CONFORMATIONAL ISOMERISM IN A SERIES OF ALKYLPIPERAZONIUM DERIVATIVES INVESTIGATED BY HIGH-FIELD H-1 AND C-13 NMR-SPECTROSCOPY

Changes in the 1H and 13C NMR spectra over the temperature range -30 to +60°C are used to investigate conformational isomerism in N,N′-dimethylpiperazine, the N,N,N′,N′-tetramethylpiperazonium dication and trialkylpiperazonium monocations of the type MeN(CH2CH2)2N(Me)R+, where R = Me, Et, n-Pr, i-Pr or CH2X (X = Cl, Br or I). Measurements of the rate of ring inverson of the heterocyclic cations (1-4) from line width data yield values of the enthalpy and entropy of activation; ΔH≠ = 64 to 75 ± 2 kJ mol-1 and ΔS≠ = 30 to 80 ± 10 J K-1 mol-1. These are compared with the values derived from the activation energy (ΔG≠ = 47 to 56 ± 1 kJ mol-1) obtained from the coalescence temperatures of NMR signals from appropriate sets of indicating nuclei. The related seven-membered heterocycle N,N,N′,N′-tetramethylhomopiperazonium dication (2) exhibits a higher barrier to ring inversion than those of the alkylpiperazonium derivatives. For the N,N,N′-trialkylpiperazonium ions (4-7) with unlike substituents on the quaternary nitrogen, the NMR spectra show that 70-75% of the ground state molecules have the bulkier group in the equatorial situation. Also reported are some 14N coupling constants, 1J(14N, 13C) of 2.5-4.0 Hz, and 3J(14N, 1H) of 1.5-2.0 Hz, involving nitrogen atoms of the quaternary centres. © 1990.