J . Org. Chem. 1999, 64, 7407-7411
7407
Mech a n istic Stu d y on Th er m a l Isom er iza tion of
1-Meth ylben zocyclobu ten ol to 2-Meth yla cetop h en on e
Katsumi Iida, Kaori Komada, Masaichi Saito, and Michikazu Yoshioka*
Department of Chemistry, Faculty of Science, Saitama University, Shimo-ohkubo, Urawa,
Saitama 338-8570, J apan
Received April 27, 1999
Heating 1-trideuteriomethylbenzocyclobutenol 7 in benzene-d6 at 160 °C gave 2-monodeuterio-
methyl- and 2-methylacetophenone 9 and 10 in a ratio of 96:4. Thermolysis of 7 in nonpolar solvents
(hexane, toluene, mesitylene) gave similar results. On the contrary, heating 7 in polar solvents
(ethanol, acetonitrile, chloroform) or in benzene-d6 in the presence of proton source (PhCO2H) gave
10 as the major product. However, heating a mixture of 7 and N-phenylmaleimide at 160 °C in
benzene-d6 or acetonitrile-d3 gave adduct 12 of N-phenylmaleimide and the dienol generated by
ring opening of 7 almost quantitatively. These results indicate that 1-methylbenzocyclobutenol
undergoes selective thermal opening to the E-dienol. The resulting E-dienol isomerizes to
2-methylacetophenone by 1,5-sigmatropic shift of hydrogen from the methyl group in nonpolar
solvent. In polar solvent, the E-dienol isomerizes to 2-methylacetophenone by both intra- and
intermolecular processes. The kH/kD value for isomerization was 1.13. Since this relatively low value
is a secondary kinetic effect, overall reaction is governed by the ring opening step. The selective
opening to the E-dienol was supported by calculation.
In tr od u ction
(MA) in refluxing toluene gave the adduct 2 together with
2-methylacetophenone 3 in a ratio of 2:1 (Scheme 1),
implying that at least 67% of the benzocyclobutenol 1 was
converted to the E-dienol which reacted with maleic
anhydride to give 2.6b However, heating 1 alone gave
cleanly 2-methylacetophenone 3. Therefore, compound 3
can arise from both Z- and E-dienols. Although the
Z-dienol is converted to 3 with a rapid 1,5-sigmatropic
hydrogen shift from the OH group, the E-dienol can also
afford 3 by a sigmatropic hydrogen shift from the methyl
group followed by ketonization or as a result of intermo-
lecular hydrogen transfers. Since 1-methoxy-1-methyl-
1,2-dihydrobenzocyclobutene was smoothly and quanti-
tatively converted into 2-methyl-R-methoxystyrene on
heating via a 1,5-sigmatropic hydrogen shift from the
E-diene,6b 2-methylacetophenone 3 from 1 was assumed
to arise by an intramolecular process from the E-dienol.
However, the mechanism for ketonization of the E-dienol
in polar solvent has remained obscure, because the
lifetime of E-dienols is shortened under acidic or basic
conditions.8
Aryl ketones with o-alkyl groups are known to undergo
a highly efficient photoinduced enolization.1 The triplet
states of these ketones are transformed into triplet
biradicals via γ-hydrogen abstraction, which decay to Z-
and E-isomers of the dienol in the electronic ground
state.2 These enols have quite similar spectroscopic
properties but are widely different in their kinetic
behavior.2 In nonpolar solvent, the Z-dienols undergo a
rapid 1,5-sigmatroic hydrogen shift to regenerate the
starting ketones (>106 s-1).2 On the contrary, the E-
dienols are sufficiently long-lived to cyclize to benzocy-
clobutenols3 or to react with various dienophiles such as
olefins4 and O2.5
The dienol species can also be generated by thermal
electrocyclic ring opening of benzocyclobutenols.1a,6 The
dienols thus generated are trapped with various dieno-
philes to give cycloadducts.7 Sammes and co-workers
reported that the benzocyclobutenols underwent selective
ring opening of the cyclobutene ring to the E-dienol on
the basis of trapping reactions.6a Heating 1-methyl-1,2-
dihydrobenzocyclobuten-1-ol 1 with maleic anhydride
We report here that 1-methylbenzocyclobutenol un-
derwent highly selective opening to the E-dienol regard-
less of solvent polarity and that the resulting E-dienol
isomerized to 2-methylacetophenone by intra- and inter-
molecular processes.
