Electrochemical oxidation of some 5-substituted barbituric acids at the pyrolytic graphite electrode

J. Electroanal. Chem., 79 (1977) 391--399

391

© Elsevier Sequoia S.A., Lausanne -- Printed in The Netherlands

ELECTROCHEMICAL OXIDATION OF SOME 5-SUBSTITUTED BARBITURIC ACIDS AT THE PYROLYTIC GRAPHITE ELECTRODE ELECTROCHEMICAL SYNTHESIS OF 5,5'-SUBSTITUTED HYDURILIC ACIDS

S. KATO * and GLENN DRYHURST **

Department of Chemistry, University of Oklahoma, Norman, Okla. 73069 (U.S.A.) (Received 9th July 1976)

ABSTRACT 5-Alkyl substituted barbituric acids are electrochemically oxidized at the pyrolytic graphite electrode by way of a single, major, le voltammetric oxidation peak to form a barbiturate radical. This dimerizes to give the corresponding 5,5'-substituted hydurilic acid. Under conditions of controlled potential electrolysis the reaction proceeds in very high yield and the hydurilic acid derivative may be readily separated and purified.

INTRODUCTION

Recent reports from this laboratory have outlined the routes of electrochemical oxidation of barbituric acid and its 1-alkyl and 1,3-dialkyl derivatives at low pH in the presence of chloride ion [ 1 ] at the pyrolytic graphite electrode (PGE). The electrooxidation of these same compounds in the absence of chloride ion and at higher pH at the PGE has also been investigated [2]. The basic reaction scheme for these molecules involves an initial l e oxidation at the C-5 position to give a barbiturate radical. In the absence of nucleophiles such as chloride ion the radical then dimerizes to give the corresponding hydurilic acid. This, being more readily oxidized than the barbituric acid itself, is immediately further electrooxidized to give a variety of complex products [1,2]. It was suspected that if one of the two hydrogens available at C-5 in the latter barbituric acids was substituted with, for example, an alkyl or aryl group, then it might be possible to form substituted hydurilic acids that are n o t further electrochemically oxidizable. That is, a general m e t h o d could be devised for the electrochemical synthesis of 5,5'-substituted hydurilic acids. Relatively few 5,5'-substituted hydurilic acids have been synthesized chemically [3] and electrochemical syntheses have never before been reported. In this paper, the basic voltammetry and electrode reactions for electrochemical oxidation of 5-substituted barbituric acids, as well as the techniques for electrochemical synthesis df 5,5'-substituted hydurilic acids will be outlined. * Present address: Tokuyama Soda Co. Ltd., 1-1 Mikage-cho, Tokuyama City 745, Japan. ** To whom further correspondence and reprint requests should be directed.

