CONCERTED HYDROXYL ION ATTACK AND PSEUDOROTATION IN THE BASE-CATALYZED-HYDROLYSIS OF METHYL ETHYLENE PHOSPHATE

The gas-phase and solution free energy profiles for the base-catalyzed hydrolysis of methyl ethylene phosphate (MEP) were obtained using ab initio molecular orbital calculations and continuum dielectric methods. The correlation energy is estimated with second-order Moller-Plesset theory and the 6-31+G* basis using fully optimized 3-21+G* geometries. In vacuum and in solution, OH- attack on MEP is concerted with pseudorotation to form intermediate 2a, which undergoes ring opening faster than exocyclic cleavage of intermediate 2b (Scheme II); the apical attack of OH- and the apical departure of the ring oxygen in 2a are in accord with an in-line mechanism. This new mechanism is consistent with the experimental observation that MEP hydrolyzes exclusively with ring opening in dilute alkaline solution (pH 8-11).