GENERAL ACID CATALYSIS AND KINETIC ISOTOPE EFFECTS FOR INTRAMOLECULARADDITION-ELIMINATION BETWEEN AMINO AND AMIDE GROUPS IN 1-AMINO-8-TRIFLUOROACETYLAMINONAPHTHALENE - OBSERVATION OF A BIPHASIC BRONSTED PLOT

The intramolecular addition-elimination of 1-amino-8-trifluoroacetylam inonaphthalene to 2-trifluoromethylperimidine in 70% (v/v) Me2SO-H2O i s catalysed by general acids, and catalytic coefficients (k(HA)) for 1 1 buffer acids and hydronium ion have been measured. A Bronsted plot o f logk(HA) against pK(a) gives a slope close to zero for catalysis by acids with pK(a) < ca. 8.5 and a slope close to unity for acids with p K(a) > ca. 8.5. The results are explained by a mechanism involving int ramolecular addition of the amino group to the amide carbonyl to give a zwitterionic intermediate present in low concentration followed by r ate-limiting protonation of the intermediate by hydronium ion and by b uffer acids. The break point in the plot of logk(HA) against pK(a) whi ch occurs at pK(a) 8.5 corresponds to the pK value of the hydroxy grou p of the zwitterionic intermediate. Kinetic solvent isotope effects on the values of k(HA) have been measured and an average value k(HA)/k(D A) 1.52 +/- 0.2 is found. The absence of a maximum in the value of k(H A)/k(DA) with pK(a) confirms that the rate-limiting protonation is con trolled by diffusion steps. The proposed mechanism is preferred to one in which the biphasic Bronsted plot is explained by a change in rate- limiting step from acid-catalysed intramolecular addition in the prese nce of strong acids to acid-catalysed elimination from the addition in termediate in the presence of weak buffer acids.