Electrophilic aromatic substitutions: reactions of hydroxy- and methoxy-substituted benzenes with 4,6-dinitrobenzofuroxan: kinetics and mechanism

✍ Scribed by François Terrier; Marie-José Pouet; Jean-Claude Halle; Elyane Kizilian; Erwin Buncel

• Publisher: John Wiley and Sons
• Year: 1998
• Tongue: English
• Weight: 223 KB
• Volume: 11
• Category: Article
• ISSN: 0894-3230
• DOI: 10.1002/(sici)1099-1395(1998100)11:10<707::aid-poc37>3.0.co;2-v

✦ Synopsis

Rate constants have been determined in aqueous Me 2 SO 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 weakly basic aromatics with DNBF, from the family of indoles previously studied with pK a values ranging from -1 to -6. The overall rate constants for the reactions of DNBF as the electrophile are at least one order of magnitude greater than for the reactions of H 3 O with the same series of aromatics. This lends further credence to the notion that DNBF possesses super-electrophilic properties. An LFER is observed between logk DNBF 50%Me 2 SO and pK H 2 O a with slope 0.54. In the case of 1,3,5-trimethoxybenzene a significant kinetic isotope effect (KIE) is observed (k H /k D = 3.71 in 50% Me 2 SO). This system hence affords one of the few instances in which a KIE has been observed in S E Ar reactions. It follows from the observed KIE that the addition of DNBF to the aromatic is not rate-limiting and that reversion to reactants and proton loss from the arenonium intermediate occur at comparable rates. Structures of the products of electrophilic substitution have been confirmed by 1 H NMR. In all cases the regiochemistry of the reactions was identical to that observed in protonation studies of the starting aromatics.