A Nuclear Magnetic Resonance Kinetic and Product Study of the Ring Opening of Propylene Oxide. Nucleophilic and General Catalysis by Phosphate

Article abstract of DOI:10.1021/ja00546a015

Systematic kinetic and product studies have been performed in an examination of the ring opening of propylene oxide over the entire pH region.The NMR kinetic method, which involves the integration of reactant and product resonances as a function of time, provided the rate data.The plots of log (area_t - area_infinite) vs. time were linear to better than three half-life times of reactiom.Reproducibility error for rate measurements run under identical conditions wes less than 5percent.Products were identified by 1H and 13C NMR spectroscopy and with gated decoupler techniques.The latter was used to quantitatively determine the product composition.The validity of this method of product analysis was carefully established with a series of control experiments.The error in these determinations was shown to be less than 4percent.Emphasis was placed on the search for general and nucleophilic mechanisms of catalysis.Kinetic and product analyses were performed on reaction solutions in both aqueous formate and aqueous phosphate buffers.The glycol monoformate esters proved to be labile under kinetic conditions.By contrast the glycol monophosphate esters permitted a complete dissection of the buffer catalytic components into nucleophilic and general modes.Thus careful analysis of the reaction species present in the buffer matrix shows that glycol monophosphate esters are always produced in amounts less than the buffer contribution to the overall rate and furthermore arise almost exclusively from HPO4^2- attack upon neutral epoxide.This nucleophilic catalysis accounts for 80 +/- 6percent of k_HPO42- and leads to almost equal amounts of 1,2-propanediol-1-phosphate and 1,2-propanediol-2-phosphate.The remaining 20 +/- 6percent of k_HPO42- and the total k_H2PO4- represent general catalysis.These contributions arise mechanistically though hydrogen bonding which operates to increase the nucleophilic capacity of rear side water molecules or to enhance the electrophilic character of the epoxide ring, respectively.