Ring Opening Reactions of β‐Propiolactam in Superacidic Media

The reaction of β‐propiolactam in the superacidic systems HF/MF(5) (M=Sb, As) led to the formation of monoprotonated 3‐aminopropanoyl fluoride in the form of [C(O)F(CH(2))(2)NH(3)][SbF(6)] and [C(O)F(CH(2))(2)NH(3)][AsF(6)]. In the presence of traces of water, the diprotonated species β‐alanine [C(OH)(2)(CH(2))(2)NH(3)][AsF(6)](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)F(CH(2))(2)NH(3)][SbF(6)] and [C(OH)(2)(CH(2))(2)NH(3)][AsF(6)](2). By using SO(2) instead of HF as the solvent, the salt [C(OH)(2)(CH(2))(2)NHSO][SbF(6)](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 β‐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 β‐propiolactam.