaction. A positive shift of the Co(II) peak proportional to log λ occurs as λ increases. (25) Nicholson, R. S.; Shain, I. Anal. Chem. 1964, 36, 706-723. (26) Parker, V. D. In Topics in Organic Electrochemistry; Fry, A. J., Britton, W. E., Eds.; Plenum Press: New York, 1986; pp 35-79.
This is illustrated in Figure 3a, which shows the shift in the peak as λ is increased by increasing the concentration of benzyl bromide. Computer simulations27 of cyclic voltammograms for the ErCir mechanism qualitatively reproduced the peak shifts (Figure 3b). However, simulations were successful representations of experimental cyclic voltammograms only when k°′ was made larger than the values found in the absence of benzyl bromide. While the cause of this finding is presently obscure, a similar apparent increase in k°′ for vitamin B12 on glassy carbon has been observed during catalytic reduction of 1,2dibromobutane in DMF.27c Average rate constants were estimated for a range of γ and scan rates. They had relative standard deviations within (20% in most cases (Table 2). The reaction involving CoI(salen) was by far the fastest and approached diffusion control in the microemulsion. An ErCir electrode reaction with sufficiently high secondorder rate constants causes a characteristic split of the CV reduction into two peaks in the presence of a limiting amount of reactant RX.28 This behavior was found for Co(salen) and vitamin B12 at γ < 0.5. Once all of the benzyl bromide is consumed in the reaction domain close to the electrode, the reversible Co(II)/Co(I) peak appears slightly negative of the positively-shifted wave. Thus, two cathodic peaks for the same Co(II) reduction appear (Figure 4). The relative heights of the two waves depend primarily on the concentration of benzyl bromide and the scan rate. As illustrated (Figure 4a) for Co(salen) in DMF, the “shifted” peak at about -1.15 V vs SCE increases with increasing γ at the expense of the reversible Co(II)/Co(I) peak at -1.28 V. At γ g 0.5 the -1.15 V peak is the only Co(II) reduction peak. The most negative peak at -1.42 V represents reduction of the organocobalt complex. Simulated cyclic voltammograms for the ErCir pathway using experimentally-estimated parameters and the conditions of Figure 4a showed qualitative agreement with experimental results (Figure 4b). The organocobalt reduction was omitted to simplify the calculations. The alkylation of CoIPcTS was the slowest of the coupling reactions examined (Table 2). The anodic peak for CoIPcTS disappears only in the presence of a large excess of benzyl bromide (Figure 5). Steady state current plateaus on the second wave confirm in this case that Co-C bond formation is the rds.25 Cleavage of Benzyl-CoIIIL. Alkylation of CoIL with benzyl bromide gives benzyl-CoIIIL,8-10,29 which may be somewhat labile.12,30 This instability has little influence on the CV time scale.12 The organocobalt compound is reduced at a more negative potential than CoIIL (Figures 1 and 2). The height of this reduction peak depends on the concentration of benzyl bromide (Figures 2 and 5). Electroreductive cleavage of R-CoIIIL can be either concerted or stepwise.8b,9b In general, electron transfer to R-CoIIIL significantly weakens the Co-C bond. For Me-CoIIIB12 and coenzyme B12 (adenosyl-CoIIIB12), one(27) (a) Digisim 2.0 (BAS) was used for CV simulation.27b (b) Rudolph, M.; Reddy, D. P.; Feldberg, S. W. Anal. Chem. 1994, 66, 589A-600A. (c) Connors, T. F.; Arena, J. V.; Rusling, J. F. J. Phys. Chem. 1988, 92, 2810-2816. (28) Andrieux, C. P.; Blocman, C.; Dumas-Bouchiat, J.-M.; M’Halla, F.; Saveant, J.-M. J. Electroanal. Chem. 1980, 113, 19-40. (29) (a) Day, P.; Hill, H. A. O.; Price, M. G. J. Chem. Soc. A 1968, 90-93. (b) Eckert, H.; Lagerlund, I.; Ugi, I. Tetrahedron 1977, 33, 2243-2247. (c) Eckert, H.; Ugi, I. Angew. Chem., Int. Ed. Engl. 1975, 14, 825-826. (d) Costa, G. Coord. Chem. Rev. 1972, 8, 63-75. (30) (a) Home, F.; Rezende, M. C.; Saboia, C. M.; Da Silva, A. C. Can. J. Chem. 1987, 65, 2095-2099. (b) Ogoshi, H.; Watanabe, E.-i.; Koketsu, N.; Yoshida, Z.-i. Bull. Chem. Soc. Jpn. 1976, 49, 2529-2536.
