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

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

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 × 102 M−1 s−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.

Citation

Massey, R. S., Quinn, P., Zhou, S., Murphy, J. A., & O'Donoghue, A. C. (2016). Proton transfer reactions of a bridged bis-propyl bis-imidazolium salt. Journal of Physical Organic Chemistry, 29(12), 735-740. https://doi.org/10.1002/poc.3567

• Journal Article Type: Article
• Acceptance Date: Mar 14, 2016
• Online Publication Date: May 5, 2016
• Publication Date: Dec 1, 2016
• Deposit Date: Apr 15, 2016
• Publicly Available Date: May 5, 2017
• Journal: Journal of Physical Organic Chemistry
• Print ISSN: 0894-3230
• Electronic ISSN: 1099-1395
• Publisher: Wiley
• Peer Reviewed: Peer Reviewed
• Volume: 29
• Issue: 12
• Pages: 735-740
• DOI: https://doi.org/10.1002/poc.3567
• Public URL: https://durham-repository.worktribe.com/output/1414437