Keto-enol tautomerism and vibrational spectra of phenylpyruvic acids

J . Phys. Chem. 1989, 93, 6013-6016

6013

Keto-Enol Tautomerism and Vibrational Spectra of Phenylpyruvic Acids Kazuhiko Hanai,*qt Akio Kuwae,* Satoshi Kawai,t and Yoko Onot

Downloaded by CENTRAL MICHIGAN UNIV on September 10, 2015 | http://pubs.acs.org Publication Date: August 1, 1989 | doi: 10.1021/j100353a016

Gifu Pharmaceutical University, Mitahora-higashi, Gifu 502, Japan, and College of General Education, Nagoya City University, Mizuho-ku, Nagoya 467, Japan (Received: September 16, 1988; In Final Form: January 5, 1989)

The keto-enol tautomerism of phenylpyruvic acid, @-hydroxypheny1)pyruvic acid, and salts of the former compound was investigated by IR and Raman spectroscopy. The spectral data indicate that these acids take the enol form in the solid state. The enol is predominant in organic media also. The IR strong band at 1620 cm-' in the solid acids is not originated from the C=C stretching vibration, but the weak band around 1660 cm-I, which has a very strong Raman counterpart, is assignable to it. 0-deuterated phenylpyruvic acid also shows similar bands. The 1620-cm-' band is not observed in solution. A possible structure which explains these spectra was proposed. The Raman spectra of salts of phenylpyruvic acid do not exhibit any C=C bands, indicating their existence in the keto form. However, no keto C=O band is observed in the IR spectra of the hydrated sodium and lithium salts, whereas the typical C=O band is found in the anhydrous sodium salt and the potassium and calcium salts. The drastic changes in the IR spectrum of the hydrated sodium salt on dehydration and in that of the lithium salt on 0-deuteration suggest that these hydrated salts have the gem-diol structure in the solid state. It was found by NMR that the normal keto form is more stable in aqueous solution irrespective of the nature of the metal cation involved.

Introduction a-Keto acids such as pyruvic acid, phenylpyruvic acid (PPA), and (p-hydroxypheny1)pyruvic acid (HPPA) are biologically important metabolic products.' Many report^^-^ have been published on the keto-enol tautomerism of these compounds. For PPA and HPPA a few spectroscopic s t u d i e ~have ~ , ~ been reported besides the kinetic ones.3 Josien et aL4 measured the IR spectra of PPA in the solid state and in carbon tetrachloride solution and concluded that this compound takes the enol form on the basis of the presence of an enol O H stretching band and a C=C stretching band. Sciacovelli et aLs also studied the tautomerism of PPA and HPPA by N M R , IR, and UV spectroscopy and came to the same conclusion and found that the keto form increases in water-dimethyl sulfoxide depending upon the water content. The sodium salt of PPA (PPA-Na) was found to take the keto form in the solid state and in aqueous solution.5a During the course of an investigation of the high-performance liquid chromatography of phenylpyruvic acids, one of us noticed that the tautomeric behavior was an important factor which affected the quantitative analysis. We measured the IR and Raman spectra of these compounds and found that the assignment of the above authors4*' should be corrected, although their conclusion for PPA is reasonable, and that hydrated salts of PPA show some characteristic features. The present paper deals with the IR and Raman spectroscopic study of PPA, HPPA, and the salts of PPA. The N M R spectral data are also reported. Experimental Section Materials. PPA was prepared from its sodium salt (PPANa.H20) according to Sciacovelli's method,5a since the purity of the commercial acid was doubtful owing to its instability. The purity of PPA thus obtained was checked by elemental analysis and it was kept in vacuo. The melting point was 146-155 OC, varying considerably with the rate of heating? 0-deuterated PPA was prepared by addition of heavy water (CEA 99.85%) to a solution of PPA in dry acetone and by removal of the solvent in vacuo. HPPA was a commercial product (Sigma Chem. Co., grade I and 11). PPA-Na.H,O was purchased from Tokyo Kasei Kogyo Co. and Sigma and used without further purification or after crystallization from its aqueous solution by addition of ethyl alcohol. The hydrated lithium salt of PPA (PPA-LLH,O) was obtained by passing an aqueous solution of PPA-Na.H20 through a column of Dowex 5OW(Li+) and recrystallized from water. This salt was presumed to be a monohydrate from the weight change after ion exchange. PPA-Li.D20 was prepared by evaporation Gifu Pharmaceutical University. *Nagoya City University.