392 EXPERIMENTAL Chemicals Substituted barbituric acids were synthesized according to the sources quoted: 5-ethylbarbituric acid (Fischer and Dilthey [4] ), 5-benzylbarbituric acid (Kast [5] ); 5-ethyl-l-methylbarbituric acid, 1,5-dimethylbarbituric acid, 1,3-dimethyl-5-ethylbarbituric acid and 1,3,5-trimethylbarbituric acid (Cope et al. [6] ). 5,5'-Diethylhydurilic acid and 5,5'-dibenzylhydurilic acid were synthesized by the methods of Aspelund [3]. 5-Ethyl-l-methyldialuric acid was prepared according to Aspelund [ 7 ]. Buffer solutions or other supporting electrolyte solutions were prepared from reagent grade chemicals and, unless otherwise specified, had an ionic strength of 0.5 M. Deaeration of electrolysis solutions was accomplished with water-saturated nitrogen. Apparatus The apparatus and equipment employed for electrochemical, chromatographic and spectral studies have been described extensively elsewhere [1,8,9]. Preparation of pyrolytic graphite electrodes for voltammetric studies and for controlled potential electrolysis and coulometry has been outlined in earlier reports [10, 11]. All potentials are referred to the saturated calomel electrode (SCE) at 25°C. Electrochemical oxidation o f 5-ethylb~rbituric acid in p H 1 chloride buffer 5-Ethylbarbituric acid (500 mg, 3.202 mmol) was dissolved in chloride buffer pH 1.0 (150 ml) and electrolyzed at 1.0 V under nitrogen. A small amount of white precipitate was formed which was removed by filtration (35 mg). The filtrate was freeze-dried, passed through a column of Dowex 50W-X8 (H ÷ form, methanol pre-washed) and eluted with methanol. The first 100 ml of eluent was collected and evaporated to give an off-white solid. This was combined with the initially filtered white solid to give 474 mg of solid material. This was recrystallized from aqueous methanol to give white crystals (217 rag, 0.699 mmol, 43.7%), m.p. 334--335°C. Analysis calculated for C12H14N406 (310.26): C, 46.45%; H, 4.56%; N, 18.06%. Found: C, 46.27%; H, 4.52%, N, 17.89%. I.r. spectrum (KBr pellet, cm -1 ): 3530 (broad), 3300, 3270, 3080, 2960, 2850, 1755--1670 (broad and strong), 1415 (broad), 1350, 1300, 1230, 1140, 1100, 1020, 835, 760 and 650. Mass spectrum (70 eV, 150°C, m/e): 195(25), 167(5), 157(8.5), 156(81.5, 1/2 M+ + 1), 155(100, 1/2 M+), 154(11), 149(5.5), 142(7), 141(80.5), 128(90), 112(28), 98(23), 88(40.5), 86(12.5), 84(13), 70(21), 69(32.5), 68(12.5), 55(35.5), 53(12.5), 39(23), and 27(12). These analytical and spectral results agreed exactly with those observed for authentic 5,5'-diethylhydurilic acid. Electrochemical oxidation o f 5-benzylbarbituric acid in aqueous methanol 5-Benzylbarbituric acid (300 mg, 1.375 mmol) was dissolved in a mixture of 1 M acetic acid (120 ml) and methanol (30 ml) and electrolyzed at 0.9 V under

393 nitrogen. A white precipitate was formed as the electrolysis proceeded. After completion of the electrolysis, the entire product solution was concentrated to about 20 ml under reduced pressure and then stored overnight at 5 ° C when a white solid crystallized (192 rag, 0.442 mmol, 64.3%), m.p. 343--345 ° C(d). Analysis calculated for C22HlsN406 (434.44): C, 60.82%; H, 4.18%; N, 12.90%. Found: C, 60.48%; H, 4.26%; N, 12.90%. I.r. spectrum (KBr pellet, cm -1 ): 3280 (strong), 3240, 3120, 2820, 1770, 1740 (strong), 1700 (strong), 1490, 1410 (strong), 1340 (strong), 1295, 1250, 1190, 1070, 1010, 800, 755, 725 and 690. Mass spectrum (70 eV, 190°C, re~e): 219(6), 218(40), 217(1/2 M+, 31), 216(17), 215(22), 172(21), 146(8), 131(7), 129(7), 128(6) 103(1), 102(8), 92(9.5), 91(100), 78(16), 77(10), 65(8), 51(14) and 54(5). Comparison of these analytical and spectral results with those of authentic 5,5',-dibenzylhydurilic acid revealed them to be identical.

