6432
J. Am. Chem. SOC.1981, 103, 6432-6436
Hydrolysis of Spirobicyclic Oxyphosphoranes. Associative and Dissociative Reactions Robert A. McClelland,* Geeta Patel, and Corrado Cirinna Contribution from the Department of Chemistry, University of Toronto, Toronto, Ontario, Canada MSS 1 A I . Received March 16, 1981
Abstract: 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 p H 9.54 sharply changing there to an OH--catalyzed reaction. The acid reaction is general acid catalyzed (a = 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 p 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 p value (+2.5) and a significantly negative entropy of activation. General base catalysis is also observed; this also appears to be a n associative process on the basis of a positive p value.
The hydrolysis of phosphate is thought to proceed in many cases by way of a pentacoordinate intermediate. A large number of analogues of this intermediate, commonly termed oxyphosphoranes, have now been prepared (see ref 3-10 for some recent examples including some observable hydroxyphosphoranes). The structural characteristics of these species have been studied extensively, particularly with respect to their tendency to undergo permutational isomerization, and a reasonable understanding of this aspect of their chemistry has emerged.1J1s12 The hydrolysis reactions of oxyphosphoranes,on the other hand, have seen little mechanistic study, principally because of the speed of the r e a ~ t i o n . ' ~ , 'A~ detailed investigation involving pentaaryloxyphosphoranes (P(OAr)5) has been reported by Archie and Westheimer,14although a mixed aqueous organic solvent mixture was required to keep the rates in an observable range. We report here a kinetic study of the hydrolysis of the spirobicyclic oxyphosphoranes Ia-c. We in particular felt it of interest CH-
O/-
