Application of biphasic reaction procedure using ferric chloride dissolved in an imidazolium salt and benzotrifluoride (FeIm-BTF procedure) to aza-Prins cyclization reaction

Aza-Prins cyclization reaction of N-tosyl-3-butenylamine with aliphatic and aromatic aldehydes was performed using a combination of FeCl3 and 1-butyl-3-methylimidazolium hexafluorophosphate (BmimPF6) or 1-butyl-3-methylimidazolium tetrachloroferrate (BmimFeCl4) in benzotrifluoride (BTF). The desired N-tosyl-4-chloro-2-substituted piperidines were obtained from aliphatic aldehydes in comparable yields to those for the previously reported reactions in which FeCl3 was used in CH2Cl2. On the other hand, significant progress for the piperidine synthesis from aromatic aldehydes has been achieved, particularly when BmimFeCl4 was used with FeCl3 in BTF.