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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. The Role of the Fused Ring in Bicyclic Triazolium Organocatalysts: Kinetic, X-ray, and DFT Insights. J Org Chem 2022; 87:4241-4253. [PMID: 35230109 PMCID: PMC8938951 DOI: 10.1021/acs.joc.1c03073] [Citation(s) in RCA: 4] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/18/2021] [Indexed: 11/30/2022]
Abstract
Bicyclic triazolium scaffolds are widely employed in N-heterocyclic carbene (NHC) organocatalysis. While 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 the 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 influence the experimental protofugality order.
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Affiliation(s)
- Jiayun Zhu
- Department
of Chemistry, Durham University, South Road, Durham DH1 3LE, U.K.
| | - Inmaculada Moreno
- Department
of Chemistry, Durham University, South Road, Durham DH1 3LE, U.K.
- Dpto.
de Química Física, Facultad de Ciencias y Tecnologías
Químicas, Universidad de Castilla
- La Mancha, Avda. Camilo
José Cela s/N, 13071 Ciudad Real, Spain
| | - Peter Quinn
- Department
of Chemistry, Durham University, South Road, Durham DH1 3LE, U.K.
| | - Dmitry S. Yufit
- Department
of Chemistry, Durham University, South Road, Durham DH1 3LE, U.K.
| | - Lijuan Song
- School
of Science, Harbin Institute of Technology
(Shenzhen), Shenzhen, 518055, China
| | - Claire M. Young
- EaStCHEM, School of Chemistry, University
of St Andrews, North Haugh, St Andrews, Fife KY16 9ST, U.K.
| | - Zhuan Duan
- EaStCHEM, School of Chemistry, University
of St Andrews, North Haugh, St Andrews, Fife KY16 9ST, U.K.
| | - Andrew R. Tyler
- Chemistry,
School of Natural and Environmental Sciences, Newcastle University, Newcastle
upon Tyne NE1 7RU, U.K.
| | - Paul G. Waddell
- Chemistry,
School of Natural and Environmental Sciences, Newcastle University, Newcastle
upon Tyne NE1 7RU, U.K.
| | - Michael J. Hall
- Chemistry,
School of Natural and Environmental Sciences, Newcastle University, Newcastle
upon Tyne NE1 7RU, U.K.
| | - Michael R. Probert
- Chemistry,
School of Natural and Environmental Sciences, Newcastle University, Newcastle
upon Tyne NE1 7RU, U.K.
| | - Andrew D. Smith
- EaStCHEM, School of Chemistry, University
of St Andrews, North Haugh, St Andrews, Fife KY16 9ST, U.K.
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