Electrochemical Reduction of Benzyl Bromide
Langmuir, Vol. 12, No. 12, 1996 3071
Figure 3. CoII(salen) reduction peaks enlarged from Figure 2 (0.035 V s-1) with background subtraction in the presence of various equivalent ratios (γ) of benzyl bromide: (a) experimental data on glassy carbon electrodes in DDAB microemulsion; (b) computer simulated voltammograms showing peak shift for ErCir pathway for the experimental conditions in Figure 3a. Experimental k1 ) 1 × 109 L mol-1 s-1, and the diffusion-kinetic parameters (see Table 1 and text) were used for simulations, except k°′ ) 1 cm s-1.
Figure 4. Cyclic voltammograms at 0.2 V s-1 for 0.6 mM Co(salen) in 0.1 M TBABr-DMF with small equivalent ratios (γ) of benzyl bromide showing the split in the Co(II) reduction peak: (a) experimental data on glassy carbon electrodes; (b) computer-simulated voltammograms showing the peak split for the ErCir pathway for the experimental conditions in Figure 4a. Experimental k1 ) 2.7 × 107 L mol-1 s-1, and the diffusion-kinetic parameters (see Table 1 and text) were used for simulations. Table 2. Rate Constants for Alkylation of CoIL by Benzyl Bromidea and Potentials for Reduction of Benzyl-CoIIIL Co(I)Lb CoI(salen) vitamin B12s CoIPcTS
mediumc DMF DDAB µE DMF-H2O DMF DDAB µE DMF-H2O DMF DMF-H2O
methodd I I I I I I II II
Ne
k1 (mean ( SD),f M-1 s-1
-Epc/2,g V vs SCE
∆E,h V
13 12 5 6 11 3 3 6
(2.7 ( 0.9) × 109 (1.2 ( 0.2) × 107 (4.2 ( 0.4) × 103 (2.0 ( 0.4) × 104 (7.8 ( 2.2) × 104 88 ( 4 82 ( 13
1.345 1.168 1.381 1.102 1.112 1.123 0.714 0.673
0.120 0.083 0.151 0.392 0.297 0.338 0.390 0.343
107
a See Experimental Section for exact conditions. b For simplicity, charge has been omitted. c DMF: 0.1 M TBABr/DMF. µE ) microemulsion. DMF-H2O: 0.1 M TBABr/(4:1 v) DMF-H2O. d For estimating k1, method I is based on the peak shift of Co(II)/Co(I) for the ErCir mechanism and II is based on the reversibility (ipa/ipa°) of Co(II)/Co(I) for the ErCir mechanism. e Number of trials. f Obtained from cyclic voltammograms with different γ ) [PhCH2Br]/[Co(II)L] at a series of scan rates. g Peak attributed to the reduction of benzyl-CoIIIL at 100 mV s-1. h ∆E ) E°′(CoIIL/CoIL) - Epc/2(PhCH2-CoIIIL).
electron reduction decreased the Co-C bond strength by half, and homolysis rates are enhanced31 by g1012. The reduction of benzyl-CoIIIL may involve a series of electrode and chemical reactions (eqs 3-5) and may be concerted or stepwise.8b,9b Electron transfer to PhCH2CoIIIL generates either [PhCH2CoIIIL]•- or [PhCH2CoIIL]-, depending on the initial electron-accepting site (eq 3). This site is controlled by the relative energies of the Co-C σ* and the ligand delocalized π* molecular orbitals (MOs), which are in turn related to the nature of the Co-bound residue PhCH2 and the ligand L.9b An intramolecular thermal electronic transition from the π*MO to the σ*-MO eventually yields the transient [PhCH2CoIIL]-, through which the Co-C bond cleavage occurs (31) (a) Martin, B. D.; Finke, R. G. J. Am. Chem. Soc. 1992, 114, 585-592. (b) Martin, B. D.; Finke, R. G. J. Am. Chem. Soc. 1990, 112, 2419-2420. (c) Finke, R. G.; Martin, B. D. J. Inorg. Biochem. 1990, 40, 19-22.
