Reactivity of phosphate monoester monoanions in aqueous solution.: 1.: Quantum mechanical calculations support the existence of "anionic zwitterion" MeO+(H)PO3 2- as a key intermediate in the dissociative hydrolysis of the methyl phosphate anion

The dissociative hydrolysis reaction of the methyl phosphate monoanion has been studied for the reactant species CH3OPO3H- (1) and CH3OPO3H-.H2O (1a) in the gas and aqueous phases by density functional theory (B3LYP) calculations. Nonspecific solvation effects were taken into account with the polarizable continuum model PCM either by solvating the gas-phase reaction paths or by performing geometry searches directly in the presence of the solvation correction. In agreement with previous theoretical studies, our gas-phase calculations indicate that proton transfer to the methoxy group of 1 is concerted with P-O bond cleavage. In contrast, optimizations performed with the PCM solvation model establish the existence of the tautomeric form CH3O+(H)PO32- (2) as an intermediate, indicating that proton transfer and P-O bond cleavage become uncoupled in aqueous solution. The dissociative pathway of la is energetically favored over the dissociative pathway of 1 only when the added water molecule plays an active catalytic role in the prototropic rearrangement 1 <----> 2. In that case, it is found that the collapse (via P-O bond cleavage) of the hydrated zwitterionic form CH3O+(H)PO32-.H2O (2a) is rate-determining. This collapse may occur by a stepwise mechanism through a very short-lived metaphosphate intermediate (PO3-), or by a concerted S(N)2-like displacement through a loose metaphosphate-like transition state. The present calculations do not allow a distinction to be made between these two alternatives, which are both in excellent agreement with experiment. The present study also reveals that PO3- reacts selectively with CH3OH and H2O nucleophiles in aqueous solution. However, the observed selectivity of metaphosphate is governed by solvation effects, not nucleophilicity (water being much more effective than methanol in capturing PO3-). This arises from a better solvation of the addition product H2O+PO32- as compared to CH3O+(H)PO32-.