Acid-catalyzed rearrangement of ethynylcycloheptatriene to phenylallene

Article abstract of DOI:10.1021/ol006328e

7-Ethynylcycloheptatriene (1) cleanly isomerizes to phenylallene in the presence of acid A machanism involving the protonation of ethynylnorcaradiene, which is in equilibrium with 1, followed by the cleavage of a three-membered ring to give an arenium ion, is proposed. The rearrangement is accelerated by a factor of 370 by introducing tert-butyl groups on C-2 and C-5, indicating the importance of the equilibrium concentration of the norcaradiene form as a rate-controlling factor.

Full text of DOI:10.1021/ol006328e

ORGANIC
LETTERS
2000
Vol. 2, No. 19
3011-3013
Acid-Catalyzed Rearrangement of
Ethynylcycloheptatriene to Phenylallene
Toshikazu Kitagawa,* Jun Kamada, Shinya Minegishi, and Ken’ichi Takeuchi
Department of Energy and Hydrocarbon Chemistry, Graduate School of Engineering,
Kyoto UniVersity, Sakyo-ku, Kyoto 606-8501, Japan
kitagawa@scl.kyoto-u.ac.jp
Received July 13, 2000
ABSTRACT
7-Ethynylcycloheptatriene (1) cleanly isomerizes to phenylallene in the presence of acid. A mechanism involving the protonation of
ethynylnorcaradiene, which is in equilibrium with 1, followed by the cleavage of a three-membered ring to give an arenium ion, is proposed.
The rearrangement is accelerated by a factor of 370 by introducing tert-butyl groups on C-2 and C-5, indicating the importance of the equilibrium
concentration of the norcaradiene form as a rate-controlling factor.
The acetylene-allene rearrangement1 is an important method
for the synthesis of allenes, since acetylenes which contain
a variety of substituents are readily available. This reaction,
formally a 1,3-hydrogen shift, is subject to both acid and
base catalysis. However, most of the synthetic and mecha-
nistic studies have been performed under basic conditions
(Scheme 1), and only a few examples of isomerization in
form a resonance-stabilized carbocation, the reaction would
become irreversible, giving an allene in a high yield under
relatively mild conditions. We now report the clean trans-
formation of 7-ethynylcycloheptatriene and its derivative into
arylallenes via an arenium ion in the presence of acid.
When heated at 60 °C in a solution of THF containing
2.2 M (17 vol %) of trifluoroacetic acid (TFA), 7-ethynyl-
cycloheptatriene 13 isomerized to phenylallene 2 with a half-
life of 9 days. After 50 days, 2 was obtained in nearly
quantitative yield.4 The isomerization was much faster in
methanol containing 0.5 M HCl, wherein the reaction was
complete in 9 h at 60 °C.
Scheme 1
(1) (a) Olah, G. A.; Molna´r, AÄ . Hydrocarbon Chemistry; Wiley-
Interscience: New York, 1995; pp 120-122. (b) Landor, P. D. In The
Chemistry of the Allenes; Landor, S. R., Ed.; Academic Press: London,
1982; Vol. 1, Chapter 2. (c) The Chemistry of Ketenes, Allenes, and Related
Compounds; Patai, S., Ed.; John Wiley & Sons: Chichester, 1980; Part 2,
Chapters 15 and 20. (d) Buehler C. A.; Pearson D. E. SurVey of Organic
Syntheses; Wiley-Interscience: New York, 1970; Vol. 1, Chapter 3. (e)
Taylor, D. R. Chem. ReV. 1967, 67, 317.
acidic media have been reported.2 One of the practical
disadvantages of the acetylene-allene rearrangement is its
reversibility, the result of which is that the product is often
a mixture of the desired allene and isomeric acetylenes. One
would expect that, if the formation of an allenyl group is
accompanied by a concomitant skeletal rearrangement to
(2) Barry, B. J.; Beale, W. J.; Carr, M. D.; Hei, S.-K.; Reid, I. J. Chem.
Soc., Chem. Commun. 1973, 177.
(3) 7-Ethynylcycloheptatriene 1 was synthesized in 68% yield by the
reaction of tropylium tetrafluoroborate with 0.97 equiv of ethynyllithium
in THF at -78 °C. A byproduct, 1,2-bis(2,4,6-cycloheptatrien-1-yl)ethyne
was separated by MPLC and obtained in 5.6% yield. The 1H NMR spectra
of both products were identical with those published: Hoskinson, R. M.
Aust. J. Chem. 1970, 23, 399.
10.1021/ol006328e CCC: $19.00 © 2000 American Chemical Society
Published on Web 08/17/2000

