A nonconventional, water-mediated catalytic mechanism was proposed to explain the effects of residual water on the reactivity and regioselectivity of tris­(pentafluorophenyl)­borane catalyst in the ring-opening reaction of 1,2-epoxyoctane by 2-propanol. This nonconventional mechanism was proposed to operate in parallel with conventional Lewis acid-catalyzed ring-opening. Microkinetic modeling was conducted to validate the proposed reaction mechanism, with all kinetic and thermodynamic parameters derived from density functional theory (DFT) calculations. Experimental data at a variety of temperatures and water contents were captured by the model after adjustments within reasonable limits set by experimental benchmarking and accuracy of theory of a small subset of parameters. In addition, the microkinetic model was able to generate accurate predictions at reaction conditions that were not used for parameter estimation. Detailed analysis of the net reaction rates showed that >95% of the reaction flux passed through conventional Lewis-acid pathways at water levels <500 ppm, even though the borane-epoxide adduct never accounted for more than 30% of the catalyst speciation under reaction conditions. With increasing water, as much as 80% of the reaction flux utilized water-mediated reaction intermediates. Within the water-mediated mechanisms, different hydrogen bond acceptors (HBAs) influenced the reaction regioselectivity. Overall, this validated mechanism and microkinetic model provided better understanding of industrially important ring-opening catalysis with this catalyst in the presence of water and could facilitate future improvement of catalyst regioselectivity and reactivity.

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Epoxide alcoholysis is extensively employed in the synthesis of polymers and chemical intermediates, and it generally requires an acid catalyst for high rates and selectivity. Tris­(pentafluorophenyl)­borane [B­(C6F5)3] is among few catalysts that are selective to primary alcohol products of terminal aliphatic epoxides that do not possess any directing groups. We have previously observed that under many conditions, the reaction regioselectivity increases with conversion. Here, we confirm a prediction from our earlier computational model, and we experimentally demonstrate that this increase is due to a selectivity-enhancing role of the reaction products. We then show that deliberate addition of catalytic amounts of certain diols increases the reaction regioselectivity. Cis-1,2 or 1,3-diols are required to enhance selectivity, consistent with a mechanism where extended hydrogen-bonding networks preferentially organize the reactants. This work presents a route to tune regioselectivity without altering the catalyst backbone and provides another example of the role of H-bonding networks in reactions taking place in protic media.

Density functional theory (DFT) calculations, experimental data, and microkinetic modeling are used to extend a triple-pathway (Lewis acid, water-mediated, and alcohol-mediated) mechanism for tris­(pentafluorophenyl)­borane-catalyzed ring opening of 1,2-epoxyoctane by alkyl alcohol nucleophiles previously applied to 2-propanol to 1-propanol. Although simpler models may capture overall rates, the reaction schemes proposed here are required to explain the increasing regioselectivity to the primary product with conversion and the dependence of the overall regioselectivity on residual water concentration and additives as a function of reaction conditions. The model indicates that the different reaction conditions (nucleophile, water concentration, temperature, and conversion) lead to different amounts of flux through alcohol-mediated pathways, different speciation of tris­(pentafluorophenyl)­borane adducts, and differences among the inherent selectivities of water-mediated mechanisms.

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