A 13C-labelling study conclusively rules out a simple valence
bond isomerization as the major pathway by which azulene rear- .
ranges thermally to naphthalene

L. T. Scott

M. A . Kirms

M. A . Minton

English

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CROATICA CHEMICA ACTA CCACAA 53 (4) 643-647 (1980) CCA-1250 YU ISSN 0011-1643 UDC 547.65 Conference Paper Thermal Rearrangements of Aromatic Compounds. 2. Isomerization of Azulene-3a-13C to Naphthalene-13C1* L. T. Scott, M. A . Kirms, and M . A . Minton Department of Chemistry, University of Nevada, Reno, Nevada 89557, USA Received September 3, 1979 A 13C-labelling study conclusively rules out a simple valence bond isomerization as the major pathway by which azulene rear- . ranges thermally to naphthalene. Thermal rearrangements, though commonplace among unsaturated ali-phatic hydrocarbons,2 have only rarely been observed in completely aromatic systems.3 The thermal isomerization of azulene (1) to naphthalene (2) undoub-tedly qualifies as the oldest and best known example of such a skeletal reorga-nization and has stood as a mechanistic puzzle for many decades.aa-i In parti-cular, the question has arisen4 as to whether or not this rearrangement occurs via sequential, concerted, pericyclic reactions5 analogous to those which so often prevail in nonaromatic systems.2 Scheme I depicts one plausible me-chanism.4 08 ! .'s + ,2, [©] The simplicity and symmetry-allowed nature of this mechanism make it most appealing. Rudimentary thermochemical calculations6 place the bicyclo-butane intermediate 3 above azulene (t1t1Hr) by 47.4 kcal/mol. This number should be compared to the activation enthalpy for the isomerization of azulene to naphthalene, which has been reported1c,d (with reservation)1c,d as t1 H =I== 48.8 kcal/mole but may be higher.7 On thermochemical grounds, then, compound * Presented in part at the International IUPAC Symposium on Aromaticity, held in Dubrovnik, Croatia, Yugoslavia, September 3-5, 1979. 644 L. T. SCOTT ET AL. 3 may be considered a permissible intermediate. The stereochemical mode by which the bicyclobutane moiety in 3 would be constrained to open corresponds to the preferred mode observed in the opening of other bicyclobutanes8 and to the mode predicted by theory.5,9 This mechanism can be tested. Since it involves only skeletal reorgani-zations without the breaking of C-H bonds, this mechanism requires that a 13C-label originating in the angular posiHon of azulene would have to appear exclusively in the angular position of the naphthalene product; any label found in the a.- or B-positions would imply the making and breaking of C- H bonds. We have synthesized angularly-labeled azulene (azulene-3a-13C) by the in-tramolecular carbene addition route developed in our laboratory (Scheme II).10 CuCl Quantitative dilution of highly enriched precursor gave azulene with a calcu-lated abundance of 13C in the angular position of 190/o. This value was con-firmed experimentally by comparison of the 13C-NMR spectra from natural abundance and enriched azulene (Figure 1.). Thermal isomerization of the enriched azulene was conducted in a base--washed, quartz flow system. The azulene was slowly sublimed at atmospheric pressure (ca 5- 10 mg/h) into a stream of nitrogen gas which carried the sample through the hot zone and into a liquid nitrogen trap. The hot zone was maintained at 820 °C with a contact time of 93 s; shorter contact times or lower temperatures resulted in incomplete disappearance of the azulene. The naphthalene was purified by sublimation (330/o yield) and examined by 13C-NMR spectroscopy (Figure 2.). It is immediately obvious from the 13C-NMR spectrum of the isolated naphthalene that the 13C-label does not all remain in the angular position. From a quantitative treatment of the data, we find the 13C-label distributed over the y, a, and B positions of naphthalene in a ratio of 3 : 2 : 1 respectively. We, therefore, conclude that the bicyclobutane mechanism (Scheme I) cannot be the major pathway by which azulene isomerizes to naphthalene; it may not be involved at all. From extensive studies on a series of methylated azulenes, Alder et al. have proposed a complicated, dual-track, free radical chain mechanism for the isomerization of azulene to naphthalene.1f-h Intermediates similar to those proposed by Alder et al. have also been suggested by Prislopski on the basis ISOMERIZATION OF AZULENE TO NAPHTHALENE I I 150 3a,sa; 5,7 l 3 I I 100 Figure 1. Proton decoupled ·13c-nmr spectra of natural abundance azul ene (--) and of enriched azul ene from the synthetic route in Scheme II (--.... ·). 