7416
Langmuir 1999, 15, 7416-7417
Electrochemical Phase-Transfer Catalysis in Microemulsions: Carbene Formation Colin J. Campbell and James F. Rusling* Chemistry Department, U-60, University of Connecticut, Storrs, Connecticut 06269-3060 Received June 17, 1999. In Final Form: September 10, 1999 Dicholocarbene can be generated at a carbon cathode from dichlorodibromomethane in microemulsions. The carbene then reacts with the organic constituent of the microemulsion to generate 7,7-dichlorobicycloheptane. A variety of reaction conditions were investigated, and the best conditions give a current efficiency of 80%.
Dichlorocarbene can be generated in a variety of ways,1 the most common of which is the action of a strong base on chloroform.2 Through the action of phase-transfer catalysts this reaction can be carried out in mixed, heterogeneous solvent systems containing water. In these cases sodium hydroxide (or other strong base) is used to abstract the proton from chloroform, forming a trichloromethyl anion. This ion associates with a quaternary ammonium ion (such as tetrabutylammonium) and enters the organic phase where it loses chloride to give dichlorocarbene which can partake in a variety of reactions including cyclopropanation across a double bond. To date, electrochemical generation of this intermediate followed by trapping with an alkene has been carried out in dry solvent systems.3,4 Microemulsions are microscopically heterogeneous mixtures of water, oil, and surfactants. They are macroscopically homogeneous, thermally stable, clear, colorless, and good solvents for substrates of a range of polarities. Conductive microemulsions are well suited to electrosynthesis.5 Since they are relatively cheap, of low toxicity, and relatively easily recycled, microemulsions hold potential for clean industrial applications. As mixtures of organic and aqueous solvents, they are also very similar to the systems in which some phase-transfer-catalyzed reactions are carried out.6 Herein we report initial investigations into the applicability of phase-transfer catalysis in microemulsions using carbon tetrachloride or dibromodichloromethane as the precursor of dichlorocarbene. Several types of microemulsions were used in this work (Table 1). An irreversible reduction wave for carbon tetrachloride was seen starting at around -1.0 V vs SCE in microemulsion 1. Controlledpotential electrolysis at -1.3 V vs SCE, however, gave rise to no cyclopropane derivatives when cyclohexene was used as a trapping agent for the carbene. Since pentanol is a cosurfactant in microemulsion 1 and would be potentially reactive with dichlorocarbene, the rest of the microemulsions utilized were alcohol-free. Using microemulsion 2, carbon tetrachloride (5.8 mM) was electroreduced at -1.3 V vs SCE in the presence of cyclohexene (1.2 mM). The product (7,7-dichlorobicycloheptane) was obtained with 40% current efficiency, (1) Jones, M.; Moss, R. A. Carbenes; John Wiley and Sons: New York, 1973; Vol. 1. (2) Sasson, Y.; Neumann, R. Handbook of phase transfer catalysis; Blackie Academic and Professional: London, 1997; p 151. (3) Fritz, H. P.; Kornrumpf, W. Justus Liebigs Ann. Chem. 1978, 1416. (4) Wawzonek, S.; Duty, R. C. J. Electrochem. Soc. 1961, 108, 1135. (5) Rusling, J. F.; Zhou, D. L. J. Electroanal. Chem. 1997, 439, 89. (6) Halpern, M. E. Phase-transfer catalysis; American Chemical Society: Washington, DC, 1997.
Figure 1. Cyclic voltammograms recorded with a pyrolytic graphite electrode in microemulsion 5: (a) without dibromodichloromethane and (b) with dibromodichloromethane (0.01 M). Scan rate 50 mV/s.
computed as a two-electron cleavage of chloride from carbon tetrachloride (eq 1) to the corresponding anion which forms dichlorocarbene (eq 2). 2e-
CCl4 9 8 CCl3-(at electrode) -Cl
-Cl-
CCl3-(org) 98 :CCl2(org)
(1) (2)
(Current efficiency is the theoretical charge required to form the recovered amount of product, divided by the actual current passed over the course of electrolysis, multiplied by 100.) Further experiments using microemulsions 2-5 and carbon tetrachloride gave no further optimization of the yield. As an alternative source of dichlorocarbene, we employed dibromodichloromethane. As the dibromo analogue of carbon tetrachloride it not only should be more easily reduced7 but also should more easily lose bromide to form the divalent carbon species. Voltammetry of dibromodichloromethane (not shown) in microemulsions 2 and 4 gave a small increase in current over the background although no obvious peaks were observed. The onset of reduction can more easily be seen (Figure 1) in voltammetry in microemulsion 5 (containing 10% water). The onset of reduction appears at -0.25 V vs SCE and increases steadily as one scans to more negative potentials. An identical voltammetric response was seen (7) Fry, A. J. Synthetic Organic Electrochemistry, 2nd ed.; John Wiley & Sons: New York, 1989; p 148.
