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

✍ Scribed by Jan M. Coddington; Michael J. Taylor

• Publisher: Elsevier Science
• Year: 1990
• Tongue: English
• Weight: 895 KB
• Volume: 46
• Category: Article
• ISSN: 1386-1425
• DOI: 10.1016/0584-8539(90)80062-4

✦ Synopsis

Changes in the 'H and "C NMR spectra over the temperature range -30 to +6OO"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),N(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 (l-4) from line width data yield values of the enthalpy and entropy of activation; AH' = 64 to 75 + 2 kJ mol-' and AS' = 30 to 80 + 10 J K-' mol-'. These are compared with the values derived from the activation energy (AC' = 47 to 56 + 1 kJ mot-') 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 "N coupling constants, 'J("N, '"C) of 2.5-4.0 Hz, and 'J("N, 'H) of 1.5-2.0 Hz, involving nitrogen atoms of the quaternary centres.

* The rate data are deposited as Supplementary Material.