Equilibria of C-Substituents in Piperidine

The conformational free energies of C-methyl groups in piperidine and yV-methylpiperidine have been estimated183 by low temperature ( — 80 to 

Takeuchi, H. Suzuki, and Y. Nomura, Chem. Lett., 697 (1981). 

Conformational Free Energy of Methyl Groups in Piperidines  

The AG° value for 3-Me appears to be somewhat smaller than that for the 4-Me as expected for nonbonded interaction in the axial form with the nitrogen atom rather than with a CH bond. Most striking is the variation in conformational free energy of the 2-Me substituent in piperidine and its N-methyl derivative. The lower value in the N-methylpiperidine must be a result of an interaction between 2-Meeq and N-Me, caused by puckering at the nitrogen end of the chair. 

The preference for the axial position of 2-alkyl substituents is dramatically increased by the presence of N-nitro-,184 N-acetyl-,185,186 N-benzoyl-,187 and related substituents (cf. N-nitroso-2,6-cis-dimethy piperidine,178 Section 

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