0022-3654/89/2093-6013$01.50/0

of a heavy water solution of the hydrate in vacuo at 35 OC. The potassium salt (PPA-K) was also prepared by the same method and recrystallized from aqueous ethyl alcohol. The calcium salt (PPA-Ca) was a commercial product (Tokyo Kasei). The anhydrous sodium salt was obtained by heating PPAN a - H 2 0at 100 "C for 5 h in vacuo. It was confirmed by measurement of the weight change that the hydrate lost one molecule of water, transformed into the anhydrous salt, and by conversion to its free acid (PPA) that the salt obtained was not a decomposed product. However, PPA-Li.H,O was not dehydrated under the same conditions. Spectra. IR spectra were recorded in KBr disks, Nujol mulls, and solutions on a JASCO DS-403G IR spectrometer. The spectra in KBr disks were almost identical with those in Nujol mulls. Raman spectra were measured in capillaries for the solid samples and in ampules for solutions on a Spex Ramalog 9 spectrometer (excitation: 514.5 nm of an argon ion laser). N M R spectra were obtained on a JEOL GX-270 FT-NMR spectrometer by using tetramethylsilane and sodium 3-(trimethylsily1)- 1propanesulfonate as internal references. Solvents used were methanol, acetonitrile, dimethyl sulfoxide (Me,SO), water, chloroform, and their deuterated analogues and carbon tetrachloride. They were purified by ordinary methods or used without further purification.

Results and Discussion PPA and HPPA. The IR spectrum of PPA has two strong bands in the C=O stretching region as in Figure 1A. The 1693-cm-' band is due to the carboxyl group. The very strong band at 1620 cm-' seems to correspond to the C=C stretching (1) Meister, A. Biochemistry ofrhe Amino Acids; Academic: New York, 1965; Vol. 11. (2) (a) Bougault, J.; Hemmerle, R. Compr. Rend. 1915, 160, 100. (b) Painter, H. A.; Zilva, S. S . Biochem. J. 1947, 41, 520. (c) Knox, W. E.; Pitt, B. M. J. Biol. Chem. 1957, 225, 615. (d) Schwarz, K. Arch. Biochem. Biophys. 1961, 92, 168. (e) Kaper, J. M.; Gebhard, 0.;van den Berg, C. J.; Veldstra, H. Ibid. 1963, 103,469. (f) Schellenberger, A.; Hubner, G. Chem. Ber. 1965,98, 1938. (g) Nazario, G.; Schwarz, K. Arch. Biochem. Biophys. 1968, 123,457. (h) Ray, W. J.; Katon, J. E.; Phillips, D. B. J. Mol. Srrucr. 1981, 74, 75. (3) (a) Bucher, T.; Kirberger, E. Biochim. Biophys. Acra 1952,8,401. (b) Larsen, P. 0.;Wieczorkowska, E. Acra Chem. Scand. B 1974, 28, 92. (4) Josien, M.-L.; Joussot-Dubien, M.; Vizet, J. Bull. Soc. Chim. Fr. 1957, 1148. (5) (a) Sciacovelli, 0.; Dell'Atti, A.; De Giglio, A.; Cassidei, L. Z . Naturforsch. 1976, 31c, 5. (b) Cassidei, L.; Dell'Atti, A.; Sciacovelli, 0.Ibid. 1976, 31c, 641; 1980, 3.52, 1. (6) Herbst, R. M.; Shemin, D. Organic Syntheses; Blatt, A. H., Ed.; Wiley: New York, 1966; Collect. Vol. 2, p 519.

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Figure 1. IR (A) and Raman spectra (B) of PPA in the solid state.

