Ring Opening and Closure Reactions of β‐Propiolactone in the Superacids HF/MF5 (M=Sb, As)

Abstract

AbstractTransformations of β‐propiolactone in the binary superacidic systems HF/MF5 (M=Sb, As) were investigated at different temperatures. Salts of monoprotonated 3‐hydroxypropanoyl fluoride [H2O(CH2)2C(O)F][SbF6] (1) and [H2O(CH2)2C(O)F][AsF6] (2) were obtained by performing the reactions at temperatures lower than −40 °C. In contrast, temperatures higher than −40 °C yield the respective salts [C9H15O6][SbF6]3 (3) and [C9H15O6][AsF6]3 (4) containing a cyclic trication. The protonated species of β‐propiolactone was not observed. All salts were characterized by low temperature Raman and IR spectroscopy. In addition, single‐crystal X‐ray analyses were performed for (1) and (3). For both cations the existence of a cationic acyl species as a reactive intermediate in the form of [HO(CH2)2CO][MF6] is assumed. The formation of the acyl cation, based on protonated β‐propiolactone, was investigated by quantum chemical calculations on the MP2/aug‐cc‐pVTZ level of theory.