Conformational effects in compounds with 6-membered rings—XII : The conformational equilibrium in n-methylpiperidine |
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Authors: | PJ Crowley MJT Robinson MG Ward |
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Institution: | The Dyson Perrins Laboratory, South Parks Road, Oxford 0X1 3QY, England |
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Abstract: | The conformational equilibrium at nitrogen in N-methylpiperidines has been determined in the gas phase (ΔG°288 ? 13.2 ± 0.4 kJ mol?1) and for dilute solutions in several solvents (ΔG°293 ranging from 12.5 ± 0.4 in dodecane to 10.1 ± 0.4 in chloroform) by kinetically controlled protonation of anancomeric model compounds 6 and 8 at the interface between the pipendine-containing phase and an immiscible strong acid. The conformational energy for N-methylpiperidine determined by this method is strikingly higher than earlier estimates based on less direct methods but is supported by independent evidence from the temperature dependence of 13C NMR chemical shifts. Reconsideration of the more important of the earlier methods indicates that these involved invalid or unproven assumptions and that the low values of ΔG° for N-methylpiperidine derived from them are not reliable. |
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