Scheme 2 shows a plausible pathway for the formation of
phenylallene. Cycloheptatriene 1 initially undergoes valence
and the increased population of the NCD form (20.5% at
-120 °C, as determined by NMR7) for 5 should accelerate
the acid-catalyzed rearrangement. Heating 5 in the presence
of TFA (2.2 M) in THF at 60 °C resulted in quantitative
isomerization to (2,5-di-tert-butylphenyl)allene 6 (Scheme
3), a product predicted by the mechanism shown in Scheme
2, in 4 h.
Scheme 2
Scheme 3
For a kinetic study, the rearrangement of 1 and 5 was
conducted in THF-d8 in the presence of TFA (2.2 M) at 60
°C and monitored by 1H NMR.8 The reactions followed first-
order kinetics, giving rate constants of 8.79 × 10-7 (1) and
3.22 × 10-4 (5) s-1. Thus, an acceleration of the reaction
by a factor of 370 was achieved.
tautomerism to the norcaradiene form, the protonation of the
terminal sp carbon of which generates a vinyl cation 3.
Subsequent cleavage of the three-membered ring forms an
arenium ion containing an allenyl group, 4, which, on
deprotonation, readily affords the final product.
In general, thermal equilibrium between a cycloheptatriene
(CHT) and its norcaradiene tautomer (NCD) lies heavily in
favor of the CHT form. HF/6-31G* calculations have shown
that the difference in free energies between the parent CHT
and NCD is 5.6 kcal mol-1.5 Although the π-acceptor ability
of the ethynyl group attached at C-7 is expected to shift the
equilibrium to some extent toward the NCD side, the
concentration of the NCD tautomer at equilibrium is still
too low to be detected by NMR. We have reported that the
introduction of bulky substituents to the olefinic carbon of
the cycloheptatriene ring raises the energy level of the CHT
form.6,7 Thus, 2,5-di-tert-butyl-7-ethynylcycloheptatriene 5
is known to exist as a mixture of valence (and conforma-
tional) isomers, the populations of which have been deter-
mined by low-temperature NMR measurements (Figure 1).7
With the expectation that the intermediate vinyl cation, 3
or its tert-butylated form, can be trapped by a nucleophile,9
the products of the reaction of 1 and 5 in MeOH in the
presence of HCl (0.1-0.5 M) at 60 °C were carefully
analyzed. However, the reactions resulted in the complete
conversion to phenylallenes 2 and 6, with no methyl ether
or chlorinated compounds being detected. The failure to trap
cationic intermediates can be reasonably explained by the
rapid conversion of the vinyl cation to the arenium ion due
1
(4) Compound 2 was identified by comparison of its H and 13C NMR
spectra with those reported in the literature: Maercker, A.; Fischenich, J.
Tetrahedron 1995, 51, 10209. Okuyama, T.; Izawa, K.; Fueno, T. J. Am.
Chem. Soc. 1973, 95, 6749. Spectral data for compound 6: colorless yellow
oil; 1H NMR (270 MHz, CDCl3) δ 7.50 (d, J ) 2.3 Hz, 1H), 7.31 (d, J )
8.6 Hz, 1H), 7.17 (dd, J ) 8.6, 2.3 Hz, 1H), 6.81 (t, J ) 6.8 Hz, 1H), 5.09
(d, J ) 6.8 Hz, 2H), 1.43 (s, 9H), 1.31 (s, 9H); 13C NMR (68 MHz, CDCl3)
δ 209.5 (C), 148.5 (C), 143.6 (C), 131.2 (C), 126.9 (CH), 125.8 (CH), 123.9
(CH), 94.4 (CH), 77.5 (CH2), 35.0 (C), 34.2 (C), 31.4 (CH3), 31.2 (CH3).
HRMS (FAB) calcd for C17H24: 228.1878; found: 228.1875.
(5) Cremer, D.; Dick, B. Angew. Chem., Int. Ed. Engl. 1982, 21, 865.
(6) (a) Takeuchi, K.; Kitagawa, T.; Ueda, A.; Senzaki, Y.; Okamoto, K.
Tetrahedron 1985, 41, 5455. (b) Takeuchi, K.; Fujimoto, H.; Kitagawa,
T.; Fujii, H.; Okamoto, K. J. Chem. Soc., Perkin Trans. 2 1984, 461. (c)
Takeuchi, K.; Kitagawa, T.; Senzaki, Y.; Fujimoto, H.; Okamoto, K. Chem.
Lett. 1983, 69. (d) Takeuchi, K.; Kitagawa, T.; Toyama, T.; Okamoto, K.
J. Chem. Soc., Chem. Commun. 1982, 313. (e) Takeuchi, K.; Fujimoto, H.;
Okamoto, K. Tetrahedron Lett. 1981, 22, 4981. (f) Takeuchi, K.; Arima,
M.; Okamoto, K. Tetrahedron Lett. 1981, 22, 3081.
(7) Takeuchi, K.; Senzaki, Y.; Okamoto, K. J. Chem. Soc., Chem.
Commun. 1984, 111.
(8) The reaction solutions were prepared by dissolving 1 or 5 in a mixture
of THF-d8 and TFA (5:1 v/v). The solutions, containing 0.03-0.07 M of
the substrate, were placed in a 5 mm o.d. NMR sample tube and were heated
in a thermostated bath. At appropriate intervals the H NMR spectra were
recorded to determine the ratio of the starting compound and the product
by peak integration.
Figure 1. Equilibrium isomer populations (%) of 2,5-di-tert-butyl-
7-ethynylcycloheptatriene 5 and its norcaradiene form (ref 7, at
-120 °C in CS2-CD2Cl2 3:1 v/v). a: valence tautomerism. b: ring
inversion.
1
(9) A review of the trapping of vinyl cation intermediates under solvolytic
conditions: Kitamura, T.; Taniguchi, H.; Tsuno, Y. In Dicoordinated
Carbocations; Rappoport, Z., Stang, P. J., Eds.; John Wiley & Sons:
Chichester, 1997; chapter 7.
The CHT-NCD composition at equilibrium is a possible
rate-controlling factor of the reaction shown in Scheme 2,
3012
Org. Lett., Vol. 2, No. 19, 2000