(eqs 4 and 5). Alternatively, a concerted Co-C bond cleavage following the one-electron reduction may occur. The CoIL or CoIIL regenerated from electroreductive cleavage (eq 4 or 5) of benzyl-CoIIIL may undergo further alkylation by benzyl bromide either directly or after reduction of CoIIL to CoIL. A catalytic cycle is thus established at the reduction potential of benzyl-CoIIIL. Also, benzyl radicals formed from the homolysis of the Co-C bond can be reduced to benzyl anion at the electrode (eq 6) or by CoIL (eq 7). The catalytic peak current (ic) on the second reduction wave could provide information about reactions in eqs 1-7. Obviously, ic may depend on the γ value, the heterogeneous electron transfer rate constants of electrode reactions including eqs 1, 3, and 6, the lifetime of (32) Bard, A. J.; Faulkner, L. R. Electrochemical Methods; Wiley: New York, 1980.
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Figure 5. Background-subtracted cyclic voltammograms at 0.05 V s-1 on glassy carbon electrodes for 0.6 mM CoPcTS in 0.1 M TBABr-DMF with various equivalent ratios (γ) of benzyl bromide. Table 3. Catalytic Efficiencies (ic/id) for Mediated Reduction of Benzyl Bromide by Cobalt Complexesa mediumb
catalystc
scan rate, V
CoI(salen)
DMF
0.05 0.1 0.05 0.1 0.05 0.1 0.05 0.1 0.05 0.1 0.05 0.1
vitamin B12s DDAB microemulsion
CoI(salen) vitamin B12s
DMF-H2O
s-1
CoI(salen)e vitamin B12se
Discussion d
ic/id
1.1 1.1 1.9 1.9 7.0 6.8 4.4 4.7 1.4 1.3 6.9 6.7
a i ) catalytic peak current; i ) diffusion current of Co(II)/ c d Co(I); see Experimental Section for conditions. b See Table 1, note c b. For simplicity, the overall charge is omitted. d [PhCH2Br]/[CoL] ) 1.0. e [PhCH2Br]/[CoL] ) 1.12.
Table 4. Product Distribution for Mediated Reductions of 0.2 mM Benzyl Bromide with Cobalt Complexesa cobalt catalyst
-Eapp, electrol. % PhCH2Br % yieldc % yieldc mediumb V/SCE time, h left bibenzyl toluene
Co(salen) DMF DMF DDAB µE DMF B12 DDAB µE
1.40 1.45 1.45 1.20 1.20
1 1 2 1 1
20 12 83 29 87
39 25 0 91 100
Figure 6. Influence of the Co(II)/Co(I) formal potential E°′ (Table 1) on log k1 (Table 2) for the bimolecular reactions of Co(I) complexes with benzyl bromide in DMF (0), DMF-H2O (+), and the DDAB microemulsion (O).
48 55 90 0 0
a See Experimental Section for detailed conditions. b Composition of microemulsion (abbreviated as µE): DDAB/H2O/dodecane ) 21/ 39/40 (wt %). DMF: 0.1 M TBABr/DMF. c Yields from HPLC analyses based on amount of benzyl bromide reacted.
[PhCH2CoIIIL]•- in a stepwise process, and reactions 8-11. Despite this complexity, ic reflects the overall reaction kinetics of the mediated electroreduction of benzyl bromide to stable products.32 Thus, ic/id (Table 3) can be used for qualitative comparisons of rates of benzyl bromide reduction. Product Distributions. Controlled-potential electrolyses of benzyl bromide with the cobalt complex mediators at potentials slightly negative of the peak for benzyl-CoIIIL reduction were compared in DMF and the microemulsion. Reactions were run for only 1 or 2 h to remove complications from benzyl bromide hydrolysis and side reactions. The reaction mediated by Co(salen) at -1.45 V vs SCE gave toluene as the principal product (Table 4). While some bibenzyl was produced in DMF, electrolysis in the microemulsion yielded only toluene. When vitamin B12 was the mediator, bibenzyl was the only product in DMF and in the microemulsion.