645 of a more limited study.1e Both investigators conclude that isomerization is probably initiated by addition of a hydrogen atom (or proton) of unknown origin, the rearrangement then taking place on the C10H 9 energy su-rface. Scheme III illustrates an abbreviated adaptation of these proposals. This mechanism is compatible with our results only if accompanied by competing secondary processes such as those suggested by Alder et al. No single pathway has yet been proposed which can account for the results of our experiment, nor can we offer one at this time. Acknowledgements. - We thank the University of Nevada Research Advisory Board and the donors of the Petroleum Research Fund, administered by the American Chemical Society for financial support of this work. REFERENCES 1. For Part 1 see: L. T. Scott and G. K. Ago pi an, J. Amer. Chem. Soc. 99 (1977) 4506. 2. See for example: L. T. Scott and M. Jones, Jr., Chem. Rev. 72 (1972) 181. 646 L. T. SCOTT ET AL. I I I I 1~ 100 Figure 2. Proton decoupled 13c-nmr spectra of natural abundance naphthalene (--) and of fgriched naphthalene from therma 1 rearrangement of 3a- C-azul ene (-- " "). co co i H' -H+ t [cb] rearrange [CO] SCHEME III ISOMERIZATION OF AZULENE TO NAPHTHALENE 647 3. Rearrangements of azulene: a E. Heil.bro n n er, P. A. P 1 attn er, and K. W i e 1 and, Experientia, 3 (1947) 70; b E. Heilbronner and K. Wieland, Helv. Chim. Acta 30 (1947) 947; c H . J. Ka 11 en, PhD. Thesis, Eidgeni:issischen Technischen Hochschule, Zurich, 1958; d E. He i 1 bro n n er in Nonbenzenoid Aromatic Compounds, D. Ginsburg, Ed., Interscience, New York, N. Y., 1959, pp 263-8; e M. C. Pris 1 ops k i, M. S. Thesis, Wesleyan University, Middletown, Conn., 1973 ; f R. W. A 1 de r and G. Whittaker, J. Chem. Soc. Perkin II, (1975) 714; g R. W. A 1 de r and C. W i 1 shire, J. Chem. Soc. Perkin II, (1975) 1464; h R. W. A 1 de r, R. W. W h it e s i de, G. Whittaker, and C. W i 1 shire, J. Amer. Chem. Soc. 101 (1979) 629; i M. A. Ki rm s, M . S. Thesis, University of Nevada, Reno, Nevada, 1979; Rearrangements of benzenes: j E. D. C 1 if ton, W. T . F 1 owe rs, and R. N. H as z e 1 dine, Chem. Com-mun., (1969) 1216; k J. K. van Strate n, W. H. de W o 1 f, and F. B i ck e 1 ha up t , Tetra -hedron Lett. (1977) 4667; Rearrangements of pyridazines: 1 R. D. Ch ambers, M. C 1 ark, J. R. Mas 1 a k i e w i c z, W. K. R. Mus-g rave, and P. G. Urben, J. Chem. Soc. Perkin I., (1974) 1513; m R. D. Ch ambers, C. R. S a r gen t, and M. C 1 ark, J. Chem. Soc. Chem. Commun. (1979) 445; n R. D. Ch ambers and C. R. S argent, J. Chem. Soc. Chem. Commun. (1979) 446; Rearrangements of naphthalene: see ref. 1. 4. R. B. Woodward, research group meeting, Harvard University, 1970, and ref. 3. 5. R. B . W o o d w a r d and R. H o f f m a n n, The Conservation of Orbital Sym-metry, Verlag Chemie, Winheim, Germany, 1970. 6. S. W. Benson, Thermochemical Kinetics, 2nd ed., Wiley, New York, N. Y., 1976, chapt. 2. 7. Unpublished observations by R. W. A 1 de r and by W. C. Herndon (per-sonal communication from each) and in our own laboratory1 strongly indicate that azulene isomerizes more slowly than predicted by Kallen 's activation para-meters. 8. G. L. C 1 o s s and P. E. Pfeffer, J. Amer. Chem. Soc. 90 (1968) 2452; W. G. Daub en and F. G. W i 11 e y, Tetrahedron Lett. (1962) 893 ; W . G. Daub en and W. T. W i pk e, Pure Appl. Chem. 9 (1964) 539. 9. M. J . S. Dewar and S. Kirsch n er, J. Amer. Chem. Soc. 97 (1975) 2931. 10. L. T . Scott, J. Chem. Soc. Chem. Commun. (1973) 882; L . T . Scott and M.A. Minton, J. Org. Chem. 42 (1977) 3757 ; L. T . Scott, M.A. Minton, and M. A. Ki rm s, J. Amer. Chem. Soc. 102 (1980) 6311; M. A. Minton, PhD. Thesis, UCLA, 1977; see also ref. li. SAZETAK TermiCka pregraaivanja aromatskih spojeva. 2. Izomerizacija azulen-3a-1ac u naftalen-13C L. T. Scott, M. A. Kirms i M. A. Minton Termicka izomerizacija azulena u naftalen stoji vec vise desetljeca kao meha-nisticka zagonetka. Najprivlacnijim se cinio biciklobutanski mehanizam jer je jedno-stavan i simetrijski dozvoljen. Medutim,13C-N1VIR spektar naftalena je pokazao da se oznaka 13C, uvedena u angularni polozaj u azulen, nakon izomerizacije nalazi na y-, a- i ~-polofajima naftalena u omjeru 3 : 2 : 1. Stoga, ni jedan do sada predlozeni jedinstveni put, ukljueujuCi i biciklobutanski mehanizam, ne moze objasniti rezultate toga mehanizma. ODJEL ZA KEMIJU SVEUCILISTA NEVADE RENO, NEV ADA, SAD Prispjelo 3. rujna. 1979. 

Thermal Rearrangements of Aromatic Compounds. 2. Isomerization of Azulene-3a-13C to Naphthalene-13C

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