Thermolysis of 1,3,8-nonatriyne: Evidence for intramolecular [2+4] cycloaromatization to a benzyne intermediate

Full text of DOI:10.1021/ja972141f

J. Am. Chem. Soc. 1997, 119, 9917-9918
9917
Scheme 1
Thermolysis of 1,3,8-Nonatriyne: Evidence for
Intramolecular [2 + 4] Cycloaromatization to a
Benzyne Intermediate
Alexander Z. Bradley and Richard P. Johnson*
Department of Chemistry
UniVersity of New Hampshire
Durham, New Hampshire 03824
ReceiVed June 27, 1997
Scheme 2
Cycloaromatizations have been of intense recent interest
because of their potential involvement in the chemistry of
antitumor agents.1 In principle, aromatic rings might also be
prepared directly by diyne + alkyne cycloaddition reactions,
as exemplified by eq 1, which yields o-benzyne. This concep-
tually simple process is a logical extension of known enyne +
alkene, enyne + alkyne, and diyne + alkene cycloadditions (eqs
2-4) which have been previously demonstrated in substances
where the reacting components are held together by a three-
carbon tether.2,3 In flash vacuum thermolysis experiments, the
(1)
(2)
1,2-shifts are well-known for alkynes; one alternative mecha-
nism (Scheme 2) would pass through vinylidene 11. C-H bond
insertion and Bergman cyclization1,8 might give p-benzyne 13
and, hence, indan.9 Enediyne 12 is unknown, although larger
ring homologues have been investigated.10 To distinguish these
1
two paths, we prepared and pyrolyzed 5-d2. The H NMR
(3)
(4)
spectrum of the isolated indan cleanly showed an AB spin
system for the aromatic hydrogens; these results are consistent
with structure 10 and a [2 + 4] cycloaddition mechanism.
Similar deuterium labeling was observed in the indene isolated
from these reactions.
intermediacy of strained cyclic cumulenes4 was supported by
the observation of predictable secondary processes. We describe
here experimental evidence for 1,3-diyne + alkyne [2 + 4]
cycloaromatization (eq 1) as a new mode of Diels-Alder type
cycloaddition. Although this process might seem geometrically
improbable, ab initio computational studies support the feasibil-
ity of the parent diyne + alkyne cycloaddition step.5
1,3,8-Nonatriyne (5) was prepared in several steps as shown
in Scheme 1. Alkylation of 1,4-bis(trimethylsilyl)butadiyne and
subsequent TMS removal gave 5 in 50% overall yield. Flash
vacuum thermolysis of pure 5 at 10-2 Torr in a quartz apparatus
at 580 °C cleanly yielded two products, which were identified
by comparison to authentic samples as indan (7, 86%) and
indene (8, 14%). These accounted for >95% of the products,
and no starting material remained. Some soot also formed in
this pyrolysis but there was no evidence for the expected dimers
of benzyne 6. At 650 °C, the product ratio was 60:40, which
suggests that indene is predominantly a secondary product of
indan dehydrogenation. This has literature precedent6 and was
confirmed by pyrolysis of indan under the same conditions.
The most straightforward route to the observed product is
through intramolecular [2 + 4] cycloaddition to give benzyne
derivative 6. There is ample precedent for reduction of benzynes
under similar pyrolytic reaction conditions.7 However, thermal
Ab initio calculations were carried out to estimate the
geometric and energetic feasibility of a diyne plus alkyne
cycloaddition. Stationary points were located at the MP2(FC)/
6-31G* level, followed by analytic Hessian calculation and
single-point MP4SDTQ/6-31G* energy evaluation.11,12 This
level of theory correctly describes energetics of the parent
Diels-Alder reaction to within a few kcal/mol.13 The transition
state is predicted to have C2V symmetry, with a nascent C-C
bond distance of 2.196 Å; this is in good agreement with
geometries of other pericyclic transition states.14 The predicted
(7) Benzyne reduction has been seen under a variety of conditions. (a)
Berry, S. R.; Spokes, G. N.; Stiles, M. J. Am. Chem. Soc. 1962, 84, 3570.
(b) Corbett, T. G.; Porter, Q. N. Aust. J. Chem. 1965, 18, 1781. (c) Bove,
J. L.; Arrigo, J. R. Chem. Ind. 1984, 803. (d) Brown, R. F. C.; Coulston,
K. J.; Eastwood, F. W. Tetrahedron Lett. 1996, 37, 6819.
(8) Bergman, R. G. Acc. Chem. Res. 1973, 6, 25. Lockhart, T. P.; Comita,
P. B.; Bergman, R. G. J. Am. Chem. Soc. 1981, 103, 4082. Bharucha, K.
N.; Marsh, R. M.; Minto, R. E.; Bergman, R. G. J. Am. Chem. Soc. 1992,
114, 3120.
