J. Org. Chem., Vol. 38,No. 4, 1973 663
PYROLYSIS OF PHENYLALANINE
Pyrolysis of Phenylalanine, 3,6-Dibenzyl-2,5-piperazinedione,and Phenethylamine JOHN 14. PATTERSON,* XABEELF. HAIDAR,E. PAUL PAPADOPOULOS, AN@ WALTERT. SMITH, JR. Department of Chemistry, University of Kentucky, Lexington, Kentucky 40606 Received M a y Si', 1972
Phenylalanine and its low-temperature pyrolysis products, 3,6-dibenzyl-2,5-piperazinedione and phenethylamine, were found to undergo pyrodegradation and pyrosynthesis reactions at 650 and 850". While the pyrolysates obtained from the three substances contained the same kinds of compounds, variations in quantities were observed. The chief constituent (650') of the phenylalanine and dibenzylpiperazinedione pyrolysates was toluene, while that of the phenethylamine pyrolysate was styrene. Other substances found in moderate quantities were bibenzyl, biphenyl, stilbene, phenanthrene-anthracene, and benzene. Dibenzylpiperazinedione pyrolysate contained, in addition, moderate amounts of benzonitrile and phenylacetonitrile. Hydrogen cyanide was produced in all pyrolyses. At the higher pyrolysis temperatures yields of fluorene, pyrene, and phenanthreneanthracene increased while yields of bibenzyl and stilbene diminished or fell to zero. These data suggest that both simultaneous and nonsimultaneous multiple bond cleavages are competitive in the thermally induced breakdown of phenylalanine and that at higher temperatures the simultaneous multiple processes become more prominent.
The thermolysis of complex plant materials, such as tobacco, gives rise to a wide variety of organic substances,' most of which are produced through the processes of pyrodegradation and pyrosynthesis. Experiments in which individual components of the plant, such as proteins2 and amino a ~ i d ,have ~ , ~been pyrolyzed show that pyrolysis mixtures of similar composition are produced. The low-temperature pyrolysis of amino acids to give amines and nitriles4-' by decarboxylation and piperazinedione by cyclodehydration4-* suggests that these substances may be primary pyrolysis products at higher temperatures. The investigation described here was conducted to determine, by examination of pyrolysate composition, in what way the high-temperature pyrolysis of these intermediates contributes to the pyrolysate composition of phenylalanine. Phenylalanine, phenethylamine, and 3,6-dibenzyl2,s-piperazinedione (I) were pyrolyzed under the same conditions a t 650 and 850" in a nitrogen atmosphere. The results are reported in Tables I, 11, and 111. Another possible breakdown pathway involves deamination of phenylalanine to cinnamic acid. Products arising from the thermolysis of cinnamic acid previously described by Jones and Schmeltzg are included for comparison. Evidence that these probable primary pyrolysis products, or their precursors, are involved at least to some extent in the thermolytic decomposition of phenylalanine in the present experiments was provided by the isolation of the intermediates. Thus phenethylamine was isolated from 450 and 650" pyrolysis experiments in 6.9 and 0.5% yields, respectively, and 3,6-dibenzyl-2,5-piperazinedione(I) was isolated from a 850" pyrolysis experiment. Only trace quantities of cinnamic acid were found in 450 and 650" pyrolysis experiments. I n spite of the relatively high stability of cinnamic acid (Jones and Schmeltzg recovered 42% (1) R. Stedman, Chem. Res., 68, 153 (1968). (2) E. B. Higman, I. Schmelta, and W. S. Schlotchauer, J . Agr. Food Chem., 18, 636 (1970). (3) J. M. Patterson, M. L.Baedecker. R . Musick, and W. T. Smith, J r . , Tobacco Sei., 13, 26 (1969); J. M. Patterson, W.Y . Chen, and W.T. Smith, Jr., ihid., 16, 98 (1971). (4) E . Waser, Helv. Chim. Acta, 8, 758 (1925). ( 5 ) J. Vollumin, P. Kriemler, I. Omura, J. Seibl, and W.Simon, Micro.. chem. J . , 11, 73 (1966). (6) G . Chatelus, Bull. Sac. Chirn. Fr., 2523 (1964); 929 (1965). ( 7 ) G. P. Shulman and P. G. Simmonds, Chem. C o m m u n . , 1040 (1968). (8) S.W. Fox, Science, 132, 200 (1960). (9) T. C. Jones and I. Schmeltz, J . Org. Chem., 84, 645 (1969).
cinnamic acid on pyrolysis at 700") only very small amounts of total acid fraction (0.05-0.067,) were produced in the pyrolysis of phenylalanine a t 450-850". Because of these observations it appears that the deamination of phenylalanine probably represents a very minor decomposition pathway in the thermolysis of the amino acid. The production of the gases HCN, "3, CO, and C o n during each of the pyrolyses is consistent with the formation of the possible intermediates, but this production, with the exception of HCN formation, does not allow distinction between the paths followed. The low ammonia yields observed do not provide a reliable indication of the fraction of reaction occurring by a deamination pathway (of phenylalanine or phenethylanime) since ammonia is known to dissociate almost completely to nitrogen and hydrogen a t ea. 625".1° Although C 0 2 yields were expected to be relatively large in view of the ease with which amino acids decarboxylate at low temperatures, especially in the presence of radicals,6 the low CO, yields do not rule out this path. Carbon dioxide is readily reduced to carbon monoxide by carbon at 1000°ll or by hydrogen at 6001000°.12 In the experiments reported here, it is likely that the CO, is not significantly reduced by carbon, since no reduction of C 0 2 was observed when the gas passed over carbonized amino acid in the pyrolysis apparatus at 850". Some additional contribution to the C 0 2 and CO yields could have arisen from the thermal decomposition of cinnamic acid, the deamination product of phenylalanine, since conjugated unsaturated acids have been reported to produce COz and CO in ratio of ea. 1-2: 1 at 5OO0.l3 I n view of the COz:CO ratio observed in the present experiments and of the stability of cinnamic acid under pyrolytic conditions,G this path is probably a minor one. The quantities of carbon monoxide and carbon dioxide produced in the present experiments suggest that some of the carbon monoxide arises from interaction of water with carbon, carbon radicals, or alkanes. The intermediacy of 3,6-dibenzyl-2,5-piperazinedione (10) N. V. Sidgwick, "The Chemical Elements and Their Compounds," Vol. I, Pergamon Press, Oxford, 1950, p 658. (11) I. G . Petrenko. K h z m . Tekhnol. Topl. Aliasel, 15 (1957); Chem. Ahst?., 61,17366 (1957). (12) W ,M. Graven and F. J. Long, J . Amer. Chem. Sac., 76, 2602 (1954), and references cited therein. (13) R . L. Forman, H. M , Mrtckinnon, and R . D. Ritchie, J . Chem. sot., 2013 (1968)
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PATTERSON, HAIDAR, PAPADOPOULOS, AND SMITH TABLE I
RELATIVE CONCENTRATIONS OF COMPONENTS" PRODUCED
ITSPRIMARY PYROLYSIS PRODUCTS,
-Phenylalanine-. 650° 850°
Component
Acenaphthylene Aniline Benzene Benaonitrile Bibenzyl Biphenyl
