ISSN:
0749-1581
Keywords:
Cobalt(III) porphyrins
;
Conformational equilibria
;
Piperidines
;
Shift reagents
;
Substituent chemical shifts
;
Chemistry
;
Analytical Chemistry and Spectroscopy
Source:
Wiley InterScience Backfile Collection 1832-2000
Topics:
Chemistry and Pharmacology
Notes:
The 1H and 13C NMR spectra of piperidine and eight monosubstituted piperidines and their spectra with cobalt(III) mesotetraphenylporphyrin chloride (CoTPPCl) and cobalt(III) 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 (-ΔG°, kcal mol-1) 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 4-substituent 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 13C SCS values were in good agreement with those in the literature.With 1-, 2- and 3-methylpiperidines unusual results were obtained for both porphyrins. For 3-methylpiperidine -ΔG° was only 0.8 kcal mol-1 compared with a literature value of 1.6 kcal mol-1, whilst for 1- and 2- methylpiperidine reduction of the shift reagent to cobalt(II) occurred. With 1-methylpiperidine the reduction was complete and no complexation was observed, but only some of the shift reagent was reduced by 2-methylpiperidine and the remaining shift reagent complexed to the amine.
Additional Material:
6 Ill.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1002/mrc.1260260412
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