Enhancing the Regioselectivity of B(C₆F₅)₃-Catalyzed Epoxide Alcoholysis Reactions Using Hydrogen-Bond Acceptors

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.