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Figure 3. IR (A) and Raman spectra (B) of HPPA in the solid state.

(11 Figure 4. A possible molecular structure of PPA in the solid state.

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Figure 2. IR (A) and Raman spectra (B) of PPA in Me2S0.

band at 1628 cm-’ of trans-cinnamic acid. Furthermore, an enol OH stretching band is clearly observed at 3476 cm-I. From these findings Josien et aL4 and Sciacovelli et al.5a concluded that PPA takes the enol form in the solid state. We measured the Raman spectra of PPA. This spectroscopy is suitable for the detection of the C=C stretching band,’ since it is expected to have strong intensity. We have found that the 1620-cm-’ band is not assignable to the C=C vibration; as shown in Figure IA,B, the 1620-cm-I IR band has no corresponding strong Raman band, but the weak IR band at 1657 cm-’ has the very strong Raman (7) Freeman, S . K. Applications of Laser Raman Spectroscopy; Wiley: New York, 1974; p 11 1.

counterpart at 1658 cm-I. Figure 2 shows the IR and Raman spectra of PPA in Me,SO; no band is found around 1620 cm-’ in both spectra, and the weak IR band is observed at 1649 cm-l and the very strong Raman band at 1653 cm-I. A methanol solution also gives analogous spectra. These observations indicate that the 1657-cm-’ band in the solid spectrum should be assigned to the C=C mode. HPPA also has similar IR(1700, 1665, and 1621 cm-l) and Raman bands (1665 cm-I) in this region, as shown in Figure 3. The strong IR bands at 1607 and 1594 cm-’ and the medium Raman bands at 1611 and 1592 cm-’ are due to the benzene ring modes. There are a few possible explanations for the 1620-cm-’ band. A possibility that the sample is not pure has been ruled out by elemental analysis. Another explanation that this band is due to the OH bending vibration which appears at an extraordinarily higher frequency is excluded by the finding that the band shift is not observed in the 1700-1600-cm-’ region on 0-deuteration except for the 1657-cm-’ band, which shows a low-frequency shift of 10 cm-I. The vibrational modes expected usually in this region are the C=O stretching and the C = C stretching vibration. The findings that the 1620-cm-’ band is broad in the IR spectrum and is observed as a very weak band in the Raman spectrum suggest that this band is assigned to the former mode.’ The molecules which have different structures may coexist in the crystal. Figure 4 shows a possible molecular structure of PPA in the crystalline state, where the Z configuration is assumed according to the NMR study of Sciacovelli et al.;5a the molecule I has the trans conformation with respect to the C=C and C=O bonds, while the other molecule (11) is in the cis form. Such isomerism has been reported for some compounds8 The C 4 group of the carboxylic

The Journal of Physical Chemistry, Vol. 93, No. 16, 1989 6015

Keto-Enol Tautomerism of Phenylpyruvic Acids

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Figure 5. IR spectra of PPA in carbon tetrachloride (0.0012 M). Solid line: immediately after preparation of the solution. Broken line: after 6 h.

