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≅ 0.05 hr−1. More importantly, the reaction is enantioselective; the product PG retains the chirality of the 2C carbon in PO with ∼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.