Transformations of 5‐Hydroxy‐1‐pyrrolines in Strong Brønsted Acids: Experimental and Theoretical Study of Intermediate Cationic Species

The reaction of 5‐hydroxy‐1‐pyrrolines with arenes in TfOH at room temperature for 1 h afford 5‐aryl‐1‐pyrrolines in 55–98 % yields. 5‐Hydroxy‐1‐pyrrolines in TfOH or H₂SO₄ at room temperature for 0.5–1 h undergo dehydration with the formation of N‐protonated forms of 3H‐pyrroles, which were characterized by NMR. Upon hydrolysis, the N‐protonated species give quantitatively the corresponding 3H‐pyrroles. The intermediate cationic species derived from the protonation of 5‐hydroxy‐1‐pyrrolines in Brønsted acids have been studied by means of NMR and DFT calculations. Plausible reaction mechanisms are discussed.