acid of the molecule I is hydrogen bonded both intramolecularly and intermolecularly and that of the molecule I1 intermolecularly only. The C=O stretching vibration of the molecule I gives rise to the 1620-cm-] band and that of the molecule I1 to the 1693-cm-l band. The enol O H stretching band of the molecule I possibly shifts to a lower frequency owing to the strong intramolecular hydrogen bonding, and is overlapped with the carboxyl O H band around 3000 cm-'. The enol O H stretching band of the molecule I1 is observed at 3476 cm-I. An example of such two crystallographically independent molecules has been found in the trichloroacetamide crystaL9 In polar organic solvents such as M e 2 S 0 and methanol these hydrogen bonds are broken, and one species of the enol form is predominant. The N M R spectrum of a M e 2 S 0 solution shows that about 5% of the keto form exists. On the other hand, a few species are in the carbon tetrachloride solution (0.0012 M) where the intermolecular hydrogen bonding is probably absent between the molecules I and 11; in Figure 5 the molecule I gives the carboxyl C=O stretching band at 1673 cm-' and the molecule I1 the band at 17 15 cm-'. In addition, the carboxyl C=O band of the keto form monomer is observed at 1790 cm-'; the keto C=O band is possibly overlapped with the 1715-cm-' band, as will be discussed later. This complexity may be ascribed in part to the geometrical isomerism. However, it is difficult to interpret this spectrum definitely by vibrational spectroscopy. The geometrical isomerism about the double bond of the enol acid has been discussed already on the basis of the N M R spectra by Sciacovelli et aissa,and it has been concluded that the acid has the Z configuration in organic solvents such as Me2S0 and acetone. N o free enol O H band is observed around 3600 cm-' in the carbon tetrachloride solution. A band at 3490 cm-I corresponds to the 3476-cm-' band in the solid state and is assigned to the intramolecularly hydrogen-bonded enol O H of the molecule 11. The O H band of the molecule I is observed in the 3000-cm-' region. The monomer carboxyl O H bands of pyruvic acid and a-alkoxycarboxylic acids1° have been reported to be in the region of 3490-3390 cm-I. The two weak shoulders are observed at 3434 and 3531 cm-I. The former can be assigned to the monomer carboxyl OH of the keto form from the time dependence discussed below. The latter may be assigned to that of the molecules I and/or 11. (8) (a) Umemura, J.; Hayashi, S. Bull. Znst. Chem. Res., Kyoto Uniu. 1974, 52, 585. (b) Schiering, D. W.; Katon, J. E. J . Mol. Srruct. 1986, 144, 25; 1986, 144, 71. (c) Schiering, D. W.; Katon, J. E. Specrrochim. Acta 1986, 42A, 487. (9) Hashimoto, M.; Hamada, K.; Mano, K. Bull. Chem. SOC.Jpn. 1987, 60, 1924. (10) (a) Oki, M.; Hirota, M. J . Chem. SOC.Jpn., Pure Chem. Sect. (Nippon Kagaku Zasshi) 1960,81, 855. (b) Oki, M.; Hirota, M. Bull. Chem. SOC.Jpn. 1960, 33, 119; 1961, 34, 374.

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Figure 6. IR spectra of solid PPA-Na.H20 (A) and PPA-Na (B).

In a thloroform solution (0.012 M) similar bands are observed at 3490, 1784, 1711, and 1674 cm-'. The intensities of these bands in both solutions vary with time. The intensity of the band at 1715 cm-' in carbon tetrachloride or 1711 cm-' in chloroform does not markedly change as compared with that of the other bands. This observation suggests that the carboxyl C=O stretching band of the molecule I1 and the keto C=O band of the keto form overlap at this frequency. In carbon tetrachloride solution, the tautomeric equilibrium is reached in 6 h. However, the time dependence in chloroform solution is probably in part ascribed to decomposition of PPA, since the 1R and N M R spectra do not reach a steady state. A few drops of triethylamine were added to the chloroform solution and immediately its IR spectrum was measured. The spectrum of the enol acid changed to that of the salt of the keto acid. This finding indicates that the transformation of the salt of the enol acid to that of the keto acid is very rapid. Sciacovelli et aLs reported that the tautomeric equilibrium depends on solvent properties; PPA and HPPA take predominantly the enol form in organic media, whereas these compounds exist mostly as the keto form in water, and in a mixture of Me2S0 and water the ratio of the enol to the keto form varies with the solvent composition. We have found the existence of the hydrated keto form of PPA, in addition to the normal keto isomer, in waterMe2SO-d6(50 ~ 0 1 % by ) N M R spectroscopy. Theys have stated that a small amount of the hydrated keto form of HPPA exists in aqueous solution but have not referred to the existence of the hydrated tautomer of PPA. Becker" and Pocker et al.12 reported that an aqueous solution of pyruvic acid shows two N M R signals whose difference in chemical shift is 0.86-0.90 ppm and that the peak at a higher field arises from the methyl hydrogens of the hydrated form and the peak at a lower field from that of the unhydrated form. In our spectrum of PPA the peaks are observed at 4.12 and 3.09 ppm in addition to the -CH= proton peak of the enol form at 6.53 ppm. The former two signals, which are nearly equal in intensity, are due to the CH, protons of the normal keto form and the hydrated form (2,2-dihydroxy-3-phenylpropionic acid), respectively. The intensities of the CH2 signals reach a maximum in about 6 h; the enol content was estimated to be about 60%, which is approximately in agreement with the value of Cassidei et aLSb The corresponding peaks in heavy waterMe,SO-d6 disappear in a week although their intensities increase at first as the solution comes to equilibrium, because eventually the deuteration at the olefinic carbon is accomplished. The vibrational spectra, however, did not show unambiguous evidence for the existence of the keto form in the solution. The IR spectrum hardly changes with time. A weak shoulder only, which may be assigned to the keto form, appears at 1733 cm-' in a heavy water-Me,SO solution after 6 h. The Raman spectrum of a (11) Becker, M. Ber. Bunsen-Ges. Phys. Chem. 1964, 68, 669. (12) Pocker, Y.;Meany, J. E.; Nist, B. J.; Zadorojny C. J . Phys. Chem. 1969, 73, 2879.

