Transfer of a positively charged acyl group between substituted phenolate ion nucleophiles: The Bronsted beta for the calibrating equilibrium for N-methylisonicotinyl (4-carbonyl-N-methylpyridinium) transfer

Rate constants have been measured for the reaction of substituted phenolate ions with aryl acetate esters and with aryl N-methylisonicotinate esters dagger. in aqueous solution. A new method is demonstrated for determining beta(eq) for group transfer from 4-nitrophenyl esters; it employs the rate constant for the reaction of 2,6-difluorophenolate ion with substituted phenyl ester as a surrogate for the reactivity of the 4-nitrophenolate ion and yields beta(eq) = 1.55 for the N-methylisonicotinyl transfer reaction. The Bronsted-type plot of the rate constant for phenolate ion attack on 4-nitrophenyl N-methylisonicotinate is linear over a range of pK(a) values from 5.5 to 10 and provides good evidence for a concerted displacement mechanism for this reaction. The reactivity of the N-methylisonicotinate esters to phenolate ions is some 300 times larger than that of the corresponding acetate esters but the larger B-nuc value (0.90 compared with 0.74) suggests a 'later' transition structure. Calibration of the beta values with the corresponding beta(eq) gives a Leffler alpha(nuc) = 0.58 and 0.42 for N-methylisonicotinate and acetate respectively, which contrasts with the order expected from reactivity-selectivity. The tighter transition structure indicated by comparison of these alpha values is explained by a less favourable acylium ion in the case of the N-methylisonicotinyl transfer reaction.