(9) As an alternative mechanism, a referee has suggested that 1,2-
hydrogen shift at C-9 would give an intermediate carbene that could cyclize
to cyclononen-3,5-diyne, which might further rearrange to indan. However,
models show the divalent center in this intermediate to be >4 Å distant
from the alkyne hydrogen, thus cyclization seems highly unlikely. By
contrast, in carbene 11, this distance is ca. 1.7 Å.
(10) Nicolau, K. C.; Zuccarello, G.; Ogawa, Y.; Schweiger, E. J.;
Kumazawa, T. J. Am. Chem. Soc. 1988, 110, 4866.
(11) (a) Spartan, version 4.01; Wavefunction Inc.: Irvine, CA, 1993.
(b) Gaussian 92, revision E.1; Frisch, M. J.; Trucks, G. W.; Head-Gordon,
M.; Gill, P. M. W.; Wong, M. W.; Foresman, J. B.; Johnson, B. G.; Schlegel,
H. B.; Robb, M. A.; Replogle, E. S.; Gomperts, R.; Andres, J. L.;
Raghavachari, K.; Binkley, J. S.; Gonzalez, C.; Martin, R. L.; Fox, D. J.;
Defrees, D. J.; Baker, J.; Stewart, J. J. P.; Pople, J. A. Gaussian, Inc.:
Pittsburgh, PA, 1992.
(12) For benzyne geometry, see: Hobza, P.; Zahradnik, R.; Hess, B.
A.; Radziszewski, J. G. Theor. Chim. Acta 1994, 88, 233.
(13) (a) Bachrach, S.; Liu, M. J. Org. Chem. 1992, 57, 6736. (b) Rowley,
D.; Steiner, H. Discuss. Faraday Soc. 1951, 10, 198.
(14) Houk, K. N.; Li, Y.; Evanseck, J. D. Angew. Chem., Int. Ed. Engl.
1992, 31, 682.
(1) For reviews, see: (a) Wisniewski-Grissom, J.; Gunawardena, G. U.;
Klingberg, D.; Huang, D. Tetrahedron 1996, 52, 6453. (b) Wang, K. K.
Chem. ReV. 1996, 96, 207.
(2) Burrell, R. C.; Daoust, K. J.; Bradley, A. Z.; DiRico, K. J.; Johnson,
R. P. J. Am. Chem. Soc. 1996, 118, 4218.
(3) Danheiser, R. L.; Gould, A. E.; Fernandez de la Predilla, R.; Helgason,
A. L. J. Org. Chem. 1994, 59, 5514.
(4) Review: Johnson, R. P. Chem. ReV. 1989, 89, 1111.
(5) For preliminary report, see: Abstracts of Papers, 213th National
Meeting of the American Chemical Society, San Francisco, CA, April, 1997;
American Chemical Society: Washington, DC, 1997; ORGN 0459.
(6) Penninger, J. M. L. Int. J. Chem. Kinet. 1982, 14, 761.
S0002-7863(97)02141-0 CCC: $14.00 © 1997 American Chemical Society

9918 J. Am. Chem. Soc., Vol. 119, No. 41, 1997
Communications to the Editor
reported the thermal trimerization of acetylene to benzene.17
Concerted cyclotrimerization (eq 5) is very unlikely because
this will have large enthalpic and entropic barriers.18 Among
many possible stepwise mechanisms, Fields and Meyerson
proposed dimerization of acetylene to butadiyne followed by
[2 + 4] cycloaddition (eq 6) to give benzyne.19 In support of
(5)
(6)
Figure 1. Free energy diagram (kcal/mol, 25 °C) and MP2/6-31G*
transition state structure for the cycloaddition of acetylene with
butadiyne.
this mechanism, they observed that co-pyrolysis of acetylene
and phthalic anhydride yielded naphthalene and other aromatics.
Both the thermal dimerization of acetylene20 and the reduction
of benzyne to benzene7 are precedented, but experimental
evidence for the key cycloaddition step has been scant. The
present data are consistent with the mechanism depicted in eq
6. Beyond this, one of the enduring questions in combustion
chemistry is the mechanism for formation of aromatic rings.21
Some types of flames are rich in alkynes, and we speculate that
cycloadditions such as those shown in eqs 1-4 may play an
important role in six-membered ring formation in combustion.
