Ring Opening Reactions of β‐Propiolactam in Superacidic Media

Abstract

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 βpropiolactam. 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.