The effect of spiroadamantane substitution on the conformational preferences of N-Me pyrrolidine and N-Me piperidine: a description based on dynamic NMR spectroscopy and ab initio correlated calculations

Dynamic NMR spectroscopy and ab initio correlated calculations revealed that the attachment of a spiroadamantane entity at the C-2 position of N-methylpyrrolidine or N-methylpiperidine induces a severe steric crowding around nitrogen, which changes the conformational space of the heterocycle resulting in: (a) the complete destabilization of the N-Me(eq) conformer in spiranic structures; in contrast the N-Me(eq) conformer corresponds to the global minimum in N-methylpyrrolidine or N-methylpiperidine. The spiroadamantane structure raises the energy of the equatorial conformer because of the severe van der Waals repulsion between the N-Me(eq) group and adamantane C-H bonds. (b) The interconversion between the only populated enantiomeric N-Me(ax) conformers ax→[eq]→ax′; the interconversion eq→ax between N-Me(eq) and N-Me(ax) conformers, which are both populated, is observed in N-methylpyrrolidine or N-methylpiperidine. (c) The raising of ring and nitrogen inversion barriers ax→ts by ∼4-6 kcal mol−1. The dynamic NMR study provides evidence that the most important process required for the enantiomerization between the axial N-Me conformers in spiropiperidine 4 and spiropyrrolidine 5 are different, i.e., a nitrogen inversion in 5 (9.10 kcal mol−1) and a ring inversion in 4 (15.2 kcal mol−1). While an enantiomerization interconverts N-Me axial conformers in spiropiperidine 5 and spiropyrrolidine 4, substitution of the pyrrolidine ring of 5 with a C-Me group effects a diastereomerization between two N-Me axial conformers and reduces effectively the nitrogen inversion barrier according to the protonation experiments and the calculations. In general, all the calculations levels used, i.e., the MM3, B3LYP/6-31+G∗∗ and MP2/6-311++G∗∗//B3LYP/6-31+G∗∗, predict correctly the different stability of the local minima; however only MP2/6-311++G∗∗//B3LYP/6-31+G∗∗ was found to be reliable for the calculation of the nitrogen inversion barriers.