Participation of two carboxyl groups in phosphodiester hydrolysis. 2. A kinetic, isotopic, and 31P NMR study of the hydrolysis of a phosphodiester with carboxyl groups fixed in an attack conformation

Article abstract of DOI:10.1021/ja00154a006

The phosphodiesters of 4,4′-methylenebis(3-hydroxy-2-naphthoic acid) (4) and 3-carboxy-2,2′-dihydroxydiphenylmethane (5) are constrained into a cyclic structure such that the oxygens of the two o-carboxy groups of 4 and the single o-carboxy group of 5 have restricted stereospecific positions with an o-CO₂^- oxygen to phosphorus distance of 3.7 ?. In the hydrolysis of 4, ³¹P NMR and HPLC data show the existence of an intermediate cyclic acyl phosphate in the g,g conformation. The ¹⁸O isotopic effects on ³¹P chemical shifts show incorporation of two ¹⁸O atoms in the product H₃PO₄. This observation is consistent with intramolecular o-CO₂^- nucleophilic attack on phosphorus to provide an acyl phosphate intermediate which undergoes hydrolytic cleavage by HO^-/H¹⁸O^- attack on phosphorus {one ¹⁸O incorporation} to provide a phosphate monoester which also undergoes hydrolysis with a second ¹⁸O incorporation on phosphorus. For hydrolysis of 4, the pH vs log k_obsd profile, the values of the deuterium solvent kinetic isotope effect, and the activation entropy accord a mechanism which involves intramolecular attack of o-CO₂^- on the phosphate phosphorus assisted by the o-CO₂H as a general acid catalyst. The latter can involve o-CO₂H hydrogen bonding to the -(PO₂^-)- oxygen(s) and/or leaving phenolic oxygen. At neutrality, 4 hydrolyzes ca. 10⁴ fold faster than 5 which only has one o-carboxy group and 10⁸-10⁹-fold faster than diphenyl phosphate.