Similar catalytic behaviour of oximate and phenoxide bases in the ionization of bis(2,4-dinitrophenyl)methane in 50% water– 50% Me2SO. Revisiting the role of solvational imbalances in determining the nucleophilic reactivity of α-effect oximate bases

Abstract

Rates of proton abstraction from bis(2,4-dinitrophenyl)methane 1 by a series of oximate bases and of reprotonation of the related diphenylmethyl carbanion C-1 by the conjugate oxime acids have been measured in a 50∶50 (v/v) H₂O–Me₂SO mixture at 25 °C. The results reveal an essentially identical behaviour of oximate (Ox^-) and phenoxide (ArO^-) bases in these proton transfer reactions. The Brønsted lines for the two types of catalysts are nearly the same, showing no tendency to level off at high pK_a and providing very similar values for the intrinsic reactivity of 1: log k₀^Ox = 0.9 ± 0.1; log k₀^ArO = 0.5 ± 0.1. A comparison with previous results obtained with carboxylate ions as well as primary and secondary amines indicates that solvational imbalances due to the catalysts are important in determining the intrinsic reactivity of 1. This conclusion implies that ArO^- and Ox^- bases undergo comparable solvation changes along the coordinate of the ionization reactions of 1, a result which is indirectly supported by the finding that a transfer from water to H₂O–Me₂SO mixtures rich in Me₂SO induces similar variations in the acidity of oximes and phenols of similar pK_a^H₂O. The identical behaviour of ArO^- and Ox^- bases in the ionization of 1 is in marked contrast with the situation observed in nucleophilic addition or substitution reactions. In agreement with their α-effect character, oximates are much more reactive than phenoxides in these processes but a typical feature is that the nucleophilic reactivity of Ox^- species is subject to a very rapid levelling off at pK_a ca. 8.5. Based on the information obtained in this work, this peculiar behaviour is re-examined and suggested to be a reflection of especially large solvational imbalances in the transition states for nucleophilic reactions.