(1) (a) Sammes, P. G. Tetrahedron 1976, 32, 405. (b) Das, P. K.;
Encinas, M. V.; Small, R. D., J r.; Scaiano, J . C. J . Am. Chem. Soc.
1979, 101, 6965. (c) Scaiano, J . C. Acc. Chem. Res. 1982, 15, 252.
(2) Haag, R.; Wirz, J .; Wagner, P. J . Helv. Chim. Acta 1977, 60, 2595.
(3) (a) Wagner, P. J .; Subrahmanyam, D.; Park, B.-S. J . Am. Chem.
Soc. 1991, 113, 709. (b) Wagner, P. J .; Sobczak, M.; Park, B.-S. J . Am.
Chem. Soc. 1998, 120, 2488.
(4) (a) Arnold, B. J .; Mellows, S. M.; Sammes, P. G.; Wallace, T. W.
J . Chem. Soc., Perkin Trans. 1 1974, 401. (b) Pfau, M.; Combrisson,
S.; Rowe, J . E., J r.; Heindel, N. D. Tetrahedron 1978, 34, 3459. (c)
Durst, T.; Kozma, E. C.; Charlton, J . L. J . Org. Chem. 1985, 50, 4829.
(d) Wilson, R. M.; Hannemann, K.; Heinman, W. R.; Kirchhoff, J . R.
J . Am. Chem. Soc. 1987, 109, 4743. (e) Charlton, J . L.; Chee, G.;
McColeman, H. Can. J . Chem. 1995, 73, 1454.
(5) Yoshioka, M.; Nishizawa, K.; Suzuki, J .; Iwata, Y.; Kumakura,
S.; Hasegawa, T. J . Chem. Soc., Perkin Trans. 1 1995, 3097.
(6) (a) Arnold, B. J .; Sammes, P. G.; Wallace, T. W. J . Chem. Soc.,
Perkin Trans. 1 1974, 409. (b) Arnold, B. J .; Sammes, P. G.; Wallace,
T. W. J . Chem. Soc., Perkin Trans. 1 1974, 415. (c) Charlton, J . L.;
Alauddin, M. M. Tetrahedron 1987, 43, 2873.
Resu lts a n d Discu ssion
P r ep a r a t ion of 1-Met h yl- a n d 1-Tr id eu t er io-
m eth yl-1,2-d ih yd r oben zocyclobu ten ols. The benzo-
(7) (a) Oppolzer, W. Synthesis 1978, 793. (b) Charlton, J . L.; Plourde,
G. L.; Koh, K.; Secco, A. S. Can. J . Chem. 1990, 68, 2022. (c) Charlton,
J . L.; Koh, K.; Plourde, G. L. Can. J . Chem. 1990, 68, 2028. (d)
Charlton, J . L.; Maddaford, S. Can. J . Chem. 1993, 71, 827. (e)
Charlton, J . L.; Maddaford, S.; Koh, K.; Boulet, S.; Saunders, M. H.
Tetrahedron: Asymmetry 1993, 4, 645. (f) Charlton, J . L.; Bogucki,
D.; Guo, P. Can. J . Chem. 1995, 73, 1463. (g) Coltart, D. M.; Charlton,
J . L. Can. J . Chem. 1996, 74, 88.
(8) Porter, G.; Tchir, M. F. J . Chem. Soc. A 1971, 3772.
10.1021/jo990705n CCC: $18.00 © 1999 American Chemical Society
Published on Web 09/10/1999