Journal
JOURNAL OF THE CHEMICAL SOCIETY-PERKIN TRANSACTIONS 2
Volume -, Issue 12, Pages 2382-2392Publisher
ROYAL SOC CHEMISTRY
DOI: 10.1039/b007506j
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The H-1 NMR spectra of nipecotic acid (piperidine-3-carboxylic acid), ethyl nipecotate, nipecotamide and some of their N-methyl derivatives are reported and analysed. At -80 degreesC the interconversion between the two chair conformations is so slow that the spectra of the two conformers can be resolved and the conformer populations and free energy differences obtained. The proton couplings found for the individual conformers were used together with the measured couplings at room temperature to obtain the conformer free energy differences for these compounds in a variety of solvents. The intramolecular hydrogen bonding interaction between the side chain and the ring nitrogen atom in the axial conformation may be obtained from the conformer energy differences. For nipecotic acid in D2O DeltaG(A - E) = 0.41 kcal mol(-1). From these data the hydrogen bond energy between the CO2- and NH2+ groups in the nipecotic acid zwitterion is obtained as ca. 1.7 kcal mol(-1) in the axial conformer even in the protic solvents methanol and water. The corresponding interactions in the cation and anion are much less (ca. 1 kcal mol(-1)) and also do not show a large solvent effect. In contrast the H-bond interaction between the neutral species is very solvent dependent. In nipecotamide the CONH2. . .N hydrogen bond energy is >2 kcal mol(-1) in nonpolar solvents, in which the axial conformer is the major form (DeltaG(A - E) = -0.46 kcal mol(-1)), but decreases to almost zero in aqueous solution in which the equatorial conformer predominates (DeltaG(A - E) = 1.4 kcal mol(-1)). The C=O . . . HN hydrogen bond is less affected by the solvent varying from ca. 1.8 kcal mol(-1) in nonpolar solvents to 1.0 kcal mol(-1) in D2O in N,N-diethylnipecotamide and from 0.9 to 0.6 kcal mol(-1) in ethyl nipecotate.
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