J . Org. C h e m . 1985, 50, 1106-1107
1106
Notes Stability of Cycloadducts Obtained by High-pressure Diels-Alder Reaction between 3,4-Dimethoxyfuran and 1,4-Benzoquinones: Kinetic Studies of Retro-Diels-Alder Reaction Janusz Jurczak,' Andrzej L. Kawczyfiski, and Tomasz Koiluk Institute of Organic Chemistry and Institute of Physical Chemistry, Polish Academy of Sciences, 01-224 Warszawa, Poland
Scheme I a
endo 0 exo 0 a (1)20 kbar, room temperature, ( 2 ) 1 atm, room temperature or higher.
Scheme I1
Received M a y 29, 1984
We have recently shown that application of high-pressure conditions (22 kbar, 25 OC, ethyl ether as solvent) enabled the first successful realization of the [4 21 cycloaddition reaction between furan and 1,4-benzoquinone.l It is stressed that the earlier failure of this reaction, when performed under thermal conditions, is evident in the light of our findings;l it is in agreement with the assumption that the equilibrium is strongly shifted toward the substrates and that the cleavage reaction is very fast. In fact, the endo and exo adducts (Scheme I), obtained by highpressure [4 + 21 cycloaddition of 1,Cbenzoquinone to furan, have been found to be extremely unstable under normal pressure,l this preventing measurements of the retro-Diels-Alder kinetics. However, it is well-known that 3,4-dimethoxyfuran (1) reacts easily with various 1,4-benzoquinones 2 to give the much more stable cycloadducts 3.233 Therefore, the cycloadducts of this type seem to be proper models for testing the influence of substituents on the rates of the retro-Diels-Alder reaction. The goal of our present kinetic measurements was to verify the hypothesis that the stability of the cycloadducts in solution under atmospheric pressure mainly depends on the substituents in the furan ring.
+
Results and Discussion In order to examine the above hypothesis, we synthesized under high pressure three pure crystalline endo adducts (3a, 3b and 3c)475and observed the changes in their 'H NMR spectra measured in a chloroform-d solution at various temperatures (Scheme 11). The initial spectra of the cycloadducts 3 as well as the final spectra originating from 3,4-dimethoxyfuran (1) and 1,4-benzoquinones 2 are relatively simple and convenient for quantitative treatment. For example, the initial spectrum of 3a is as follows: 6 6.71 (s, 2 vinyl H), 5.22 (bs, 2 HA) 3.68 (s, 6 H,OCH,), 3.50 (bs, 2 HB); the final spectrum shows the signals derived form l,4-benzoquinone (1)Jurczak, J.; Koiluk, T.; Filipek, S.; Eugster, C. H. Helu. Chim. Acta 1983, 66, 222.
(2) Hofmann, A. A.; Wyrsch-Walraf, J.; Iten, P. X.; Eugster, C. H. Helu. Chim. Acta 1979, 62, 2211. (3) Jurnak, J.; Koiluk, T.; Tkam,M.; Eugster, C. H. Helu. Chim. Acta 1983, 66, 218. (4) According to the 'H NMR spectra, the bridgehead coupling constants for endo adducts 3 were evaluated to be about 3 Hz, and those for the corresponding exo adducts to be