Electrochemical oxidation of 5-ethyl-l-me thylbarbituric acid in pH 1.0 chloride buffer 5-Ethyl-l-methylbarbituric acid (500 mg, 2.938 mmol) was dissolved in pH 1.0 chloride buffer (150 ml) and electrolyzed at 0.80 V under nitrogen. The white precipitate that formed was separated by filtration. This solid was dissolved in refluxing methanol (60 ml) which on cooling gave white crystals (151 mg). The electrolyte and washings were freeze-dried, then dissolved in methanol and passed down a column of Dowex 50W-X8 (H ÷ form, methanol pre-washed) and eluted with methanol (400 ml). The methanol was evaporated and the resultant residue washed with cold water and then ether. The resultant solid was combined with the originally filtered and crystallized solid. The combined amount of white, solid product, 303 mg (0.896 mmol, 61.0%), had a melting point of 293--295°C (d). Analysis calculated for C14H1sN406 (338.35): C, 49.69%; H, 5.37%; N, 16.56%. Found: C, 49.57%; H, 5.44%; N, 16.58%. I.r. spectrum (KBr pellet, cm -1 ): 3260, 3130, 2960, 1710, 1665, 1430, 1355, 1320, 1255, 1185, 1035, 965, 760, 690 and 620. Mass spectrum (70 eV, 150°C, m/e): 171(31), 170(83, 1/2 M+ + 1), 169(1/2 M+, 100), 156(21), 155(89), 142(44), 112(59), 81(45), 70(33), 69(56), 58(41), 56(30), 55(40), 53(33), 41(57) and 39(39). N.m.r. spectrum (d6-DMSO, 5): 0.67 (t, J = 7.5 Hz, 6H, CH2CHa ), 2.28 (q, J = 7.5 Hz, 4H, CHgCHa ), 3.13 (s, 6 H, N--CHa), 12.0 (broad, s, exchangable with D20, 2H, N--H). These analytical and spectral results agreed with those expected for 5,5'-diethyl-l,l-dimethylhydurilic acid. In certain experiments, the filtrate and aqueous washings obtained after separating the second crop of 5,5'-diethyl-l,l'-dimethylhydurilic acid were reduced to a low volume by freeze-drying and then passed down a column of Sephadex G-10 (37 × 2.2 cm) and eluted with water. Following the elution of some additional 5,5'-diethyl-l,l'-dimethylhydurilic acid, a second c o m p o n e n t was eluted. After freeze-drying, this was a white solid which could be crystallized from a small amount of water to give white crystals, m.p. 134--136 ° C. Mass spectrum (70 eV, 145°C, re~e): 186(26, M÷ ), 158(61, M+--Et + H), 129(11.5, M+--CHaNCO), 86(100, M+--C~IaNCO--HNCO). This compound was identified as 5-ethyl-l-methyldialuric acid by comparison of the above results with those for the authentic compound. The maximum yield of 5-ethyl-l-methyldialuric acid obtained in this fashion was ca. 11%.

394

Electrochemical oxidation of 1,5-dimethylbarbituric acid in chloride buffer pH 1.0 1,5-Dimethylbarbituric acid (500 mg, 3.202 mmol) was dissolved in pH 1.0 chloride buffer (150 ml) and electrolyzed at 0.70 V under nitrogen. The white precipitate that formed was separated by filtration and then dissolved in refluxing methanol (200 ml) which on cooling gave white crystals (127 mg). The remaining electrolyte and washings were freeze-dried, dissolved in methanol and then passed down a column of Dowex 50W-X8 (H ÷ form, methanol prewashed) and eluted with methanol (400 ml). The methanol was evaporated, and the resultant residue crystallized from boiling methanol to give white crystals (206 mg). The total yield of this white product was 333 mg (1.073 mmol, 67.0%), m.p. 316--318°C (d). Analysis calculated for C12H14N406 (310.29): C, 46.45%; H, 4.56%; N, 18.06%. Found: C, 46.29%; H, 4.59%; N, 18.20%. I.r. spectrum (KBr pellet, cm -1 ): 3260, 3120, 2950, 1710, 1680, 1435, 1375, 1340, 1260, 1185, 1130, 1080, 1025, 870, 830, 760, 670 and 610. Mass spectrum (70 eV, 150°C, m/e): 310(0.3, M+), 266(0.5), 157(11), 156(100, 1/2 M+ + 1), 155(38, 1/2 M+) and 56(26). N.m.r. spectrum (ds-DMSO, 6): 1.63(s, 6H, C--CH3 ), 3.03(s, 6H, N--CH3 ), 11.6 (broad, s, exchangable with D20, 2H, N--H). These analytical and spectral results were in accord with those expected for 1,1',5,5'-tetramethylhydurilic acid. If the.material obtained after eluting the freeze.dried electrolysis product through a Dowex 50W-X8 column was dissolved in a small amount of water and eluted through a column of Sephadex G10 with water, then in addition to 1,1',5,5'-tetramethylhydurilic adid, a second c o m p o n e n t was eluted. After freeze-drying, this c o m p o n e n t was a white solid. Analysis calculated for C6HsN2Oa (172.14): C, 41.86%; H, 4.68%; N, 16.28%. Found: C, 41.47%; H, 4.76%; N, 16.27%. The i.r. and mass spectra of this compound agreed with those of authentic 1,5-dimethyldialuric acid prepared by oxidation of 1,5dimethylbarbituric acid with hydrogen peroxide [7].

