Triazolium Salt Organocatalysis: Mechanistic Evaluation of Unusual Ortho-Substituent Effects on Deprotonation

Quinn, Peter; Smith, Matthew S.; Zhu, Jiayun; Hodgson, David R.W.; O’Donoghue, AnnMarie C.

Abstract

Organocatalysis by N-heterocyclic carbenes is normally initiated by the deprotonation of precursor azolium ions to form active nucleophilic species. Substituent effects on deprotonation have an impact on catalytic efficiency and provide insight into general catalytic mechanisms by commonly used azolium systems. Using an NMR kinetic method for the analysis of C(3)-H/D exchange, we determined log kex–pD profiles for three ortho-substituted N-aryl triazolium salts, which enables a detailed analysis of ortho-substituent effects on deprotonation. This includes N-5-methoxypyrid-2-yl triazolium salt 7 and di-ortho-methoxy and di-ortho-isopropoxyphenyl triazolium salts 8 and 9, and we acquired additional kinetic data to supplement our previously published analysis of N-pyrid-2-yl triazolium salt 6. For 2-pyridyl triazoliums 6 and 7, novel acid catalysis of C(3)-H/D exchange is observed under acidic conditions. These kinetic data were supplemented by DFT analyses of the conformational preferences of 6 upon N-protonation. A C(3) deprotonation mechanism involving intramolecular general base deprotonation by the pyridyl nitrogen of the N(1)-deuterated dicationic triazolium salt is most consistent with the data. We also report kDO values (protofugalities) for deuteroxide-catalyzed exchange for 6–9. The protofugalities for 8 and 9 are the lowest values to date in the N-aryl triazolium series.

Citation

Quinn, P., Smith, M. S., Zhu, J., Hodgson, D. R., & O’Donoghue, A. C. (2021). Triazolium Salt Organocatalysis: Mechanistic Evaluation of Unusual Ortho-Substituent Effects on Deprotonation. Catalysts, 11(9), Article 1055. https://doi.org/10.3390/catal11091055

• Journal Article Type: Article
• Acceptance Date: Aug 25, 2021
• Online Publication Date: Aug 30, 2021
• Publication Date: 2021
• Deposit Date: Sep 15, 2021
• Publicly Available Date: Sep 16, 2021
• Journal: Catalysts
• Electronic ISSN: 2073-4344
• Publisher: MDPI
• Peer Reviewed: Peer Reviewed
• Volume: 11
• Issue: 9
• Article Number: 1055
• DOI: https://doi.org/10.3390/catal11091055
• Public URL: https://durham-repository.worktribe.com/output/1250479