Chirality Retention in Aqueous Propylene Oxide Hydration: Chirality of the Transition State

The hydration kinetics of enantiomerically pure propylene oxide (PO, CH3*CHCH2O) to chiral propylene glycol (PG, CH3*CH(OH)CH2OH) in aqueous solution have been studied using FTIR while simultaneously monitoring the net chirality of the reaction mixture. The hydration reaction appears to be first-order in the PO concentration with a rate constant of k approximately equal to 0.05 hr(-1). More importantly, the reaction is enantioselective; the product PG retains the chirality of the 2C carbon in PO with similar to 2 : 1 selectivity. The fact that there is some inversion of the chirality suggests that the dominant transition state is one in which the 2C-O bond in PO is cleaved, resulting in a close to planar transition state capable of inversion during hydration. If the transition state involved 1C-O cleavage it would retain the rigid chiral center of the PO reactant, preventing significant inversion.