Ring Opening Reactions of beta-Propiolactam in Superacidic Media

The reaction of beta-propiolactam in the superacidic systems HF/MF5 (M=Sb, As) led to the formation of monoprotonated 3-aminopropanoyl fluoride in the form of [C(O)F(CH2)(2)NH3] [SbF6] and [C(O)F(CH2)(2) NH3][AsF6]. In the presence of traces of water, the diprotonated species beta-alanine [C(OH)(2)(CH2)(2)NH3][AsF6](2) was synthesized for the first time. All salts were characterized by low-temperature infrared and Raman spectroscopy. Additionally, single-crystal X-ray analyses were conducted in the case of [C(O)(2)(CH2)(2)NH3][SbF6] and [C(OH)F(CH2)(2)NH3][AsF6](2). By using SO, instead of HF as the solvent, the salt [C(OH)(2)(CH2)(2)NHSO][SbF6](2) was obtained, and single-crystal X-ray analysis of this salt containing a thionylimide moiety was conducted. For the formation of these open-chain compounds, an acyl cationic species as intermediate is assumed, which is formed from N-protonated beta-propiolactam. Quantum chemical calculations at the B3LYP/aug-cc-pVTZ and MP2/aug-cc-pVTZ levels of theory were carried out to gain a better understanding of the formation and the structural properties of protonated beta-propiolactam.