Electrochemical Reduction of Beta-Diketones in Dimethylsulfoxide Raymond C. Buchta and Dennis H. Evans Department of Chemistry, UniGersity of Wisconsin, Madison, Wis. 53706 Electrochem'ical studies of the reduction of 1,3diphenyl-1,3-propanedione (DBMH) have been carried out i n dimethylsulfoxide (DMSO) as solvent. Controlled potential coulometry at a potential on the f i r s t polarographic wave indicated an n-value of 0.55. The products were found to be t h e enolate of DBMH and the pinacol, 1,4-dibenzoyl-2,3diphenyl-2,3-butanediol. A reaction scheme is proposed for this process. When electrolysis was continued after the n = 0.5 process, it was found that the current increased, passed through a maximum, then decreased to the residual value again. The n-value was 0.24 with respect to DBMH. This unusual behavior was caused by an electrolytic autocatalytic decomposition of the pinacol i n a base-catalyzed reverse aldol condensation to benzil and acetophenone. The current was caused by the reduction of the benzil to its radical anion, the latter being the base which catalyzes the decomposition of the pinacol. The presence of benzil radical anion was confirmed by ESR spectrometry. A simplified theoretical expression for the controlled potential current-time curve with electrolytic autocatalysis was developed and it was found t o agree satisfactorily with the experimental data. Other p-diketones were found to behave i n an analogous fashion and ESR data for radicals produced by decomposition of the pinacols are given.
THEPOLAROGRAPHIC characteristics of many 8-diketones have been reported for protic solvents, usually ethanol-water. One of the few 8-diketones whose electrochemistry has been studied more completely is 1,3-diphenyl-1,3-propanedione (dibenzoylmethane, DBMH) (1). It was found that DBMH could be reduced in 50% ethanol-water to three products, depending upon the pH of the solution and the electrode potential. The one-electron reduction product at the first polarographic wave was the pinacol, 1,4-dibenzoyl-2,3diphenyl-2,3-butanediol. This communication reports the results of studies of the electrochemical reduction of DBMH and other P-diketones in dimethylsulfoxide (DMSO) as solvent. The enolate of DBMH has been reduced by chemical and electrochemical means and the visible and ESR spectra of the resulting dianion radical have been reported ( 2 , 3). In the present study the neutral enol form was studied. The acidity of this species resulted in more complex electrochemical behavior involving the formation of radicals produced from the starting material through several reactions subsequent to the initial charge transfer. EXPERIMENTAL
Reagents. The DMSO (J. T. Baker Reagent) was dried with calcium hydride and distilled at reduced pressure just before use. Tetrabutylammonium perchlorate (TBAP) (Matheson) was used as supporting electrolyte. A 0.1M solution of this material in the distilled DMSO produced a polarogram which was free of impurity waves at cathodic potentials up to about -2.7 volts GS. SCE (aq). Airco prepurified nitrogen was used for solution deaeration. (1) D. H. Evans and E. C . Woodbury, J . Org. Chem., 32, 2158 (1967). (2) N. L. Bauld, J . Amer. Chem. Soc., 86, 2305 (1964). (3) N. L. Bauld and M. S. Brown, ibid., 89, 5413 (1967).
