Hana Navrátilová

Microwave‐Assisted Elimination Reaction of trans‐4‐(4‐Fluorophenyl)‐3‐chloromethyl‐1‐methylpiperidine on Alumina

Abstract trans‐4‐(4‐Fluorophenyl)‐3‐chloromethyl‐1‐methylpiperidine 3b was subjected to elimination reaction on alumina or KF‐alumina under solvent‐free conditions and microwave irradiation. Compared with the “classical” method of heating in the presence of an organic base, the microwave‐assisted methodology provided higher yields of 4‐(4‐fluorophenyl)‐3‐methylene‐1‐methylpiperidine 7b (65.5–71%) in considerably shorter reaction times (20–40 min). Furthermore, the exocyclic double bond in 7b underwent rearrangement to the endocyclic double bond to furnish compound 8. The effect of alumina and irradiation time on reaction conversion and the extent of isomerization was examined.

Enantiodiscrimination in NMR spectra and X-ray structures of diastereomeric salts of trans-4-(4-fluorophenyl)-3-hydroxymethyl-1-methylpiperidine with (S)-Mosher acid

Signal anisochrony (Δδ) in NMR spectra of racemic trans-4-(4-fluorophenyl)-3-hydroxymethyl-1-methylpiperidine 1 induced on the formation of diastereomeric salt complexes with (S)-Mosher acid 2 showed a strong dependence on solvent polarity, concentration, stoichiometric ratio and enantiomeric composition. X-Ray structures of the salts, 3a and b, revealed that the conformations of the protonated base in ion pairs are very similar with respect to interatomic distances, bond and torsion angles but the mutual orientation of counterions in ion pairs differs significantly. It is suggested that the conformations of transient diastereomeric complexes in solution, responsible for Δδ, resemble those in the solid state. Furthermore, ab initio calculations performed on the optimized X-ray geometries of both ion pairs showed that the free energy of 3a is lower than that of 3b. This implies that the association constant for the formation of 3a is higher than that of 3b.