Electrochemical oxidation of 1,3-dimethyl-5-ethylbarbituric acid in pH 1.0 chloride buffer 1,3-Dimethyl-5-ethylbarbituric acid (368 mg, 2.00 mmol) was dissolved in pH 1.0 chloride buffer (150 ml) and electrolyzed at 0.90 V under nitrogen. A white precipitate formed which, upon completion of the electrolysis (ca. 35 h) was separated by filtration and washed with cold water. This solid (248 mg, 0.677 mmol, 67.7%), was recrystallized from methanol to give white crystals, m.p. 157--162 ° C. Analysis calculated for C16H22N406 (366.42): C, 52.44%; H, 6.06%; N, 15.29%. Found: C, 52.35%; H, 5.89%; N, 15.39%. I.r. spectrum (KBr pellet, cm -1 ): 3420 (broad), 2960, 1675 (strong), 1440 (strong), 1365 (strong), 1295, 1180, 1150, 1070 (strong), 970, 850 and 740 (strong). Mass spectrum (70 eV, 135°C, re~e): 367(0.3), 366(0.4), 185(11), 184(96, 1/2 M+ + 1), 183(100, 1/2 M+), 160(11.5), 159(87), 126(35), 112(30), 97(25), 81(28), 69(28), 58(65), 56(37), 53(24), 42(42), 41(76) and 39(34). N.m.r. spectrum (d6-DMSO, 5): 0.67 (t, J = 7.5 Hz, 6H, CH2CH3), 2.27 (q, J= 7.5 Hz, 4H, CH2CH3) , 3.17 (s, 12H, N--CHa). These analytical and spectral results are

395 in agreement with those expected for 5,5'-diethyl-l,l',3,3'-tetramethylhydurilic acid.

Electrochemical oxidation of 1, 3, 5-trimethylbarbituric acid in pH 1.0 chloride buffer 1,3,5-Trimethylbarbituric acid (340 rag, 2.00 mmol) was dissolved in pH 1.0 chloride buffer (150 ml) and electrolyzed at 0.74 V under nitrogen. Upon completion of the electrolysis, the white precipitate present in the solution was filtered, washed with cold water and then recrystallized from methanol (150 ml) to give white crystals (232 rag, 0.686 mmol, 68.6%), m.p. 262--265°C. Analysis calculated for C14HlsN406 (338.35): C, 49.69%; H, 5.37%; N, 16.56%. Found: C, 49.61%; H, 5.31%; N, 16.61%. I.r. spectrum (KBr pellet, cm -1 ): 3400 (broad), 2990, 2940, 1670, 1440, 1375, 1275, 1205, 1150, 1070, 1035, 935, 800, and 740. N.m.r. spectrum (ds-DMSO, 5): 1.57 (s, 6H, C--CH~), 3.05 (s, 12H, N--CH3). Mass spectrum (70 eV, 130°C, m/e): 281(1), 171(12), 170(100, 1/2 M+ + 1), 169 (1/2 M+), 83(26), 58(23), 56(48), 55(40), and 54(24). These analytical and spectral results are in accord with those expected for 1,1',3,3',5,5'hexamethylhydurilic acid. RESULTS AND DISCUSSION

Stability of 5-substituted barbituric acids It was found that many 5-substituted barbituric acids slowly decompose in aqueous solution in the presence of dissolved oxygen. For example, 170 mg (1.00 mmol) of 5-ethyl-l-methylbarbituric acid was dissolved in water and stirred for 30 h at room temperature. The u.v. absorption peak of 5-ethyl-l-methylbarbituric acid (~r,~x = 269 nm in water) slowly decreased with time and finally completely disappeared. Correspondingly, a peak at ~m~x = 222 nm increased as the peak at 269 nm decreased. After 30 h the water was removed by freezedrying and the resulting white residue was characterized as pure 5-ethyl-1methyldialuric acid by means of its melting point, i.r., mass and u.v. spectrum.

