Natural-Abundance Nitrogen-15 Nuclear Magnetic Resonance Spectroscopy. Steric and Electronic Effects on Nitrogen-15 Chemical Shifts of Piperidines and Decahydroquinolines

Abstract

Natural-abundance ^(15)N-NMR chemical shifts of closely related methyl-substituted piperidines, decahydroquinolines, and their N-methyl derivatives have been measured in cyclohexane and methanol. For both solvents, the secondary amines and two groups of tertiary amines give separate linear correlations with the ^(13)C chemical shifts of their hydrocarbon analogues. Additive shift parameters for carbon substituents near nitrogen, similar to those which correlate ^(13)C chemical shifts, have been determined. Except for the N-alkylation parameters, these parameters are relatively solvent insensitive, at least for cyclohexane and methanol. Nonetheless, ^(15)N chemical-shift comparisons are best made for the same solvent or very similar solvents. A large shift effect results when substituents are changed which are antiperiplanar to the orbital of the unshared electrons of tertiary amines. The use of the additive shift parameters and the general correlation between ^(15)N and ^(13)C shifts with respect to analysis of conformational and structural changes is illustrated using N-methylpiperidine and cis-2,3-dimethylpiperidine as specific examples.

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