Conformational Analysis of N-Nitrosopiperidines by 13C Nuclear Magnetic Resonance. The Orientation Preference of an α-Methyl Group

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Abstract
The 13C shielding parameters for N-nitroso-4-phenylpiperidine and four α-methyl derivatives have been measured. From these data a number of methyl substituent parameters have been obtained. A comparison of the observed spectra of syn- and anti-N-nitroso-2-methylpiperidines with those calculated from the substituent parameters and the shieldings in N-nitrosopiperidine established the syn isomer to exist exclusively in the conformation having the methyl group axial; the anti isomer has the methyl group predominantly axial (59%). It is proposed that the large differential shielding between α carbons syn and anti to the nitroso group is caused by steric compression at the syn carbon. Large dilution effects on the carbon shieldings (up to 1.3 p.p.m.) have been encountered in these nitrosamines.