SIMILAR CATALYTIC BEHAVIOR OF OXIMATE AND PHENOXIDE BASES IN THE IONIZATION OF BIS(2,4-DINITROPHENYL)METHANE IN 50-PERCENT-WATER-50-PERCENT-ME(2)SO - REVISITING THE ROLE OF SOLVATIONAL IMBALANCES IN DETERMINING THE NUCLEOPHILIC REACTIVITY OF ALPHA-EFFECT OXIMATE BASES

Rates of proton abstraction from bis(2,4-dinitrophenyl)methane 1 by a series of oximate bases and of reprotonation of the related diphenylme thyl carbanion C-l by the conjugate oxime acids have been measured in a 50:50 (v/v) H2O-Me(2)SO mixture at 25 degrees C, The results reveal an essentially identical behaviour of oximate (Ox(-)) and phenoxide (A rO-) bases in these proton transfer reactions, The Bronsted 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 int rinsic reactivity of 1:log k(0)(Ox) = 0.9 +/- 0.1; log k(0)(ArO) = 0.5 +/- 0.1, A comparison with previous results obtained with carboxylate ions as well as primary and secondary amines indicates that solvation al imbalances due to the catalysts are important in determining the in trinsic reactivity of 1, This conclusion implies that ArO- and Ox(-) b ases undergo comparable solvation changes along the coordinate of the ionization reactions of 1, a result which is indirectly supported by t he finding that a transfer from water to H2O-Me(2)SO mixtures rich in Me(2)SO induces similar variations in the acidity of oximes and phenol s of similar pK(a)(H2O). The identical behaviour of ArO- and Ox(-) bas es in the ionization of 1 is in marked contrast with the situation obs erved in nucleophilic addition or substitution reactions,In agreement with their a-effect character, oximates are much more reactive than ph enoxides in these processes but a typical feature is that the nucleoph ilic reactivity of Ox(-) species is subject to a very rapid Ievelling 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 reflectio n of especially large solvational imbalances in the transition states for nucleophilic reactions.