Comparison of the Electrophilic Reactivities of N-Acylpyridinium Ions and Other Acylating Agents

The kinetics of the reactions of carbanions, carboxylate anions, and primary and secondary amines with acylating agents were studied in acetonitrile and dimethyl sulfoxide (DMSO) under first-order conditions by UV/Vis spectrophotometry and conductimetry. Whereas reactions of a large variety of nucleophiles with ordinary carbanions and Michael acceptors generally follow the linear free energy relationship lgk(2) (20 degrees C) = s(N)(N + E), separate correlation lines for different types of nucleophiles were found when (lg k(2))/s(N) was plotted against N for the reactions with acyl derivatives 2-5. Even when nucleophilic attack is rate determining, the reactivities of acylating agents towards nucleophiles cannot be described by a single set of nucleophile-independent electrophilicity parameters.