The effect of neighboring 1- and 2-adamantyl group substitution on the conformations and stereodynamics of N-methylpiperidine. Dynamic NMR spectroscopy and molecular mechanics calculations

When a 1-adamantyl or a 2-adamantyl substituent is introduced at the 2-position in N-methylpiperidine, four different chair conformations are possible. Experimental observation using dynamic NMR spectroscopy and molecular mechanics calculations agree that the chair conformation with an equatorial adamantyl group and an axial methyl group is by far the most stable, but in both cases a minor population of a second conformation is demonstrated and characterized. Interaction between adamantyl and methyl groups is much more conformation-determining than any preference for equatorial over axial location which predominates in simpler 2-substituted N-methylpiperidines.