TOP
I" H
SIDE
Figure 1. PolarograpPic cell The various P-diketones and other materials studied were purified if required by recrystallization until constant melting points in agreement with literature values were obtained. These substances were obtained from Eastman (White Label) 1,3-bis(p-methoxyphenyl)(1,3-diphenyl-l,3-propanedione, 1,3-propanedione, 1,3-bis(p-fluorophenyl)-l,3-propanedione, l-phenyl-l,3-butanedione, benzil, acetophenone) and Pfaltz and Bauer, Flushing, N. Y . ( p , p '-dimethoxybenzil). Apparatus and Procedure. The polarograph was a Sargent Model XV equipped with a Sargent IR Compensator. The polarographic cell is depicted in Figure 1 . This jacketed cell is ideally suited for sample volumes of about 10 cc. The top of the cell allows for the introduction of the counter electrode, reference probe, nitrogen deaeration tube, and a DME, the latter being in the center. The counter electrode was a spiral of platinum wire immersed in the test solution, hence it was not separated from the working electrode compartment. The amount of product formed at the counter electrode during a polarogram was so small that its presence did not affect the subsequent polarograms. The reference electrode was a commercial SCE with either a sleeve or asbestos fiber junction. The SCE was immersed in a supporting electrolyte solution in the bridge tube which made contact with the test solution by means of a crackedglass tip which was positioned within 1 cm of the DME. Both the DME and the reference electrode bridge tube were held in place by Teflon 14/20 T adapters with O-ring seals (Kontes Glass Co., #K-17980). These were particularly advantageous because they permitted vertical and angular adjustment of the electrodes to obtain the optimum configuration. The potentiostat for controlled potential electrolysis was a Wenking Model 61RS. The cell was a 150-cc cylindrical glass cell with five glass joints in the top. A central 24/40 joint VOL. 40, NO. 14, DECEMBER 1968
0
2181
8
Table I. Polarographic Data Compound
5
CURRENT ,JJa
-1.38 -1.69 -2.25
1,4-Dibenzoyl-2,3-diphenyl-2,3-butanediol (DBMH pinacol) Enolate of DBMH Benzil
-1.70 -1.95 -2.25
Acetophenone 1,3-Bis(p-rnethoxyphenyl)-l,3-propanedione
p,p’-Dimethoxybenzil 1,3-Bis(p-fluorophenpl)1 3-propanedione
i
~
1-Phenyl-¶,3-butanedione 0. I
- 1.5
- 2.0
- 2.9
VOLTS vs. S.C.E’. Figure 2. Polarogram of DBMH
received the counter electrode compartment which was a cylindrical 14-mm tube terminating in a IO-mm mediumporosity fritted-glass disk about 2 cm above the mercury p301 working electrode. Three peripheral 14/20 T joints provided access for the nitrogen deaeration tube, the reference electrode bridge tube (similar to that used in the polarographic cell), and a DME (or thermometer). The latter two were also inserted through the Teflon adapters. The fifth joint (10130 T) was for nitrogen escape and sample introduction. The counter electrode was a spiral of platinum wire. Both the test solution and the counter electrode compartment were continually deaerated during electrolysis. The solution was stirred with a magnetic stirrer. Electrical connection to the working electrode was made by means of a platinum wire sealed through the wall of the cell. The electrolysis current was recorded on a Sargent Model SR recorder. Integrals were obtained with the aid of a planimeter (Gelman Model 39321). The electron spin resonance (ESR) spectrometer was a Varian E-3. The Varian electrolytic cell was used for the internal generation of radicals. A Heath transistorized, variable dc power supply (EUW-17) and a microammeter were used with the electrolytic cell; a two-electrode system was employed because exact control of the reduction potential was not found necessary. To obtain samples of somewhat stable radicals produced during an electrolysis outside the ESR cavity--i.e., by the external generation method-the electrolysis cell was placed in a glove bag filled with nitrogen, and capillary tubes filled by syringe were sealed in the glove bag to avo$ oxygen attack. The tetrabutylammonium enolate of DMBH was prepared by reacting in 95% ethanol stoichiometric amounts of the DBMH and tetrabutylammonium hydroxide. The resulting yellow solution was evaporated to dryness leaving a light yellow solid which was washed and recrystallized from diethyl ether. Anal. Calcd. for C31H47N02: C, 79.95; H, 10.17; N, 3.01. Found: C , 80.01; H , 10.24; N, 3.04. The pinacol of DBMH, 1,4-dibenzoyl-2,3-diphenyl-2,3-butanediol, was prepared as described previously ( I ) . Gas chromatography experiments were performed using an F & M Model 720 gas chromatograph incorporating a column of 1 0 - 1 5 z silicone gum rubber on Analabs ABS. 2
0
ANALYTICAL CHEMISTRY
Ei/P
1,3-Diphenyl-1,3-propanedione(DBMH)
a
All potentials
tis.
-1.04 -1.76 -1.94 -1.56 -1.81 -2.5 -1.20 -1.96 -1.41 -1.73 -2.3 -1.54 -2.41 -2.7 -1.10 -2.16
aq. SCE, volts.
