Salicyl phosphate dianion hydrolysis

On the other hand, phosphorane intermediates are not expected to be involved in the hydrolysis of phosphate monoesters, so the effective observed catalysis by the carboxyl group of salicyl phosphate 3.21 [51] (Scheme 2.26) is presumed to be concerted vith nucleophilic attack. (The hydrolysis reaction involves the less abundant tautomer 3.22 of the dianion 3.21, and the acceleration is >10 -fold relative to the expected rate for the pH-independent hydrolysis of the phosphate monoester dianion of a phenol of pK 8.52.) However, this system differs from the methoxy-methyl acetals discussed above, in that there is a clear distinction between neutral nucleophiles, which react through an extended transition structure similar to 3.16 in Scheme 2.23, and anions, which do not react at a significant rate, presumably because of electrostatic repulsion. This distinction is well-established for the dianions of phosphate monoesters with good leaving groups (p-nitrophenyl [52] and... 

Several studies on intramolecular catalysis of the solvolysis of phosphate esters have been reported. The larger hydrolysis rate of the zwitterion of 8-hydroxyquinoline phosphate (20) compared with that of pyridyl 3-phosphate (21) was attributed, on the basis of the kinetic isotope effect, to intramolecular general acid catalysis. A similar general acid catalysis by the hydroxy-group seems to operate in the (3-hydroxy-2-pyridyl)methyl phosphate dianion (22), which hydrolyses faster than either the monoanion or the neutral molecule. From a study of 4- and 5-substituted derivatives of salicyl phosphate (23) it is suggested that the negligible solvent isotope effect is inconsistent with preliminary proton transfer, and that here also intramolecular general acid catalysis by the... 

The requirement for the carboxyl group to lie in the plane of the aromatic ring in order to provide intramolecular catalysis appears to be more stringent here than in the hydrolysis of the acetals. Thus the dianion of salicyl phosphate is hydrolysed about thirty three times... 

Intramolecular catalysis may occur in the hydrolysis of the dianion of 8-carboxy-l-naphthyl phosphate, but the rate enhancement is not as large as that found in the hydrolysis of salicyl phosphate. The rate constant is only about 7.5 times greater than that for hydrolysis of the dianion of m-carboxyphenyl phosphate [118]. This result is similar to that found with the analogous acetals (see p. 345) and presumably the smaller rate enhancement arises from the greater difficulty of proton transfer via a seven-membered ring in this stereochemical situation. 

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