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.

No abstract