Our results thus support the existence of a sixth mode of
Diels-Alder cycloaddition, thus completing this series of
pericyclic reactions. This new mode of cycloaromatization
complements the many electrocyclizations that have been
reported.1,22
free energy changes are summarized in Figure 1. Intramolecular
reaction should decrease ∆Gq by ca. 5 kcal/mol because of the
smaller T∆Sq component. The large predicted reaction exo-
thermicity is consistent with the formation of a new aromatic
ring, while the high activation energy must derive from the
dramatic molecular distortion necessary to reach the transition
state geometry. Most notably, the predicted activation free
energy is only 9.0 kcal/mol higher than that calculated for the
cycloaddition of ethyne with butadiene.2 The electronic features
of this cycloaromatization transition state are unusual because
two orthogonal orbital arrays must come together simulta-
neously. In this process, the out-of-plane π orbitals simply
merge to form the aromatic ring π system, while the in-plane
π bonds transform into σ bond orbitals and the in-plane benzyne
π bond. Both transition state arrays are aromatic in this doubly
symmetry allowed process. A stepwise cycloaddition is also
possible; we have not yet explored the energetics of this process.
Polyynes are known to be explosive,15 and Vollhardt has
recently shown that such explosions might generate ordered
carbon arrays.16 One sample of triyne 5 underwent a mild
explosion during reduced pressure distillation, with instantaneous
formation of heavy soot. Similarly, purification by preparative
gas chromatography at 130 °C resulted in an injector port filled
with fine black powder; this material is currently being analyzed.
Both our experiments and model ab initio calculations are
thus consistent with intramolecular [2 + 4] cycloaromatization
of triyne 5 to benzyne 6. Further reaction gives indan and
indene. This cycloaddition has little precedent, but might
potentially be involved in diverse high-temperature processes
such as the Berthelot benzene synthesis. In 1866, Berthelot
Acknowledgment. We are grateful to the National Science Founda-
tion for support of this research and to the Pittsburgh Supercomputer
Center for a generous allocation of supercomputer time. We thank
Professors Curt Wentrue and Charles Zercher for helpful discussions.
Supporting Information Available: Selected characterization data
and spectra (5 pages). See any current masthead page for ordering
and Internet access information.
JA972141F
(18) For theoretical studies, see: (a) Houk, K. N.; Gandour, R. W.;
Strozier, R. W.; Rondan, N. G.; Paquette, L. A. J. Am. Chem. Soc. 1979,
101, 6797. (b) Bach, R. D.; Wolber, G. J.; Schlegel, H. B. J. Am. Chem.
Soc. 1985, 107, 2837. (c) Wagenseller, P. E.; Birney, D. M.; Roy, D. J.
Org. Chem. 1995, 60, 2853.
(19) (a) Fields, E. K.; Meyerson, S. Tetrahedron Lett. 1967, 571. (b)
Fields, E. K.; Meyerson, S. In AdVances in Physical Organic Chemistry;
Gold, V., Ed.; Academic: New York, 1968; Vol. 6, p 1. (c) Brown, R. F.
C. Pyrolytic Methods in Organic Synthesis; Academic: New York, 1980;
p 65.
(20) Kiefer, J. H.; Von Drasek, W. A. Int. J. Chem. Kinet. 1990, 22,
747.
(15) (a) Brandsma, L.; Verkruijsse, H. D. Synthesis of Acetylenes, Allenes
and Cumulenes; Elsevier Publishing Company: Amstrdam, 1981; references
therein. (b) For a review of polyyne structures, see: Bunz, U. H. F. Angew.
Chem., Int. Ed. Engl. 1994, 33, 1073.
(21) For example, see: (a) Miller, J. A.; Melius, C. F. Combust. Flame
1992, 91, 21. (b) Miller, J. A.; Volponi, J. V.; Pauwels, J.-F. Combust.
Flame 1996, 105, 451. (c) Melius, C. F.; Miller, J. A.; Evleth, E. M. Symp.
(Int.) Combust. 1992, 24, 621-8.
(22) As this paper was completed, a related cycloaromatization was
reported: Miyawaki, K.; Suzuki, R.; Kawano, T.; Ueda, I. Tetrahedron
Lett. 1997, 38, 3943.
(16) Boese, R.; Matzger, A. J.; Vollhardt, K. P. C. J. Am. Chem. Soc.
1997, 119, 2052.
(17) Berthelot, M. Liebigs Ann. Chem. 1867, 141, 173.