Electrochemical Reduction of a,@=UnsaturatedKetones Joseph P. Zimmer, Jeffrey A. Richards, James C. Turner, and Dennis H. Evans Department of' Chemistry, University of Wisconsin, Madison, Wis. 53706
The electrochemical reduction of several aryl a,punsaturated ketones, 1, has been studied at mercury cathodes by the techniques of polarography, controlled potential coulometry, and cyclic voltammetry. Conditions have been established under which the dimer, 2, formed by coupling at the p carbon atoms is produced in good yield. Suitable media for the reaction include pH 5 buffer in 50% ethanol-water, tetra-n-butylammonium perchlorate (TBAP) in dim methylsulfoxide (DMSO) with added lithium perchlorate, and TBAP in DMSO with 6% water. Electrolysis in anhydrous DMSO with TBAP as electrolyte i s complicated by side reactions, probably polymerization. The results of cyclic voltammetric studies are consistent with a mechanism involving reversible reduction of the a,p-unsaturated ketone to its radical anion followed by irreversible dimerization.
THEELECTR.OCHEMICALREDUCTION of a,&unsaturated ketones can lead to a variety of products. The ketone function may be reduced to the corresponding alcohol and/or saturation of the carbo?-carbon double bond may occur. In addition, various coupling reactions involving the formation of new carbon-carbon bonds must be considered. Three symmetrical dimers are possible: p-p dimer (formed by coupling at the /3 carbon atoms), a-adimer, and the pinacol formed by coupling at the carbonyl carbon. Nonsymmetrical coupling (e.g., a-p dimer) is also a possibility and many unsaturated ketones are substituted in such a way that some of the dimers will be diastereoisomeric. This rich variety of chemical properties is suggestive of the interesting electrochemical behavior of this class of compounds. Some of the possibilities outlined above were realized in early electrochemical studies (1) carried out utilizing ethanolwater as solvent. For example, two representative enones, 1, R = CH3 and R = C6Hs, were shown to be reduced to the p-p dimer 2 or the saturated ketone 3 depending on solution pH and potential of the mercury cathode. H C6H,CHCHzCOR \ I C6HjCHtCHzCOR /c=c\ CnHsCHCHKOR 3
Each of these products is the result of a multistep electrode reaction. For example, 2 requires the involverr.ent of two molecules of enone, two electrons, and two protons in its formation. The overall reactions are extremely rapid in a solvent system of high proton availability like ethanol-water. Consequently, little has been learned of the details of the processes. The reactions are much slower when an aprotic solvent is used. The work of Wawzonek and Gundersen (2), who used N,N-dimethylformamide (DMF) as solvent, showed that radical anions were intermediates in the reduction of 1, R = C H 3 and R = C6H5, and that dimer 2 could be produced under the proper conditions. Simonet (3-5) studied the (1) R.Pasternak, Helc. Chim. Acta, 31, 753 (1948). (2) S. Wawzonek and A. Gundersen, J . Electrochem. Soc., 111, 325 (1964). (3) J. Sirnonet, C. R . Acad. Sci., Paris, Ser. C , 263, 1546 (1966). (4) ]bid., 264. 1962 (1967). (5) Ibid., 267, 1548 (1968).
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ANALYTICAL CHEMISTRY, VOL. 43, NO. 8, JULY 1971
reduction in DMF both with and without proton donors added to the solution. An undesirable effect of carrying out the reductions in aprotic solvents is that polymerization of the starting material can become a serious side reaction under these conditions. The reductions of 1, R = CH3 and R = CeH5 have also been studied as examples of the general process of electrolytic reductive coupling (6, 7). In the present investigation, the reduction of various a,punsaturated ketones was studied in dimethylsulfoxide (DMSO) as solvent. Experimental conditions which permit reduction to the p-p dimers 2 without side reactions were established. The technique of cyclic voltammetry was used to characterize the kinetics of the coupling process. EXPERIMENTAL
Dimethylsulfoxide (J. T. Baker Reagent) was stirred over calcium hydride for at least eight hours and distilled under reduced pressure just before use. Tetra-nbutylammonium perchlorate, TBAP (Matheson), was vacuum dried before use. Occasional low quality batches were recrystallized from ethyl acetate or acetone-water followed by vacuum drying (mp 213-14 "C). Anhydrous lithium perchlorate (G. F. Smith) and sodium perchlorate (Baker) were vacuum dried at 120 "C for eight hours. Prepurified nitrogen (Airco) was used for solution deaeration. The trans isomers of the various a,p-unsaturated ketones were prepared by condensation of benzaldehyde with the appropriate ketone followed by purification by crystallization. Melting points were in agreement with literature values. Apparatus. Instrumentation, cells, electrodes, and procedures for polarography and controlled potential coulometry have been described (8). Some of the coulometric studies were performed with a potentiostat based on a Kepco Model KS-120-1M programmable power supply (9). Unless otherwise specified, an aqueous saturated calomel electrode was used as reference. Instrumentation and procedures for cyclic voltammetry have been described elsewhere (IO). The hanging mercury drop working electrode was a microburet-type (Brinkmann Instruments, Inc.) or an electrode utilizing a mercury-coated platinum wire support (11). In the latter case, mercury drop electrodes of known size were prepared using the microburet electrode and were later transferred to the support. A positive feedback circuit (12) was added to the instrument to provide correction of distortion due to uncompensated resistance. When a mercury drop electrode of 0.035 cmz area in 0.2M TBAP in DMSO was used, it was necessary to compensate for about 500 ohms of resistance when the end of the reference electrode probe was located Reagents.
