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.

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-n H) 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 (∆ V _r-n) of the homo-Diels-Alder reaction (1+2) is considered as 'normal': ∆ V^≠/∆ V _r-n = −25.1/−30.95 = 0.81. Copyright © 2012 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]