Zur Kupplung von 4-substituierten 2-Pyrazolin-5-onen mit p-Chinondiiminkationen; Nachweis isokinetischer Beziehungen

On the Coupling of 4-Substituted 2-Pyrazoline-5-ones with Quinone Diimine Cations; Detection of Isokinetic Relations

The temperature dependence of the rate constant k_K of the coupling reaction between pyrazoline-5-ones 1a–i and quinone diimine cations 2a, b is determined in dependence of the substituents at the coupling position (R³: H, NNAr,CHAr–pyrazoline-5-one). The activation enthalpies are between 7 and 60 kJ mol⁻¹ and the activation entropies between −132 and −19 J mol⁻¹ K⁻¹. An isokinetic relation with β = 106°C is found and a rate determining combination of the carbanions of 1a–i with 2a, b is proposed. Contrary to this, the 2-pyrazolin-5-ones 1j–l, n (R³: SCNH₂NR) obey an isokinetic relation with β = −45°C. Taking into consideration the activation parameters, the low dependence of k_K on the ionic strength, and the decrease of k_K with the increase of the pK values of the couplers, a rate determining splitting off of R³ is assumed.