Co-C Bond Formation. Rate constants for the coupling of benzyl bromide with the CoI complexes (Table 2) are consistent with our previous finding7 of nucleophilic reactivity in the order CoI(salen) > CoIB12 > CoIPcTS. This can be explained by viewing the coupling reaction as an electron transfer, as discussed previously.7 By using radical scavengers, the oxidative addition of benzyl bromide to CoIB12 in DMF was concluded to be an inner sphere electron transfer (i.s.e.t.).9 The reactions of other sterically less hindered cobalt complexes are SN2 processes, which can be treated as inner sphere electron transfers.33 The process leading to the formation of the Co-C bond with CoIB12 is thus
CoIL + PhCH2Br f CoIIL + PhCH2• + Br- (i.s.e.t.) (13) CoIIL + PhCH2• f PhCH2CoIIIL (coupling) (14) This overall process is kinetically equivalent to an SN2 reaction, so all the Co-C bond formations here can all be treated as if they follow the same pathway. A plot of log k1 for the benzyl bromide reactions vs E°′Co(II)/Co(I) is reasonably linear (Figure 6), as predicted for an electron transfer reaction.33 Rate constants for the microemulsion, DMF, and DMF-H2O all fall on the same line. A plot of log k1 vs ∆E ) E°′Co(II)/Co(I) - Epc/2(PhCH2Br) gave a similar correlation. Benzyl bromide resides primarily in the oil phase. This was confirmed by diffusion coefficients estimated electrochemically in the three media, which were 8.1 × 10-6 cm2 s-1 in DMF, 8.8 × 10-6 cm2 s-1 in the microemulsion, and 9.4 × 10-6 cm2 s-1 in DMF-H2O at 25 °C. If benzyl bromide resides at the oil/water interface with the surfactant, or is partitioned into the water phase, a much smaller diffusion coefficient is expected.1 The cobalt complexes reside in the water phase.7 Points on the plot of log k1 vs E°′Co(II)/Co(I) fall on the same regression line (Figure 6) for the microemulsion and homogeneous solvents. If the phase distribution of reactants exerted a large influence on the observed kinetics of the coupling reactions, we would expect the points for the microemulsions to fall well below the line for the homogeneous solvents. We conclude that the rates are under significant control by the intrinsic activation free energy of the reaction, which depends on the formal (33) Saveant, J.-M. Adv. Phys. Org. Chem. 1990, 26, 1-130 and references cited therein.
Electrochemical Reduction of Benzyl Bromide
Figure 7. Correlations between catalytic activity as ic/id at 50 mV s-1 (Table 3) and the logarithm of the rate constant for reaction of CoIL with benzyl bromide for vitamin B12 and Co(salen) in DMF (0), DMF-H2O (+), and the DDAB microemulsion (O).
potential of the Co(II)/Co(I) couple. This is consistent with conclusions about reactions of CoIL with n-alkyl halides in the same microemulsion.7 Reductive Cleavage of Co-C Bond. Electron transfer from the electrode to benzyl-CoIIIL leads to cleavage of the Co-C bond (eqs 3-5), responsible for the new peak which appears negative of the Co(II) peak in the presence of benzyl bromide (Figures 1 and 2). Electron transfer, if any, to the intermediate radical PhCH2• (eqs 6 and 7) also contributes to the current of this second peak. The ic/id value thus reflects the kinetics of the overall electrocatalytic reduction of benzyl bromide mediated by CoIIL/ CoIL. Electroreductions of all benzyl-CoIIIL species in all media were chemically irreversible at scan rates up to 10 V s-1. No primary reduction products could be detected by voltammetry. Therefore, either the reductive cleavage is concerted or a transient intermediate with a very short lifetime is formed. The ic/id data (Table 3) gave correlations with log k1 which are different for vitamin B12 and Co(salen) (Figure 7). The coupling reaction (eq 2) may exert control on the current by controlling the rate of benzyl-CoIIIL formation. Co(salen) gave the largest k1 values but much smaller ic/id values than vitamin B12 for a given k1. This suggests that coupling (eq 2) may not be the rate-determining step in the catalytic reduction of benzyl bromide with Co(salen). Reductive cleavage rates (eqs 4 and 5) are directly related to the strength of the Co-C bond following oneelectron reduction.34 Upon taking up one electron, the lifetimes of the reduced intermediate for CH3-CoIII(salen) reduction at room temperature were 5-70 ms,34b significantly longer than 0.6 ms for CH3-CoIIIB12 at -30 °C.34a Thus, the reduced benzylcobalt species is likely to be more stable for Co(salen) than for vitamin B12. This reduction intermediate may be stabilized by the (salen) ligand, which allows more extensive delocalization of the charge34b than the corrin ring in vitamin B12. If this is the case, the reduction rate of benzyl bromide mediated by vitamin B12 is probably controlled by the rate of the initial coupling reaction (eq 2), since once benzyl-CoIIIB12 is reduced it breaks apart rapidly. However, slower decomposition of the benzyl-CoIII(salen) would create a bottleneck in the complex catalytic cycle for benzyl bromide reduction. This is the most likely reason for the smaller ic/id with Co(salen). (34) (a) Lexa, D.; Saveant, J. M. J. Am. Chem. Soc. 1978, 100, 32203222. (b) Costa, G.; Puxeddu, A.; Reisenhofer, E. Bioelectrochem. Bioenerg. 1974, 1, 29-39. (c) The lifetimes also depend on the nature of the electrolyte.34b
Langmuir, Vol. 12, No. 12, 1996 3073
While Co(salen) provides the fastest rates of the initial coupling reaction (eq 2), it is actually a poorer catalyst than vitamin B12 because of slower Co-C bond cleavage (eqs 4 and 5). The one-electron catalyst vitamin B12, with a much smaller rate of oxidative addition (eq 2), facilitates faster catalytic reduction than the two-electron catalyst Co(salen). Among the three media, the microemulsion provides the best rate of benzyl bromide reduction for Co(salen) and the second best rate for vitamin B12. The competition between homolysis (eq 4) and heterolysis (eq 5) of the Co-C bond should depend upon the redox potentials of the CoIIL/CoIL and PhCH2•/PhCH2couples which govern the free energy of the electron exchange reaction in eq 7. The reduction potential of benzyl radical in acetonitrile is -1.43 V vs SCE.35 All of the CoIIL/CoIL standard potentials (Table 1) are well positive of this, suggesting that formation of the benzyl radical by homolysis is thermodynamically favored.36 On the basis of the E°′ values of Co(II)/Co(I), the homolysis (eq 4) of benzyl-CoIIIL following electron transfer should be favored in the order CoPcTS > vitamin B12 > Co(salen). The competition between homolytic and heterolytic cleavage may further depend on the availability of protons in the solvent, as shown experimentally.37 Note that the largest values of ic/id occur in media containing water (Figure 7), i.e. the microemulsion and DMF-water. The water phase of the bicontinuous microemulsion is slightly acidic (ca. pH 4.8).1 Thus, either the polarity or acidity of the medium may play a role in the specific catalytic activity. Synthetic Implications. Mediation of the reduction of benzyl bromide by vitamin B12 gave exclusively bibenzyl in high yields in both the microemulsion and DMF (Table 4). When Co(salen) was the mediator, toluene was the only reduction product in the microemulsion, while a mixture of toluene and bibenzyl was obtained in DMF. The amount of benzyl bromide conversion per unit time was smaller in the microemulsion than in DMF. This may be related to the large bulk viscosity of the microemulsion, which limits convective mass transport, or to limitations caused by conductivity. We recently found greatly improved conversion efficiencies of mediated reactions using a cetyltrimethylammonium bromide microemulsion,38 which is more conductive and less viscous compared to the present DDAB microemulsion. Since benzyl radical is reducible (eqs 6 and 7), the reduction potential of benzyl-CoIIIL influences the final reaction products in electrolyses at this potential. As shown in Table 2, Epc/2 of benzyl-CoIIIL is medium- and ligand-dependent. The primary product of the reductive cleavage of benzyl-CoIIIL can be a neutral benzyl radical and/or a benzyl anion. Mediated electrolyses of benzyl bromide were done at potentials which break the Co-C bond (Tables 2 and 4). Product distributions were relatively independent of the medium, with Co(salen) giving mostly toluene and vitamin B12 giving exclusively bibenzyl. At potentials negative of its E1/2 (-1.43 V vs SCE in acetonitrile35) benzyl radical is reduced to the anion (eq 6). Moreover, direct electroreduction of benzyl bromide may compete with the mediated process at very negative potentials. Direct electrolysis of PhCH2Br gave toluene as the principal product,13,15 with benzyl anion as an intermediate.14-16 (35) Sim, B. A.; Griller, D.; Wayner, D. D. M. J. Am. Chem. Soc. 1989, 111, 754-755. (36) Zhou, D.-L.; Tinembart, O.; Scheffold, R.; Walder, L. Helv. Chim. Acta 1990, 73, 2225-2241. (37) Tinembart, O.; Walder, L.; Scheffold, R. Ber. Bunsen-Ges. Phys. Chem. 1988, 92, 1225-1231. (38) Gao, J.; Rusling, J. F.; Zhou, D.-L. J. Org. Chem., submitted.