( 6 ) M. M. Baizer and J. D. Anderson, J. Org. Chem., 30, 3138 (1965). (7) J. P. Petrovich, M. M. Baizer, and M. R. Ort, J . Electrochem. SOC.,116, 743 (1969). (8) R. C. Buchta and D. H. Evans, ANAL.CHEM., 40, 2181 (1968). (9) D. R. Tallant and D. H. Evans, University of Wisconsin, Madison, unpublished results, 1968. (10) R. C. Buchta and D. H. Evans, J . Electrochem. SOC.,117, 1494 (1970). (11) J. W. Ross, R. D. DeMars, and I. Shain, ANAL. CHEM.,28, 1768 (1956). (12) E. R. Brown, D. E. Smith, and G. L. Booman, ibid., 40, 1411 (1968).
a t a distance from the working electrode greater than ten electrode radii, A d q u a t e compensation could be achieved for scan rates up to about fifty volts/sec. Electrolysis Products. The meso-dimer 2, R = CHs, was prepared by controlled potential reduction of 0.23 g 1, R = CHI, a t a mercury pool cathode in p H 5 acetate buffer in 50 % ethanol-water. The control potential was -1.1 volt us. SCE and 0.99 equivalent of electricity per mole of 1,R = CH3, was required to complete the electrolysis. The product precipitated during the electrolysis. Recrystallization from ethyl ether produced colorless needles, mp 161-162 "C [lit. (13) 163.5-164 "C]. N M R (CDClJ 6 7.18 (lOH, s, aryl), 6 3.3 (2H, m, methine), 6 2.5 (4H, m, methylene) and 6 1.80 (6H, s, methyl). The reduction of 1.01 grams of 1, R = tert-C4Hg, was performed using a mercury pool cathode and 0.5M sodium acetate, 0.5M acetic acid in 50% ethanol-water. The control potential was -1.15 volts us. SCE. Total electrolysis time was three and a half hours. The products precipitated as a white powder (0.86 gram), mp 170-177 "C. The electrolysis required 0.96 equivalents of electricity per mole of 1, R = jerr-C4Hg. Crystallization of the crude product from hexane produced 40z meso 2, R = tert-C4Hg, mp 212-213 "C [lit. (14) 212212.5 "C], IR (CHClJ 1700 cm-l, N M R (CDCIJ 6 7.30 (lOH, m, aryl), 6 2.0-3.8 (ca. 6H, m, methylene and methine) and 6 0.73 (18H, s, tert-C4Hg). Anal. Calcd for C&&: C, 82.49; H , 9.05. Found: C, 82.47; H, 9.16. Evaporation of the hexane filtrate produced a white powder which after two crystallizations from pentane afforded 30 mg of racemic 2, R = tert-CdH9, mp 145-147 "C [lit. (14) 147-148 "C]. Ana/. Calcd for C26H3402: C , 82.49; H , 9.05. Found: C, 82.24; H , 9.01. Reduction of 1.01 grams of 1, R = tert-C4Hg,in 0.1M lithium perchlorate in DMSO-water (93:7 by volume) was performed at a mercury pool cathode at - 1.7 volts us. SCE. Complete electrolysis required 0.96 equivalent per mole. The amber electrolysis solution was treated with one mole of perchloric acid per mole of 1, R = tert-CaHg. The color was discharged and a white powder (0.8 gram) precipitated, mp 144-145 "C. Crystallization of the crude product from ethanol afforded 75 racemic 2, R = tert-C4Hs,mp 146-147 "C, N M R (CDC13) 6 6.8-7.4 (lOH, m, aryl), 6 2.8-3.8 (6H, m, methylene and methine) and 6 1.00 (18H, s, tert-C4H9). N o meso 2, R = tert-C4Hg,could be isolated. Reduction of 0.50 gram of 1, R = tert-C4Hg, in 0.1M TBAP, 0.075M lithium perchlorate in 100 cc of DMSO was carried out at - 1.7 volts us. SCE. The molar ratio of lithium perchlorate to 1 was 3. Complete electrolysis required 1.05 equivalents per mole. Neutralization with perchloric acid discharged the color of the electrolysis solution but no precipitate formed nor did precipitation occur upon addition of 5 % water. Evaporation of the solution yielded an oil which crystallized on standing overnight. Extraction with hot hexane produced a solution which yielded after slow cooling 0.24 gram of colorless needles of racemic 2, R = tert-C4Hg, mp 146-147 "C.
