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Conformational Analysis via NMR Isotope Shifts. Side-Chain Equilibria in N-Alkylpiperidines
Forsyth, David A.,Prapansiri, Vichukorn
, p. 4548 - 4552 (2007/10/02)
Conformational equilibria involving the N-alkyl side chain of N-alkylpiperidines were detected via NMR isotope shifts.Introduction of a single deuterium at C1' of the side chain causes a gauche gauche equilibrium to be nondegenerate and observable by the separation of signals for C2 and C6 in 13C NMR spectra at low temperature.The separation is temperature dependent, as expected for an equilibrium isotope effect.Actual Kiso and ΔG0iso for the isotopically perturbed equilibrium cannot be directly calculated by the application of Saunders' equation because the equilibration process is too rapid to obtain the maximum chemical shift difference between C2 and C6 at the slow exchange limit.Nonetheless, some useful estimates of these values can be made.The observations can be accounted for by the presence of both gauche and anti conformers in the equilibrium.MM2 calculations using Allinger's parameters for amines also support the conclusion that both gauche and anti conformers are present, with the proportion of gauche conformers declining in the sequence N-ethylpiperidine > N-propylpiperidine, N-butylpiperidine > N-benzylpiperidine.