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As discussed above, benzyl radical is the thermodynamically preferred product of reduction of benzyl-CoIII(salen) because the E1/2 of benzyl radical is more negative than E°′Co(II)/Co(I). Thus, a stepwise reduction of benzyl-CoIII(salen) leading to the benzyl anion according to eq 4 followed by eqs 6 and 7 is consistent with our results. The rate constant for dimerization of benzyl radical in benzene is 4 × 109 M-1 s-1.39 However, if electrochemical reduction of benzyl bromide occurs at sufficiently negative potentials, the initial radical will be reduced at a diffusioncontrolled rate to a benzyl anion. Thus, the radicals are too short-lived even for such a fast dimerization to compete successfully.40 Since reduction of benzyl-CoIIIB12 occurred at about -1.1 V vs SCE, controlled-potential electrolysis of benzyl bromide can be mediated by vitamin B12 at potentials well positive of the reduction of benzyl radical. Reductive homolysis of the Co-C bond in benzyl-CoIIIB12 is thermodynamically more favorable than that for benzyl-CoIII(salen). Benzyl radicals formed are stable at the potential at which benzyl-CoIIIB12 is reduced. Radical dimerization produces bibenzyl (Table 4). Similar reasons for potential-dependent product distributions were proposed by Fry and co-workers in the double catalytic reduction of benzal chloride by Co(salen).18 Scheme 1 shows that two different pathways can lead to the formation of toluene (eqs 8 and 9). Hydrogen-atom abstraction of benzyl radicals from the medium could also afford toluene (eq 8). This path appears less favored, since catalytic electrolysis at more positive potentials with vitamin B12 gave bibenzyl as the exclusive product (Table (39) Burkhart, R. D. J. Am. Chem. Soc. 1968, 90, 273-277. (40) (a) Fry, A. J.; Mitnick, M. A. J. Am. Chem. Soc. 1969, 91, 62076208. (b) Jacobus, J.; Eastham, J. F. J. Chem. Soc., Chem. Commun. 1969, 138-139.
Zhou et al.
4). No toluene was found, as when Co(salen) was used as a mediator at the more negative potentials where benzyl anion is produced. Therefore, H-atom abstraction by benzyl radicals does not seem viable in the microemulsion or DMF. Thus, for mediation by Co(salen), toluene is most probably produced by the protonation of benzyl anion. The microemulsion used behaves like an organic solvent with proton-donating ability, as shown previously.1,2b,22 Thus, only toluene is found with the microemulsion for Co(salen), but a mixture of toluene and bibenzyl is found with DMF, in which only poor proton donors are available. There is also competition between dimerization and further reduction under the conditions used in DMF. This may be related to a parent-anion dimerization (eq 11), or both radicals and anions might be produced at the potential used. Conclusions The bicontinuous microemulsion used behaves similarly to homogeneous solvents as media for mediated organic reductions. A radical or anionic pathway for benzyl bromide reduction can be chosen by controlling the redox potential of the mediator and the reactant-mediator adduct. The facile formation of bibenzyl shows that the bicontinuous microemulsion should be applicable for carbon-carbon bond formation by electroorganic synthesis using radicals as intermediates. Mediated radical reactions in microemulsions forming unsymmetric C-C bonds are currently being pursued in our laboratory. Acknowledgment. This work was supported by Grant No. CTS-9306961 from the NSF. The authors thank Al Fry of Wesleyan University for helpful discussions. LA9515175