z
RESULTS AND DISCUSSION
Certain characteristics of the electrochemical reduction of a,B-unsaturated ketones are revealed by polarographic studies. In agreement with previous work ( I ) , it was found that 1, R = CHI, exhibits two polarographic waves in 50% ethanol-water. The first wave represents the one-electron (13) I. K. Traore, B. Furth, and J. Wiemann, C. R. Acad. Sci., Paris, Ser. C , 264, 1079 (1967). (14) H. 0. House, R. W. Giese, K. Kronberger, J. P. Kaplan, and J. F. Simeone, J . Amer. Chem. Soc., 92, 2800 (1970).
reduction producing the dimer 2 (R -- CHJ). I n the p H range 1-8, the half-wave potential of the first wave is a linear function of p H suggesting that the process is proton-assisted. The half-wave potentials for eleven values of the p H define a line whose equation is = -0.62-0.078 p H volt us. SCE (as) with all points falling within 0.02 volt of the line. The height of the second wave is approximtely equal to that of the first and its half-wave potential [-1.42 0.04 volts us. SCE (as)] is relatively independent of p H from the point where it first becomes discernible at p H 4 to the point where it merges with the first wave (pH 8). The product at the second wave is mainly the two-electron product, the saturated ketone 3 (R = CH3). The assignment of reaction products for the two waves is supported by controlled potential coulometry and isolation of the products after large scale electrolysis. For example, at p H 1.3 a coulometric n-value of 1.15 was obtained upon electrolysis at a potential o n the diffusion plateau of the first wave and the dimer 2 was isolated (1). In the present investigation, an average n-value of 1.01 f 0.03 was obtained for three electrolyses at -1.15 volts us. SCE (aq) in p H 5.1 acetate buffer (50 % ethanol-water) for ketone concentrations ranging from 5 x lO-4M to 1 x 10-2M. The dimer 2 (R = CHI) was obtained in good yield. Electrolysis at the second wave has been reported to give a coulometric n-value of 1.55 and both dimer 2 (R = CH3) and monomer 3 (R = CH,) as products ( 1 ) . This behavior has been verified in the present study. Coulometric studies of the reduction of 1 (R = CH,) at a potential [- 1.55 volts us. SCE (aq)] o n the diffusion plateau of the second wave in the p H 5.1 acetate bufter produced concentration-dependent ketone to n-values. They ranged from 1.58 at 1.0 X 1OW2M 1.98 at 5.1 x 10-4M ketone. This suggests that relatively more dimer is formed at high concentrations where second order processes are favored. Assuming only 2 and 3 (R = CH,) are produced, the n-value of 1.58 obtained at the highest concentration corresponds to a mole fraction of the dimer of 0.42 in the product mixture. Significant amounts of dimer were isolated from the final electrolysis solutions. As mentioned above, the pH-dependence of the first halfwave potential and the invariance of the second causes the two waves to merge into a single two-electron wave in the pH range of about 9-10. Above p H 10, the wave again divides into two waves of approximately equal height with the half-wave potential of the first being pH-independent [-1.36 f 0.04 volts us. SCE (aq)] and the second becoming more negative as the p H is increased. Controlled potential coulometry using a dilute sodium hydroxide solution in 50 ethanol-water (pH 12.5) suggests that the major product at the first wave [-1.42 volts us. SCE (as)] is the dimer (n = 1.30) while the product at the second wave [-1.76 volts us. SCE (as)] is probably the reduced monomer 3 (n = 2.01). Of particular significance to the present investigation is the fact that the half-wave potential of the first wave is independent of p H in the range 10-13 where the activity o f protons has become too low for them to be effective in the potential-determining reaction. The process can now be thought of as direct electron trarisfei to the ketone producing a radical anion which undergoes protocatiori by the solvent or buffer constituents followed by dimerization and other chemical and electrochemical reactions. The overall reactions are still rather rapid. Only a small anodic wave for the oxidation of the radical is found in a cyclic voltaminogram of lOP3M1 (R = CH,) in 0.1M NaOH in 507; ethanol-water at a scan rate of 100 volts per second.
