M.J.T. Robinson

The conformational analysis of saturated heterocycles. N-inversion in hindered piperidines

Results from a new dynamic n.m.r. method, applicable to heavily biased equilibria, and kinetically controlled protonation studies agree when applied to 1,2,2,6-tetramethylpiperidine, giving the free energy difference for N-inversion [ΔG°(1E→1A)] as 1–9± 0·2 at 213 K (d.n.m.r.) and 1·95 ± 0·1 kcal mol–1 at 293 and 373 K (kinetic protonation); the free energy of activation [ΔG‡(1E→1A)] is 11·0 ± 0·3 kcal mol–1 at 213 K.

Conformational equilibria in N,3- and N,4-dimethylpiperidine

The conformational equilibrium free energy difference for the C-methyl group in N,3- and N,4-dimethylpiperidine, and therefore in 3- and 4-methylpiperidine to a good approximation, have been estimated to be 6.3 ± 0.3 and 8.3 ± 0.3 kJ mol–1 at 293 K in dodecane as solvent, using the ratios of diastereomeric ions formed by the kinetically controlled protonation of 13C isotopically enriched derivatives.

Proton-coupled carbon-13 nuclear magnetic resonance spectra from individual carbon sites in a molecule: the rotameric equilibrium in menthone

Vicinal carbon-proton coupling constants have been measured for menthone by a new technique and show that the most stable rotamer is (1a) rather than (1b).