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NMR spectra of the porphyrins. 34 —Determination of the conformational equilibria of monosubstituted piperidines at room temperature using cobalt(III) porphyrin shift reagents
Author(s) -
Abraham Raymond J.,
Medforth Craig J.
Publication year - 1988
Publication title -
magnetic resonance in chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.483
H-Index - 72
eISSN - 1097-458X
pISSN - 0749-1581
DOI - 10.1002/mrc.1260260412
Subject(s) - chemistry , porphyrin , substituent , conformational isomerism , cobalt , piperidine , steric effects , reagent , proton nmr , nmr spectra database , crystallography , chloride , stereochemistry , medicinal chemistry , spectral line , photochemistry , organic chemistry , molecule , physics , astronomy
Abstract The 1 H and 13 C 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 (−Δ G °, 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 13 C 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 −Δ G ° 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.