Proton transfer reactions of a bridged bis-propyl bis-imidazolium salt

Richard S. Massey, Peter Quinn, Shengze Zhou, John A. Murphy, Annmarie C. O'Donoghue

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Abstract

Tetraazafulvalene 1 has found broad application in reduction and other related transformations and is conveniently generated from bis-propyl bis-imidazolium salt 4 with a strong base in a non-protic solvent. The proposed mechanism for the formation of 1 involves initial deprotonation at C(2) to give a mono-carbene 9 followed by intramolecular reaction at the second azolium centre. Herein, we report the second-order rate constants for deuteroxide-catalysed exchange in aqueous solution of the C(2)-hydrogens of bis-propyl bis-imidazolium di-iodide salt 4 and related monomeric dipropyl imidazolium iodide 10 of kDO=1.37×104 and 1.79×102M-1s-1, respectively, and used these data to calculate pKa values of 21.2 and 23.1. The greater C(2)-H acidity of the doubly bridged bis-propyl bis-imidazolium salt 4 relative to 10 may be attributed to the inductive or electrostatic destabilization of the conjugate acid dicationic azolium ion 4 relative to the monocationic carbene 9, which is enhanced by bis-tethering. Formation of tetraazafulvalene 1 was not observed under the aqueous conditions employed highlighting that carbene reprotonation significantly outcompetes dimerization under these conditions.

Original languageEnglish
Number of pages6
JournalJournal of Physical Organic Chemistry
Early online date5 May 2016
DOIs
Publication statusE-pub ahead of print - 5 May 2016

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Keywords

  • acidity
  • bis-imidazolium
  • deuterium exchange
  • N-heterocyclic carbene
  • tetraazafulvalene

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