Journal of the Chemical Society. Perkin Transactions 2 (2001) p. 2285 - 2291 (1996)
Update date:2022-08-03
Topics:
Colthurst, Matthew J.
Nanni, Matilde
Williams, Andrew
Rate constants have been measured for the reaction of substituted phenolate ions with aryl acetate esters and with aryl N-methylisonicotinate esters? in aqueous solution. A new method is demonstrated for determining βeq 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 βeq = 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 pKa values from 5.5 to 10 and provides good evidence for a concerted displacement mechanism for this reaction. The reactivity of the N-methylisonicorinate esters to phenolate ions is some 300 times larger than that of the corresponding acetate esters but the larger βnuc value (0.90 compared with 0.74) suggests a 'later' transition structure. Calibration of the β values with the corresponding βeq gives a Leffler αnuc = 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 α values is explained by a less favourable acylium ion in the case of the N-methylisonicotinyl transfer reaction.
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