Voltammetry The voltammetric oxidation of six, 5-substituted barbituric acid derivatives was studied at the PGE and each compound gave at least one oxidation peak. The variation of the voltammetric peak potential, E~, with pH for each barbituric acid is shown in Fig. 1. The most obvious fact about the Ep vs. pH plots shown in Fig. I is that the peak labelled Ia for each compound is strongly pH dependent up to about pH 4--5 at which point the peak becomes independent of pH. In view of the fact that the first pKa of these 5-substituted barbituric acids is generally about 4.8--5.0 [12], it is clear that at pH values below ca. 5, the neutral barbituric acid is the electrooxidized species, while at pH values greater than the pKa the monoanion is oxidized. It is also clear from the data in Fig. 1 that there is appreciable scatter in the experimental points, and that several compounds exhibit more than one

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voltammetric oxidation peak at certain pH values. The scatter of the points is due primarily to the fact that none of the barbituric acids gave well-formed voltammetric peaks. Rather, rounded, drawn-out peaks were observed on which it was difficult to measure peak potentials. The additional peaks observed for some compounds were observed as ill-defined inflections on the voltammograms. Because of the ill-defined shape of all peaks, sweep rate studies gave very little useful information regarding the processes responsible for peaks more positive than peak I a. However, after controlled potential electrolysis at peak Ia potentials, all peaks were eliminated (vide infra). Accordingly, it was assumed that the peaks observed at more positive potentials than peak Ia were due to various adsorption phenomena or, on occasion, perhaps to certain acid-base equilibria. Cyclic voltammetry of the various 5-substituted barbituric acids at sweep

397

rates up to about 20 V s-1 gave no evidence for reversibility of any of the voltammetric oxidation peaks. At high sweep rates the distortion of the various peaks became extreme.

Coulometry, mass electrolysis and product isolation and identification Coulometry at controlled potential of several 5-substituted barbituric acids at potentials close to the Ep value of peak Ia gave n-values close to 1 (Table 1). After eliminating peak Ia all other peaks also disappeared. The n-values appeared to be essentially independent of concentration. The results shown in Table 1 were obtained generally at pH 1 although similar results were obtained at higher pH. Product isolations were normally somewhat easier at pH 1, hence this medium is utilized extensively in this study. Electrolysis of sufficient amounts of the 5-substituted barbituric acids to allow product separation and identification revealed in all cases that the major product was the corresponding hydurilic acid derivative (Table 2). On occasion small quantities of 5-substituted dialuric acids were isolated and identified (Table 2). However, in view of the fact that air oxidation of many 5-substituted barbituric acid occurs quite rapidly in aqueous solution, it is probable that the 5-substituted dialuric acids are formed via the latter mechanism, perhaps predominantly at the time of initial dissolution of the barbituric acids when buffer solutions normally were air-saturated. There is also the possibility of partial further le electrochemical oxidation of the initial radical species formed by le oxidation of the 5-substituted barbituric acids to give a dialuric acid. However, the experimental n-values gave no real evidence to support this suggestion.

Table 1 E x p e r i m e n t a l n-values o b s e r v e d o n c o n t r o l l e d p o t e n t i a l electrolysis o f 5 - s u b s t i t u e d b a r b i t u r i c acids at p e a k I a a t the P G E Compound

pH

Concentration/ mM

Controlled potential/V

n-Value

5 - E t h y l - b a r b i t u r i c acid

1.0 a

5 - B e n z y l b a r b i t u r i c acid

2.3 b

1 - M e t h y l - 5 - e t h y l b a r b i t u r i c acid

1.0 a

1 , 5 - D i m e t h y l b a r b i t u r i c acid

1.0 a

1.13 21.35 0.98 11.46 1.30 19.60 1.10 21.35

1.00 1.00 0.90 0.90 0.80 0.80 0.70 0.70

1.2 1.2 0.9 1.2 0.7 0.9 1.1 0.9

1,3-Dimethyl-5-ethylbar bituric acid

1.0 a

1 , 3 , 5 - T r i m e t h y l b a r b i t u r i c acid

1.0 a

1.58 13.32 1.33 13.32

0.90 0.90 0.74 0.74

0.88 1.23 1.2 0.9

a Chloride b u f f e r (KC1 + HC1). b 4 parts 1 M HOAc + 1 part methanol.

