(337a) Computational Study of Reaction Mechanisms in Epoxide Ring-Opening Reactions By Aryl Borane Catalysts

With their versatility, epoxide ring-opening reactions have played a crucial role in organic synthesis. For example, aliphatic epoxides can be utilized to prepare polyols, the building blocks of polyurethanes (PU), by undergoing epoxide ring-opening reactions. Epoxide ring-opening can produce two regio-isomers: a primary alcohol (P1) and a secondary alcohol (P2). For PU applications, P1 has enhanced reactivity with isocyanates and is much preferred to P2. Therefore, catalysts that provide better selectivity and rates to P1 are of great interest. To date, only B(C₆F₅)₃ (also known as BCF), favors P1 over P2. Since BCF is thermally stable and possesses chemical versatility, it has also been used in a wide range of applications, such as olefin oxidation reactions and those using frustrated Lewis pairs. Despite the utility of this catalyst, mechanisms for epoxide ring opening, catalyst speciation under reaction conditions, and catalyst decomposition pathways are not fully understood.

We have previously established a model of epoxide ring-opening of epoxyoctane using a variety of alcohols as nucleophiles. Depending on reaction conditions, decomposition (protodeborylation) of BCF and isomerization of epoxide are side reactions that may occur, and thus, we expanded the model to encompass these additional reaction pathways. Density functional theory (DFT) calculations were used to map reaction mechanisms and quantify thermodynamics and kinetics characterizing this complex reaction system. Gaussian 16 was used as the computational chemistry software for all calculations. Reaction energies were calculated at the B3LYP-D3BJ/6-31+G(d,p) level of theory. Solvation effects were incorporated using the SMD solvation model with 1-propanol as the implicit solvent. Transition state energies (∆G^≠, ∆𝐻^≠, ∆𝑆^≠) were calculated using standard statistical mechanics formulae once the electronic energies of reactant(s) and transition state were quantified.

Transition states were verified by vibrational frequencies (a single imaginary frequency) and intrinsic reaction coordinate (IRC) calculations. Explorations of the potential energy surface to confirm minimum energy geometries were conducted via dihedral scans and conformational searches.

DFT calculations for the additional reaction pathways leveraged the mechanism for epoxide ring-opening by BCF that we previously unraveled, which revealed that additional hydrogen bond acceptors such as water and 1-propanol played an important role on both rate and regioselectivity. Incorporating reaction kinetics from DFT calculations into the expanded microkinetic model, the core reaction model was modified to establish a model for propylene oxide ring opening and its side reactions, including catalyst decomposition.