Homo‐Diels–Alder reaction of a very inactive diene,bicyclo[2,2,1]hepta‐2,5‐diene,with the most active dienophile, 4‐phenyl‐1,2,4‐triazolin‐3,5‐dione. Solvent,temperature, and high pressure influence on the reaction rate |
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Authors: | Vladimir D. Kiselev Ilzida I. Shakirova Dmitry A. Kornilov Helen A. Kashaeva Lubov N. Potapova Alexander I. Konovalov |
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Affiliation: | Department of Physical Chemistry, Butlerov Institute of Chemistry, Kazan Federal University, , Kazan, 420008 Russian Federation |
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Abstract: | Solvent, temperature, and high pressure influence on the rate constant of homo‐Diels–Alder cycloaddition reactions of the very active hetero‐dienophile, 4‐phenyl‐1,2,4‐triazolin‐3,5‐dione (1), with the very inactive unconjugated diene, bicyclo[2,2,1]hepta‐2,5‐diene (2), and of 1 with some substituted anthracenes have been studied. The rate constants change amounts to about seven orders of magnitude: from 3.95.10?3 for reaction (1+2) to 12200 L mol?1 s?1 for reaction of 1 with 9,10‐dimethylanthracene (4e) in toluene solution at 298 K. A comparison of the reactivity (ln k2) and the heat of reactions (?r‐nH) of maleic anhydride, tetracyanoethylene and of 1 with several dienes has been performed. The heat of reaction (1+2) is ?218 ± 2 kJ mol?1, of 1 with 9,10‐dimethylanthracene ?117.8 ± 0.7 kJ mol?1, and of 1 with 9,10‐dimethoxyanthracene ?91.6 ±0.2 kJ mol?1. From these data, it follows that the exothermicity of reaction (1+2) is higher than that with 1,3‐butadiene. However, the heat of reaction of 9,10‐dimethylanthracene with 1 (?117.8 kJ mol?1) is nearly the same as that found for the reaction with the structural C=C counterpart, N‐phenylmaleimide (?117.0 kJ mol?1). Since the energy of the N=N bond is considerably lower (418 kJ/bond) than that of the C=C bond (611 kJ/bond), it was proposed that this difference in the bond energy can generate a lower barrier of activation in the Diels–Alder cycloaddition reaction with 1. Linear correlation (R = 0.94) of the solvent effect on the rate constants of reaction (1+2) and on the heat of solution of 1 has been observed. The ratio of the volume of activation (?V≠) and the volume of reaction (?Vr‐n) of the homo‐Diels–Alder reaction (1+2) is considered as “normal”: ?V≠/?Vr‐n = ?25.1/?30.95 = 0.81. Copyright © 2012 John Wiley & Sons, Ltd. |
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Keywords: | activation volume bicyclo[2,2,1]hepta‐2,5‐diene heat of reaction homo‐Diels– Alder reaction rate constants reaction volume 4‐phenyl‐1,2,4‐triazolin‐3,5‐dione |
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