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ANALYTICAL CHEMISTRY, VOL. 43, NO. 8, JULY 1971
1001.
Table I. Controlled Potential Coulometry in DMSO" Run
Compound
1 2 3
1 (R 1 (R 1 (R
4 5 6
1 (R CH3) 1 (R CHI) 1 (R = CHI) 1 (R = CH3) 1 (R = CH3) 1 (R = CsHJ 1 (R = CsHj)
7 8 9 10
11 12 13 14 15 16
17
1 (R 1 (R 1 (R 1 (R
1 (R 1 (R 1 (R
= =
= = = = = = =
CHI)
CH,) CH3)
C&)
CsHj) fert-C4H9) tert-CrH9) te;t-C4H9) tert-C4H9) tert-C4Ho)
Medium O.1MTBAP 0 . 1 M TBAP 0 . IMTBAP
LiC104
Concentration, M
n
m*
1.0 x 10-8
0.27 0.31 0.77
7.4 6.4 2.6
x 10-3 1 . 2 x 10-2
0.89 0.43 1.05 0.96 1.01 0.54 0.85
2.2 4.6 1.9 2.1
2.2
+ 4mM
0 . 1 M LiC104 0 , l M LiC10, pH 5 , l acetate pH 5.1 acetate pH 5.1 acetate 0 . 1 M TBAP 0.1MTBAP
1 . 0 x 10-8 1 . 4 x 10-3 1.3
bufferc bufferc bufTerc
+ 4mM
LiC104 0 . 1 M LiC104 pH 5 . 1 acetate bufferc 0 1MTBAP 0 . IMTBAP 0 . 1 M TBAP 0 . 1 M TBAP 0.1M TBAP
+ 3mM 0 . 1 M TBAP + 6mM LiCI04 0 . 1 M TBAP + 4mM NaCIOl 0.lMTBAP + llmM NaCI04 O.1MTBAP + 0 . 6 z H 0 . 1 M TBAP + 6 z
5.1 x 2.1 x 9.9 x 1.5 x 1.1 x
lo-' 10-3 10-3
1.2 x 2.5 x 1.6 x 1.0 x 4.3 x 9.8 x
x
10-3 10-2 10-3 10-3 10-3 10-4 10-4
0.93 0.99 0.56 0.65
0.87 0.99
3.6 3.1 2.9 2.3 2.0
x
10-3
0.99
2.0
2.7 x 10-4
0.84
2.4
1 . 1 x 10-3
0.83
2.4
1 . 2 x 10-3
0.78
2.6
10-3
0.98
2.0
5 . 3 x 10-4 1 . 9 x 10-3 6 . 4 x 10-3
1.05 0.96 1.02
2.1
1.0
LiC104
18
1 (R
=
tert-C4Hg)
19
1 (R
=
tert-C4Ho)
20
1 (R
=
tert-C4He)
21
1 (R
=
tert-CdH9)
2.1
1 0
22 23 24 25
1 (R = terf-CaHo) 1 (R 1 (R 1 (R
= = =
terf-CaHe) fert-C4Hg) tert-GHg)
H 10
pH 5 , 6 acetate bufferc pH 5.6 acetate bufferc pH 5 , 6 acetate bufferc
1.1
x
10-3
0.70
2.0 3.7 2.4
2.0
1.9
2.0
Cathode potential controlled at a value on the diffusion plateau of the first wave. * Calculated average number of monomer units in polymeric products (see text). 50% ethanol-water solvent. a
Thus in alkaline solution, the behavior of the a,p-unsaturated ketone begins to resemble what is expected in a dipolar aprotic solvent, ciz., the primary electrode reaction is the one-electron reversible reduction to the radical anion. Nevertheless, the lifetime of the radical anion in alkaline ethanol-water solution is exceedingly short. The lifetime of the radical anions may be increased considerably if an anhydrous dipolar aprotic solvent such as acetonitrile, N,N-dimethylformamide (DMF) or DMSO is used. In dry (