to resonance stabilization of the latter cation and the
subsequent irreversible deprotonation to form an aromatic
ring.10,11
The present result suggests the possible transformation of
cycloheptatrienes bearing a directly attached multiple bond
(XdY) other than the ethynyl group into a benzene with a
conjugated CdXsYH group under acidic conditions (Scheme
4). In search of the rearrangement of other 7-substituted
Acknowledgment. This work was supported by a Grant-
in-Aid for Scientific Research on Priority Areas (A) “Cre-
ation of Delocalized Electronic Systems” from the Ministry
of Education, Science, Sports and Culture, Japan.
Supporting Information Available: Spectroscopic data
for 6. This material is available free of charge via the Internet
at http://pubs.acs.org.
Scheme 4
OL006328E
(11) In connection with this finding, the ring opening of the norcaradi-
enylmethyl cation has been proposed to explain the formation of styrene in
the solvolysis of (2,4,6-cycloheptatrien-1-yl)methyl 3,5-dinitrobenzoate:
Sargent, G. D.; Lowry, N.; Reich, S. D. J. Am. Chem. Soc. 1967, 89, 5985.
See also Warner, P. M.; Lu, S.-L. J. Am. Chem. Soc. 1980, 102, 331.
Thompson, G. L.; Heyd, W. E.; Paquette, L. A. J. Am. Chem. Soc. 1974,
96, 3177. It has been also reported that dialkoxy- and alkoxyamino-
substituted norcaradienylmethyl cations rearrange to 1,1-disubstituted-2-
phenylethyl cations under stable-ion conditions: Betz, W.; Daub, J.; Rapp,
K. M. Liebigs Ann. Chem. 1974, 2089.
cycloheptatrienes via a protonation-deprotonation mecha-
nism, isomerization of vinyl and cyanocycloheptatrienes is
now underway.
(10) DFT calculations (B3LYP/6-31G*) indicated that the rearrangement
of 3 to 4 is exothermic by 2.3 kcal mol-1
.
Org. Lett., Vol. 2, No. 19, 2000
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