Hydrolysis of Spirobicyclic Oxophosphoranes. Associative and Dissociative Reactions

Article abstract of DOI:10.1021/ja00411a029

The hydrolysis reactions of three aryl-substituted spirobicyclic oxyphosphoranes (Ia-c) have been studied in aqueous solution.These are characterized by an H⁺-catalyzed reaction extending to about pH 9.54 sharply changing there to an OH^--catalyzed reaction.The acid reaction is general acid catalyzed (α=0.74) and appears to be a dissociative process directly analogous to the general acid catalyzed hydrolysis of an acetal or ortho ester.Features indicative of this type of reaction include a negative ρ value based on the variation of the aromatic substituent, a near-zero entropy of activation, and a slightly inverse solvent isotope effect.The hydroxide ion reaction appears to be an associative process proceeding through a hexacoordinated phosphoroxanide ion.Evidence for this includes a positive ρ value (+2.5) and a significantly negative entropy of activation.General base catalysis is also observed; this also appears to be an associative process on the basis of a positive ρ value.