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The Role of the Fused Ring in Bicyclic Triazolium Organocatalysts: Kinetic, X-ray and DFT Insights

Lookup NU author(s): Dr Paul Waddell, Dr Michael HallORCiD, Professor Mike ProbertORCiD



This work is licensed under a Creative Commons Attribution 4.0 International License (CC BY 4.0).


Bicyclic triazolium scaffolds are widely employed in N-heterocyclic carbene (NHC) organocatalysis. Whilst the incorporation of a fused ring was initially for synthetic utility in accessing chiral, modular triazolyl scaffolds, recent results highlight the potential for impact upon reaction outcome with underpinning origins unclear. The common first step to all triazolium-catalyzed transformations is C(3)-H deprotonation to form the triazolylidene NHC. Herein, we report an analysis of the impact of size of the fused (5-, 6- and 7-membered, n=1, 2 and 3 respectively) ring on the C(3) proton transfer reactions of a series of bicyclic triazolium salts. Rate constants for the deuteroxide-catalyzed C(3)-H/D-exchange of triazolium salts, kDO, were significantly influenced by the size of the adjacent fused ring, with the kinetic acidity trend, or protofugalities, following the order: kDO (n=1) > kDO (n=2) ≈ kDO (n=3). Detailed analyses of X-ray diffraction (XRD) data for 20 triazolium salts (including 16 new structures), and of computational data for the corresponding triazolylidene NHCs, provide insight on structural effects of alteration of fused ring size. In particular, changes in internal triazolyl NCN angle and positioning of the most proximal CH2 with variation in fused ring size are proposed to explain the experimental protofugality order.

Publication metadata

Author(s): Zhu J, Moreno I, Quinn P, Yufit DS, Song L, Young CM, Duan Z, Tyler AR, Waddell PG, Hall MJ, Probert MR, Smith AD, O'Donoghue AC

Publication type: Article

Publication status: Published

Journal: Journal of Organic Chemistry

Year: 2022

Volume: 87

Issue: 6

Pages: 4241–4253

Print publication date: 18/03/2022

Online publication date: 01/03/2022

Acceptance date: 03/02/2022

Date deposited: 16/02/2022

ISSN (print): 0022-3263

ISSN (electronic): 1520-6904

Publisher: American Chemical Society


DOI: 10.1021/acs.joc.1c03073


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