NMR spectra of the porphyrins. 34—Determination of the conformational equilibria of monosubstituted piperidines at room temperature using cobalt(III) porphyrin shift reagents

✍ Scribed by Raymond J. Abraham; Craig J. Medforth

• Publisher: John Wiley and Sons
• Year: 1988
• Tongue: English
• Weight: 851 KB
• Volume: 26
• Category: Article
• ISSN: 0749-1581
• DOI: 10.1002/mrc.1260260412

✦ Synopsis

The 'H and I3C NMR spectra of piperidine and eight monosubstituted piperidines and their spectra with cobalt(II1) mesotetraphenylporphyrin chloride (CoTPPCl) and cobalt(II1) octaethylporphyrin chloride (CoOEPCl) were assigned using a range of NMR techniques. When complexed to the porphyrin shift reagent the axial and equatorial conformers were in slow exchange at room temperature, allowing the determination of conformational equilibria and substituent chemical shifts (SCS).

Conformational free energy differences (-AG", kcal mol-') obtained for 4-substituted piperidines using

CoOEPCl at 298 K were 0.5 (X = Br), 0.7 (X = OH), 0.8 (X = carbethoxy), 1.8 (X = Me) and >1.6 (X = Ph).

These were in good agreement with literature values. For CoTPPCl the corresponding values were -0.2 (axial conformation favoured), 0.3, 0.7, 1.4 and > 1.6, indicating that an axial orientation of the kubstituent was more favoured than with CoOEPCl. This difference was rationalized in terms of steric and electrostatic interactions between the piperidine substituent and the phenyl rings of CoTPPCl. In both cases proton and I3C SCS values were in good agreement with those in the literature. With 1-, 2-and fmethylpiperidines unusual results were obtained for both porphyrins. For fmethylpiperidine -LIP was only 0.8 kcal mol-' compared with a literature value of 1.6 kcal mol-', whilst for 1and 2methylpiperidine reduction of the shift reagent to cobalt(I1) occurred. With 1-methylpiperidine the reduction was complete and no complexation was observed, but only some of the shift reagent was reduced by Zmethylpiperidine and the remaining shift reagent complexed to the amine.