ELECTROPHILIC AROMATIC SUBSTITUTIONS - REACTIONS OF HYDROXY-SUBSTITUTED AND METHOXY-SUBSTITUTED BENZENES WITH 4,6-DINITROBENZOFUROXAN - KINETICS AND MECHANISM

Rate constants have been determined in aqueous Me2SO mixtures for the reaction of super-electrophilic 4,6-dinitrobenzofuroxan (DNBF) with a series of hydroxy- and methoxy-substituted benzenes whose pK(a) values range between -3 and -9. The study extends the reactivity range of we akly basic aromatics with DNBF, from the family of indoles previously studied with pK(a) values ranging from -1 to -6. The overall rate cons tants for the reactions of DNBF as the electrophile are at least one o rder of magnitude greater than for the reactions of H3O+ with the same series of aromatics. This lends further credence to the notion that D NBF possesses super-electrophilic properties. An LFER is observed betw een logk(50%Me2SO)(DNBF) and pK(a)(H2O) with Slope 0.54. In the case o f 1,3,5-trimethoxybenzene a significant kinetic isotope effect (KIE) i s observed (k(H)/k(D) = 3.71 in 50% Me2SO). This system hence affords one of the few instances in which a KIE has been observed in SEAr reac tions. It follows from the observed KIE that the addition of DNBF to t he aromatic is not rate-limiting and that reversion to reactants and p roton loss from the arenonium intermediate occur at comparable rates. Structures of the products of electrophilic substitution have been con firmed by H-1 NMR. In all cases the regiochemistry of the reactions wa s identical to that observed in protonation studies of the starting ar omatics. (C) 1998 John Wiley & Sons, Ltd.