ORTHO-ESTER HYDROLYSIS - DIRECT EVIDENCE FOR A 3-STAGE REACTION-MECHANISM

Direct evidence is supplied for the existence of 1, 3-dioxolenium ion and hydrogen ortho ester intermediates in the acid-catalyzed hydrolysis of a series of 2-aryl-(and 2-cyclopropyl-) 2-alkoxy-l, 3-dioxolanes. These two intermediates require a three-stage reaction path: (1) loss of the exocyclic alkoxy group from the substrate to generate the dioxolenium ion, (2) reaction of this ion with water to form hydrogen ortho ester, and (3) breakdown of the hydrogen ortho ester to carboxylic acid products. Rate constants for all three stages and equilibrium constants for stage 2 were measured in some cases and were estimated in others. Differential substituent effects of phenyl and cyclopropyl on stages 1 and 3 are shown to be responsible for the changes in rate-determining step which these systems undergo between low (stage 3 rate determining) and high (stage 1 rate determining) pH. The purely aliphatic ortho esters 2-methoxy-l, 3-dioxolane and 2-methyl-2-methoxy-l, 3-dioxolane were found not to give this change in rate-determining step. © 1979, American Chemical Society. All rights reserved.