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Figure 7. Raman spectrum of solid PPA-Na.H20.

water-Me2S0 solution does not show the C=O stretching band due to the keto form. However, the finding that the intensity ratios of the observed Raman C=C band to the solvent bands decrease with time suggests that the enol content decreases and possibly the keto form increases. Lithium, Sodium, Potassium, and Calcium Salts of PPA. The IR spectrum of commercial PPA-Na-H20 shows characteristic features around 3000 and 1600 cm-I, as in Figure 6A. The H20 band is not found in the region expected, but a broad and strong band appears around 3000 cm-l. The very strong band at 1595 cm-' is assigned to the antisymmetric stretching mode of the carboxylate group. The C=O stretching band, which is expected to occur if this salt has the keto form, is not observed; a weak band at 1707 cm-I does not occur in a Nujol mull and arises from a small amount of the anhydrous salt formed during the sample preparation. Although Sciacovelli et also reported the IR spectrum of this compound, our spectrum is quite different from theirs. Their spectrum has a medium intensity band at 1710 cm-I. Therefore, they assigned this band to the keto carbonyl group and concluded that the salt in the solid state takes the keto form. Probably, their sample contained some decomposed products, judging from the spectral feature. No strong Raman band is observed around 1650 cm-', as shown in Figure 7. This finding indicates that the salt does not have the enol structure. Accordingly the problem of the absence of the C=O stretching band must be solved. PPA-Li.H20 also gives a similar IR pattern (Figure 8A). We have found that PPA-Na.H20 is dehydrated by heating at 100 "C for 5 h in vacuo and that the dehydrated salt shows the normal spectral pattern; the broad band at 3000 cm-I disappears and the strong keto C=O band appears at 1709 cm-l, as shown in Figure 6B. Such a drastic spectral change cannot be explained in terms of simple removal of the water molecule. This behavior may be caused by some structural changes in the molecule. The hydrate of PPA-Li was difficult to be dehydrated under the same conditions. On the other hand, PPA-K and PPA-Ca, which seem to have water of crystallization, show the normal spectra. The 0-deuteration is expected to afford useful information on the structure of the water molecule in the crystal. In Figure 8B the pronounced spectral shift is observed on 0deuteration of PPA-Li-H20. This change suggests that the hydroxyl group involved is not that of the water molecule.

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(13) (a) Bellamy, L. J.; Williams, R. L. Biochem. J . 1958, 68, 81. (b) Anderson, D. M . W.; Bellamy, L. J.; Williams, R. L. Spectrochim. Acta 1958, 12, 233. (14) Long, D. A.; George, W. 0. Trans. Faraday Sot. 1960, 56, 1570. (15) Jencks, W. P.; Carriuolo, J. Nature 1958, 182, 598.

Keto-enol tautomerism and vibrational spectra of phenylpyruvic acids

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