Conformational isomerism in a series of alkylpiperazonium derivatives investigated by high field 1H and 13C NMR spectroscopy

Abstract

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.