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2-Carboxyphenyl diphenyl phosphate is rapidly hydrolysed to salicylic acid and diphenyl phosphate, in a reaction subject to highly efficient catalysis by the ionised carboxy-group. A rate enhancement of the order of 10" is observed, and there is convincing evidence that the reaction involves intramolecular nucleophilic catalysis. The stereochemistry of this non-linear displacement a t phosphorus is in striking contrast to the corresponding reaction of the monophenyl ester, in which selective exocyclic displacement occurs, but is consistent with Westheimer's rules for pseudorotation. Kinetic and product studies with five aryl 2.3-dicarboxyphenyl phenyl phosphates support the proposed mechanism, and show that intramolecular displacements on these phosphate triesters depend exclusively on the basicity of the leaving group, and not a t all on geometry. Linear free-energy relationships show that the sensitivity to the basicity of the leaving group is the largest yet measured for a displacement reaction. and allow order-of-magnitude calculations of rate constants for the breakdown of pentacovalent intermediates.WE have shown that the hydrolysis of 2-carboxyphenyl phenyl phosphate (1) involves intramolecular nucleophilic catalysis by the ionised carboxy-group. Exocyclic displacement of phenoxide accounts for over 99% of the reaction, although this is the poorer of the two possible leaving groups. The hydrolysis of dialkyl Z-carboxyphenyl phosphates (2) also involves intramolecular nucleophilic catalysis by the carboxylate group,2 but in this case displacement is exclusively endocyclic, and the products are salicylic acid and the dialkyl phosphate.We consider that displacement is stereospecific in the first case because reaction proceeds by way of a quinquel S.

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