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.
| Language | English |
|---|---|
| Number of pages | 6 |
| Journal | Journal of Physical Organic Chemistry |
| Early online date | 5 May 2016 |
| DOIs | |
| Publication status | E-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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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.
In: Journal of Physical Organic Chemistry, 05.05.2016.Research output: Contribution to journal › Article
TY - JOUR
T1 - Proton transfer reactions of a bridged bis-propyl bis-imidazolium salt
AU - Massey, Richard S.
AU - Quinn, Peter
AU - Zhou, Shengze
AU - Murphy, John A.
AU - O'Donoghue, Annmarie C.
N1 - This is the peer reviewed version of the following article: Massey, R. S., Quinn, P., Murphy, J. A., & O'Donoghue, A. (2016). Proton transfer reactions of a bridged bis-propyl bis-imidazolium salt. Journal of Physical Organic Chemistry. , which has been published in final form at [Link to final article using the DOI]. This article may be used for non-commercial purposes in accordance with Wiley Terms and Conditions for Self-Archiving.
PY - 2016/5/5
Y1 - 2016/5/5
N2 - 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.
AB - 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.
KW - acidity
KW - bis-imidazolium
KW - deuterium exchange
KW - N-heterocyclic carbene
KW - tetraazafulvalene
UR - http://www.scopus.com/inward/record.url?scp=84965120712&partnerID=8YFLogxK
U2 - 10.1002/poc.3567
DO - 10.1002/poc.3567
M3 - Article
JO - Journal of Physical Organic Chemistry
T2 - Journal of Physical Organic Chemistry
JF - Journal of Physical Organic Chemistry
SN - 0894-3230
ER -