RESULTS AND DISCUSSION
Polarogeaphy. The polarography of the various substances was studied briefly in order to establish the appropriate potentials for controlled potential coulometry and to gain some insight into the probable course of the reactions, A polarogram for the reduction of 2 X 10-aM DBMH in 0.1M TBAP in DMSO is given in Figure 2. The current on the plateau of the first wave is not proportional to the concentration of DBMH, being relatively lower at high concentrations. The current is also not proportional to h1’2,being relatively lower at low I? (long drop times). These data are indicative of processes other than diffusion limiting the current and suggest that coupled chemical reactions are involved. The plot of log [(id - i)/i] us. E for the first wave had a slope of 70 mV. The small wave at - 1.69 volts DS. SCE appears at the same potential as the first wave for the reduction of DBMH pinacol and could be caused by its reduction or the reduction o f a similar substance formed in the first step. This wave is relatively smaller at low concentrations of DBMH. The wave at -2.25 volts appears at the same potential as the wave for the enolate of DBMH. It is also relatively smaller at low concentrations of DBMH. These data suggest that DBMH is reduced at the potential of the first wave to a product which may give DBMH pinacol and enolate by further chemical reaction. In Table I are summarized the polarographic data for the various starting materials and suspected products and intermediates. Controlled Potential Coulometry. The current-time curve for the reduction of 50 mg of DBMH at -1.50 volts us. SCE is shown in Figure 3. In the first part of the electrolysis the current decreases from its initial value to a minimum in about 40 minutes. Various shades of green color develop in solution though the final color is yellow. The integrals of the first part (to the minimum) of the current-time curves of a series of electrolyses like that of Figure 3 gave an average n-value of 0.55. This nonintegral n-value may be explained most efficiently
2ol
E=-1.50~VS. SCE. 2 m M DBMH 0.1M TBAP i n DMSO
\
CURRENT]
ma io
Figure 3. Controlled potential reduction of DBMH
Figure 4. ESR spectrum of benzil radical anion obtained by electrolysis of DBMH
by the following overall reaction scheme in which DBMH is written in the enol form which is the predominant form of P-diketones ( 4 ) .
evident when it was discovered that the only paramagnetic material observed by ESR spectrometry with internal generation was the benzil radical anion.
OH
0
I
II
4 CGH~-C=CH-C-C~H~
+ 2e
OH
0
II
1
II 5
I
C~H~--C-CH~-C-CGHS
I
II
CGH~-C-C-C~H~ f e
C6H ~-C--CH~-C-CGH
OH
0 0-
0 0 --t
+
Jl
0 0-
0
I
11
2 CGH~-C=CH-C-CGH~
(1)
Though the mechanism for this process is not known, it would seem most probable that DBMH is first reduced to its radical anion which in turn is protonated by another molecule of DBMH leaving a neutral radical (which dimerizes to DBMH pinacol) and the enolate. Therefore, the final electrolytic products should be the pinacol and enolate. A polarogram was obtained with the electrolysis solution. Waves were observed with half-wave potentials of - 1.69, -1.95, and -2.25 volts. As may be seen in Table I, these are in excellent agreement with the half-wave potentials of DBMH pinacol and the enolate. Furthermore, the yellow color of the electrolysis solution (indicative of the enolate) was completely discharged by the addition of dilute perchloric acid which converted the enolate to DBMH. The resulting solution had a polarographic wave at - 1.38 volts, indicating the presence of DBMH. The half-wave potential for hydrogen ion is - 1.05volts us. SCE in DMSO. Electrolytic Autocatalysis. The most striking characteristic of the electrolysis depicted in Figure 3 is the current-time maximum observed between 50 and 100 minutes. The solution develops a deep blue color during this period, and near the final tailing area it is deep green in color. The currenttime integral corresponds to an average n-value of 0.24 with respect to the initial amount of DBMH. The cause of this unusual current-time curve first became (4) E. S. Gould, “Mechanisms and Structure in Organic Chemistry,” Henry Holt and Co., Inc., New York, N.Y., 1959, p 376.
