Publication Date:
2012-09-26
Description:
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 k 2 ) 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. The rate constants change on about seven orders of magnitude: from 3.95 10 −3 L mol −1 s −1 in reaction (1+2) to 12200 L mol −1 s −1 in reaction of (1+4e) in toluene solution at 298 K. The heat of reaction (1+2) is −218±2 kJ mol −1 , (1+4e) is −117.8±0.7 kJ mol −1 , and (1+4f) is −91.6 ±0.2 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 energy of breaking bonds can be the main reason of unusual high reactivity of 1 in the Diels–Alder cycloaddition reaction.
Print ISSN:
0894-3230
Electronic ISSN:
1099-1395
Topics:
Chemistry and Pharmacology
,
Physics
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