Carbon-13 nuclear magnetic resonance spectroscopy of methyl-substituted piperidines and their hydrochloride salts

Carbon-13 chemical shifts and ¹J(CH) coupling constants of piperidine, 4-t-butylpiperidine, five C-methyl substituted piperidines and of the corresponding N-methyl compounds have been measured. These NMR parameters have also been determined for the fourteen corresponding hydrochlorides. The N-methyl-C-methyl piperidinium salts showed two stereoisomers on the positive nitrogen atom. The effect of protonation is discussed in relation to charge and steric effects and the methyl substituent effects are compared with those of methyl-substituted cyclohexanes. Deviations from the general tendencies in the series are correlated with changes in ring conformations around the highly crowded substituted carbons.