I 1
e C~H~-C=C-CGH~
(2)
The ESR spectrum of the benzil radical anion obtained by internal generation with a DMSO solution of DBMH is given in Figure 4. The ESR spectrum obtained by electrolysis of benzil or DBMH pinacol was identical to that in Figure 4. Furthermore, the same spectrum was obtained from samples taken from an electrolysis solution during the anomalous current increase (Figure 3). These ESR data strongly suggest that this increase in current is related to a reaction which produces benzil which in turn is reduced at the control potential to benzil radical anion, thus causing the observed current. The observed splitting constants were 1.10 gauss for the para protons, 0.35 gauss for the meta protons, and 0.98 gauss for the ortho protons compared to 1.12, 0.36, and 0.99 gauss, respectively, which have been reported for benzil radical anion in dimethylformamide (5). The reaction which most satisfactorily explains these observations is the decomposition of DBMH pinacol. Noting that DBMH pinacol is the formal aldol condensation product of benzil and two molecules of acetophenone, we write the decomposition reaction as the reverse aldol condensation. OH
0
I
II
C gH ~-C!-CH2-C-C6H
j
I CaH~-c-cH2-c-C6Hj
I
OH
base +
‘1
0 0
C gH ;-C
I1
( 5 ) R. Dehl and G. K. Fraenkel, J. Chem. Phys., 39,1793 (1963). VOL. 40, NO. 14, DECEMBER 1968
2183
Figure 5. ESR spectrum of p,p'-dimethoxybenzilradical anion
The equilibrium for Reaction 3 is expected to favor the products benzil and acetophenone, and the process should be base-catalyzed. Once formed, benzil would be consumed by its reduction to benzil radical anion (cf. Table I). The one-electron reduction of benzil coupled with the stoichiometry of the dimerization and decomposition reactions results in an expected n-value of 0.25 with respect to DBMH for the current-time maximum. The presence of benzil radical anion in the electrolysis mixture is indicated by the blue color and its ESR spectrum. The presence of acetophenone is more difficult to establish because its polarographic wave occurs at an Eli2 near the second wave of benzil-i.e., the reduction wave of benzil radical anion. Nevertheless, it was established that acetophenone is easily obtained by decomposition of DBMH pinacol. A solution of the pinacol in DMSO is stable. However, the addition of 2z sodium hydroxide at room temperature immediately produces the distinctive fragrance of acetophenone. In an attempt to demonstrate more convincingly the identity of acetophenone as a product, it was found that thermal decomposition of DBMH pinacol also produces acetophenone. Injection of a 9 % DMSO solution of the pinacol in a gas chromatograph (injection temperature: 230 "C; m.p. of the pinacol: 201-3 "C; column temperature: 140 "C)produced acetophenone in 75% theoretical yield as ascertained by comparison of peak areas and retention times with those of standard acetophenone solutions. These experiments add credibility to the formation of benzil in the electrolysis solution via Reaction 3. However, the shape of the current-time curve merits further consideration. ne would not expect a current-time curve with a maximum to result from the simple decomposition of an electrolysis product yielding a new electroactive material. The increasing rate of the decomposition as indicated by the rising portion of the maximum suggests a reaction catalyzed by one or more of the reaction products-Le., an autocatalytic reaction. Because reverse aldol condensations me known to be basecatalyzed (@, Reaction 3 was examined with respect to the basicity of its products. It appeared that neither benzil nor acetophenone should catalyze the reaction, so the electrolytic product-the benzil radical anion-suggested itself as the catalyst. Because this catalyst is an electrolytic product, we suggest that the process be called electrolytic autocatalysis. The plausibility of the above interpretation was supported ( 6 ) V. K. LaMer and M. L. Miller, J . Amer. Chem. SOC.,57, 2674 (1935).
