(704a) Increasing the Selectivity of B(C6F5)3-Catalyzed Epoxide Ring-Opening Using Hydrogen Bonding
Epoxide ring-opening is an important reaction for synthesizing a range of intermediates and compounds. On the industrial scale, it is employed with alcohols for manufacturing polyether polyols, which are primary raw materials in the production of polyurethane products. This reaction, which occurs in the presence of a catalyst, typically yields primary (P1) and secondary alcohol (P2) products, the latter being unreactive in certain polyurethane applications. However, most conventional catalysts such as KOH and double metal cyanide âDMCâ catalysts are quite unselective and yield P1:P2 ratios < 0.1, thus requiring further polyol processing to achieve the desired P1 content. In contrast, we saw that tris(pentafluorophenyl)borane, better known as BCF, is a highly Lewis acidic catalyst that offers fast rates and selectivities as high as 3.5:1 for 1,2-epoxyoctane ring-opening with 2-propanol. Moreover, we observed that owing to its Lewis acidity, BCF presents catalytically active adducts with H2O which are described as H2O-mediated catalytic pathways. We further proposed that, while one of the protons on the BCF-OH2 adduct can participate in epoxide ring-opening, the other proton can form hydrogen bonds with a suitable hydrogen bond acceptor (HBA) and could potentially influence regioselectivity at the former. In this work, we confirm this concept using experimental observations and DFT predictions and explain the role of reaction products P1 and P2 in influencing the selectivity of BCF-catalyzed epoxide ring-opening with 1-propanol. Going further, we were also able to bring about significant enhancements to regioselectivity by adding co-catalyst amounts of 1,2-diol HBAs. We also explore the role of HBA structure in selectivity enhancement based off of results from a series of diols and demonstrate its applicability across a range of substrates and nucleophiles. Having established the basis for this phenomenon, these findings will further guide the discovery of other HBA classes for this reaction.