Kinetics of Aryl Borane-Catalyzed Propylene Oxide Ring Opening by 1-Propanol

Propylene oxide (PO) ring-opening is crucial to the production of many commodity chemicals, including polyether polyols, which are solvents and intermediates in polyurethane production. In previous studies, we demonstrated that a strongly Lewis-acidic aryl borane, tris(pentafluorophenyl)borane, or BCF, catalyzed the ring-opening reactions of aliphatic epoxides and exhibited high regioselectivity to products with a primary hydroxyl group. We previously established a microkinetic model for a limited subset of reaction conditions and a model epoxide. Here, we used PO as the epoxide and 1-propanol (1-Prop) as the initiator/nucleophile, and we employed a high ratio of PO to 1-Prop to understand the reactivity under industrially relevant conditions that give oligomers and side products, primarily propionaldehyde (PA). We analyzed a wide range of temperatures, catalyst loadings, and concentrations of PO, 1-Prop, and residual water. BCF maintained its high regioselectivity but exhibited complex kinetic behaviors due to its Lewis acidity and the hydrogen bonding networks present under the reaction conditions. At low temperatures, BCF showed both an initial rate acceleration and inhibition at moderate conversion, both of which were assigned to the formation of inactive BCF complexes with oligomers, including the reaction products. The formation of these complexes was verified with NMR. We developed an enhanced microkinetic model that successfully captured this system’s unique kinetic behavior. Density functional theory (DFT) calculations for the energies of forming catalytic intermediates and transition states provided initial estimates for kinetic constants, which were then fit to the experimental data with a small number of adjustable parameters. This study shows that BCF-catalyzed reaction kinetics are dictated by the competitive formation of BCF complexes with different hydrogen bond acceptors in the reaction system, including water, polyether products, and the 1-Prop nucleophile.