4
B
ANALYTICAL CHEMISTRY
Figure 6. ESR spectrum of p,p'-difluorobenzil radical anion
by the following experiment: A solution of DBMH pinacol was prepared and electrolysis was performed at - 1.2 volts us. SCE. Because the Eli* of the pinacol is - 1.69 volts, no reduction occurred. At this point a quantity of benzil equivalent to one fourth the amount of the pinacol present was added to the solution. The electrolysis was continued at - 1.2 volts producing the blue benzil radical anion. After the electrolysis had proceeded for several minutes, the current began to increase and the solution became intensely blue. Finally the current passed through a maximum and decreased to the residual value. The quantity of electricity was equivalent to one electron per molecule of benzil plus one electron per molecule of pinacol-Le., DBMH pinacol decomposed to acetophenone and benzil and the latter was reduced. It should be noted that Reaction 3 did not commence until benzil radical anion was formed. Current-time curves similar to Figure 3 were also obtained for the symmetrical P-diketones 1,3-bis(p-methoxyphenyl)1,3-propanedione and 1,3-bis(p-fluorophenyl)-1,3-propanedione, and the color of both electrolysis solutions during the anomalous current increase again was blue. A combination technique was employed in obtaining the ESR spectra shown in Figures 5 and 6. To obtain these spectra, a solution of the P-diketone was electrolyzed to the first current minimum analogous to that in Figure 3. A sample of the electrolysis solution, which should contain the corresponding pinacol, was transferred to the ESR electrolytic cell and the spectra were obtained by the internal generation method. The spectrum in Figure 5 was obtained from 1,3-bis(pmethoxyphenyl)-l,3-propanedione and is identical to the spectrum obtained from pure p,p'-dimethoxybenzil (anisjl) upon internal generation. The observed coupling constants for anisil were 1.03 gauss for the ortho protons, 0.36 gauss for the meta protons and 0.10 gauss for the methoxy protons. Figure 6 shows the radical obtained from 1,3-bis(p-fluorophenyl)-l,3-propanedione. It is the expected p,p'-difluorobenzil radical with coupling constants of 2.32 gauss for the fluorine nuclei, 0.39 gauss for the meta protons, and 1.02 gauss for the o ~ h protons. o These results indicate that the catalyzed pinacol decomposition is not peculiar to DBMH electrochemistry. A less-pronounced anomalous current increase was obtained for the unsymmetrical P-diketone, l-phenyl-l,3-butanedione. In this case one might wonder how this molecule would dimerize if at all-Le., would two radicals undergo phenyl-phenyl
coupling, methyl-methyl coupling, or mixed phenyl-methyl coupling? The above combination electrolysis technique was employed and two radicals were detected: 1-phenyl-1,2propanedione and acetophenone radical anions. The former indicates that some mixed coupling occurs. Further research is being carried out on this system to discover whether any symmetrical coupling occurs. The observed coupling constants for l-phenyl-l,2-propanedioneare 3.35 gauss for the methyl protons, 1.81 gauss for the para proton, 1.55 gauss for the ortho protons, and 0.51 gauss for the meta protons which are quite similar to reported values (7). The acetophenone radical apparently results from the pinacol decomposition and the observed splitting constants are 6.93 gauss for the methyl protons and 4.28, 1.12, 6.52, 0.90, and 3.74 gauss for the ring protons in good agreement with reported values (8). Electrolytic Autocatalysis. THEORYAND RESULTSFOR DBMH PINACOL DECOMPOSITION. Consider an electrochemical reaction scheme of the type
A
+ nle
01
B
BSC+D D + nze 2 E
(4) (5) (6)
where, in the case of DBMH, A is the @-diketone, B is the dimeric pinacol, C is acetophenone, D is benzil, and E is the benzil radical anion. The 6's are the mass transfer rate constants and k is the decomposition rate constant. We assume that the reverse of Reaction 5 is very slow and that the potential of the electrode is in a region where the oxidation of B and/or E may be neglected and C is not electroactive. The rate of Reaction 5 is considered to be a function of both CBand CE. (7) The case in which b = 1 and e = 0 has been solved (9) but it pertains to the uncatalyzed decomposition of B. We shall consider the case in which b = 0 and e = 1-i.e., the rate being zero order with respect to substrate, first order with respect to catalyst. In addition to Equation 7 we have the rate equations
(9) Solving Equation 8 for CDand differentiating with respect to time we have
which on substitution with Equation 8 into 9 gives upon rearrangement d2eE dt
+ Pz dcE - - PzkCE = 0 dt
Equation 11 is a homogeneous linear differential equation which when solved by means of its auxiliary equation gives (7) 6.A. Russell, E. T. Strom, E. R. Talaty, and S. A. Weiner, J . Amer. Chem. Soc., 88,1998 (1966). (8) N. Steinberger and G. K. Fraenkel, J. Chem. Phys., 40, 723 (1964). (9) R. I. Gelb and L. Meites, J . Phys. Chem., 68, 630 (1964).
0.90t
LOG i ( i inma.)
0.40
-0. I 0-
86
70
9'0
MINUTES
Figure 7. Plot of data from Figure 3 according to Equations 15 and 16 C E = Xexp[(Sl
+ &)tI + Yexp[(S1 - SJt!
(12)
+
where SI = -042 and S2 = (@z2 4@2k)1/2/2. Initially no E is present so CE = 0 at t = 0 and thus X = - Y. Rearrangement of Equation 12 gives CE
=
{Xexp[(Sl
+ ~ 2 > t 1[I] - exp(-2~~t)1
(13)
