Insight into the New Catalytic Species for the Hydration of Propargyl Alcohols Catalyzed by [Bu4p+][Im-]

Abstract

The mechanism of hydration of propargylic alcohols catalyzed by [Bu4P+][Im-] has been elucidated along with the carbon dioxide (CO2) as the co-catalyst. The cation is thought to be the minor role in catalysis to adjust the basicity of anion in previous study, however, it conflicts with the experimental results, in which the product yield is related with the type of cation. The influence of [Bu4P+][Im-], especially for the [Bu4P+] cation in the catalysis is explored in this work by the density functional theory (DFT). It is found that CO2 could be absorbed by the [Bu4P+] cation to form [Bu4P-H-CO2] complex after the hydrogen atom in [Bu4P+] cation transfers to the [Im-] anion. Then, a novel pathway is constructed with the [Bu4P-H-CO2][Im+H] complex employed as the actual catalytic species achieving the better catalytic performance relative to other pathways reported in the literature. Molecule dynamic simulations indicate that the [Bu4P+] cation prefers to absorb the CO2 as compared with the [Im-] anion. Following the above mentioned pathway, the catalytic activity of five different ionic liquids for this reaction is compared to elucidate the influence of cation and anion. This work not only provides a new insight into the cation-regulated pathway in the hydration of propargylic alcohols, but also brings out new strategy for designing the ionic liquids to improve the catalytic efficiency.

Keywords: Hydration of propargylic alcohols, Ionic liquids, CO2 absorption, Reaction mechanism, Catalysis.