10.1021/la9907836 CCC: $18.00 © 1999 American Chemical Society Published on Web 10/07/1999
Letters
Langmuir, Vol. 15, No. 22, 1999 7417 Table 1. Microemulsions and Their Components by Weight microemulsion 1: microemulsion 2: microemulsion 3: microemulsion 4: microemulsion 5:
cetyltrimethylammonium bromide/pentanol/tetradecane/water 17.5/35/12.5/35 didodecyldimethylammonium bromide/cyclohexane/water 40.2/58.2/1.6 didodecyldimethylammonium bromide/cyclohexane/water 36/54/10 didodecyldimethylammonium bromide/cyclohexene/water 40.2/58.2/1.6 didodecyldimethylammonium bromide/cyclohexene/water 36/54/10 Table 2. Results of Preparative Electrolysesa
microemulsion (see Table 1)
organic phase
% water
mmol of product
current efficiency
comments
4 5 2 3 4 4
cyclohexene cyclohexene cyclohexane cyclohexane cyclohexene cyclohexene
1.6 10 1.6 10 1.6 1.6
0.06 0.18 0 0.006 0.012 0.02
78.3 18.8 0 9.5 32.2 42.3
0.12 M cyclohexene 0.12 M cyclohexene no Chloroform no TBAB
a All electrolyses were performed in divided cells (10 mL) with KBr (1 M) in the counterelectrode compartment. Electrodes were cut from carbon cloth (2 × 2 cm2). Reaction mixtures consisted of microemulsion, dibromodichloromethane (0.06 M), chloroform (0.06 M), and tetrabutylammonium bromide (0.006 M) unless otherwise stated. Electrolyses were performed at -1 V vs SCE for 3 h.
in microemulsion 3. Voltammetric curves were independent of the presence of either chloroform or tetrabutylammonium bromide. A series of preparative electrolyses was run with dibromodichloromethane as the source of dichlorocarbene (Table 2). Unless otherwise stated, the reaction mixture included dibromodichloromethane (0.06 M), tetrabutylammonium bromide (0.006 M), and chloroform (0.06 M). Tetrabutylammonium bromide was added as a phasetransfer catalyst since it is smaller and would be expected to move more easily across the oil/water interface than didodecyldimethylammonium bromide which probably remains bound to these interfaces.8 Chloroform has previously been found to improve yields in electrochemical reduction of carbon tetrahalides9 where it acts as a proton donor and also generates further perhalo anions in a catalytic fashion. Since this mechanism does not pertain in our system, the function of chloroform is less well understood and requires further investigation. For the reaction conditions stated herein, chloroform alone was not electroreduced to form carbene-derived products. The best current efficiencies were obtained when cyclohexene, the trapping agent, was present in vast excess (Table 2). This was achieved by its use as the oil phase of the microemulsion. Where cyclohexene was in lower concentration, yields and current efficiencies were less impressive. The best current efficiencies were obtained in microemulsions of lower water concentration. This suggests that under these conditions, less current is dissipated in side reactions such as proton reduction and also that, when formed, the carbene intermediate is likely to react with anything other than the desired olefin target. It has been seen previously that highly reactive intermediates can be stabilized in microemulsions of low water concentration.10 Where either chloroform or tetrabutylammonium bromide was omitted from the reaction mixture, current efficiency dropped, and the best results were found when both were used. Where cyclohexene was not in massive excess (i.e., when it was not the organic phase of the microemulsion), the chemical yield with respect to cyclohexene was only 12% (Table 2). Although TBAB is not necessary for the production of the carbene product, it leads to a 2-fold increase in current efficiency. This suggests that when TBAB is not present, the large concentration of didodecylammonium bromide (8) Evans, D. F.; Mitchell, D. J.; Ninham, B. W. J. Phys. Chem. 1986, 90, 2817. (9) Shono, T.; Kise, N. J.; Suzumoto, T. J. Org. Chem. 1985, 50, 2527. (10) Iwunze, M. O.; Rusling, J. F. J. Electroanal. Chem. 1991, 303, 267.
Scheme 1. Suggested Pathway for Electrochemical Carbene Phase-Transfer Reactiona
a Key: Q+ ) quaternary ammonium; int ) interface; org ) organic phase.
can effect some phase transfer but, due to its bulk and low solubility in the microemulsion phases, it is inferior as a phase-transfer catalyst. We suggest Scheme 1 as a possible pathway for this procedure. After formation at the electrode (eq 3), dichlorobromomethane anion is transferred across the interface by association with a quaternary ammonium ion (eq 4). When in the organic phase it loses bromide to form the very reactive carbene (eq 5) which reacts with the double bond of cyclohexene to produce 7,7-dichlorobicycloheptane (eq 6). Our results demonstrate the feasibility of electrochemical phase-transfer catalysis in microemulsions. The method does not require the use of strong base to generate dichlorocarbene as in conventional phase-transfer catalysis. Thus, it may have applications in the cyclopropanation of base-sensitive compounds. The incorporation of the olefin as a microemulsion constituent led to improved current efficiencies and may be of industrial interest since commercial electrochemical processes such as the cathodic hydrodimerization of acrylonitrile to adiponitrile utilize similar mixtures of quaternary ammonium salts and mixed aqueous-organic solvents.11 Similarly to the case of adiponitrile production, product could be removed and reactant added in a constant-feed stream. We continue to investigate this and other potential applications of microemulsions and intend to extend the scope of this synthetic methodology to incorporate other types of phase-transfer reaction. Acknowledgment. We thank the NSF (Grant No. CTS-9632391) for financial support. Thanks to Dr. Eamonn Coyne for relevant literature. LA9907836 (11) Pletcher, D.; Walsh, F. C. Industrial Electrochemistry, 2nd ed.; Blackie Academic: London, 1993; pp 298-311.