398 Table 2 Products and their yields obtained on controlled potential electrolysis of 5~ubstituted barbituric acids at peak I a potentials at the PGE Compound

pH

Controlled potential/V

Products (yield %) a

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1.0 b 2.3 c

1.00 0.90

1-Methyl-5-ethylbarbituric acid

1.0 b

0.80

5,5'-Diethylhydurilic acid (44%) 5,5'-Dibenzylhydurilic acid (64.3%) 1,1'-Dimethyl-5,5'-diethylhydurilic acid (61%) 1-Me thyl-5-ethyldialuric acid

1,5-Dimethylbarbituric acid

1.0 b

0.70

1,3-Dimethyl-5-ethylbarbituric acid 1,3,5-Trimethylbarbituric acid

1.0 b

0.90

1.0 b

0.74

(11%) 1,1 ', 5,5'-Tetramethylhydurilic acid (67%) 1,5-Dimethyldialuric acid (13%) 1,1',3,3'-Tetramethyl-5,5'-diethylhydurilic acid (68%) 1,1',3,3',5,5'-Hexamethylhydurilic acid (69%)

a These are the yields obtained by gravimetric crystallization techniques. The total yields of these products would be appreciably greater. b Chloride buffer (KC1 + HC1). c 4 parts 1 M HOAc + 1 part methanol. REACTION SCHEME

The very distorted shape of the voltammetric oxidation peaks of 5-substituted barbituric acids at the PGE, and the totally irreversible nature of the electrode processes precluded any meaningful investigation of the electrode mechanism

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by use of cyclic voltammetry, chronoamperometric or potentiostatic experiments. Nevertheless, the basic reaction scheme is quite obvious in view of the le nature of the process and the formation of substituted hydurilic acids as the almost sole product. Accordingly, the 5-substituted barbituric acid (Ia, eqn. 1) at pH values less than pKa or its monoanion (Ib, eqn. 1) at pH values greater than PKa undergoes a le reaction to give a barbiturate radical (II, eqn. 1). This then undergoes a very rapid dimerization to the corresponding 5,5'-substituted hydurilic acid (III, eqn. 1). The ease of controlled potential electrooxidation of 5-substituted barbituric acids to the corresponding 5,5'-substituted hydurilic acids along with the simplicity of separation and purification of the latter compounds clearly recommends this electrochemical process as a synthetic method. With the exception of 5,5'-diethylhydurilic acid and 5,5'-dibenzylhydurilic acid, all the other hydurilic acids reported in this paper are new compounds. ACKNOWLEDGEMENT

The authors would like to thank the National Science Foundation for financial support of the work described. REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12

S. Kato and G. Dryhurst, J. Electroanal. Chem., 62 (1975) 415. S. Kato and G. Dryhurst, J. Electroanal. Chem., in press. H. Aspelund, J. Prakt. Chem., 136 (1933) 329. E. Fischer and A. Dilthey, Ann. Chem., 335 (1904) 334. H. Kast, Chem. Ber., 45 (1912) 3124. A.C. Cope, D. Heyl, D. Peck, C. Elde and A. Arroyo, J. Amer. Chem. Soc., 63 (1941) 356. H. Aspelund, Acta Acad. Aboensis, Math. Phys., 10 (1936) 20, through Chem. Abstr., 31 (1937) 6630. G. Dryhurst, M. Rosen and P.J. Elvmg, Anal. Chim. Acta, 42 (1968) 143. G. Dryhurst, J. Eleetrochem. Soe., 116 (1969) 1097. G. Dryhurst and P.J. Elving, J. Electrochem. Sot., 115 (1968) 1014. G. Dryhurst, Anal. Chim. Acta, 57 (1971) 137. G. Korttim, W. Vogel and K. Andrussow, Dissociation Constants'of Organic Acids in Aqueous Solution, Butterworths, London, 1961.