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Experimental and Computational Studies on the Acetate-Assisted C–H Activation of N-Aryl Imidazolium Salts at Rhodium and Iridium: A Chloride Additive Changes the Selectivity of C–H Activation

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posted on 2024-04-16, 15:09 authored by N Tamosiunaite, LC Logie, SE Neale, K Singh, DL Davies, SA Macgregor
Combined experimental and computational mechanistic studies of the reactions of unsymmetrical, para-substituted N-aryl imidazolium salts, L2-R1,R2, at [MCl2Cp*]2 (M = Rh, Ir) in the presence of NaOAc are reported. These proceed via intermediate N-heterocyclic carbene complexes that then allow an internal competition between two differently substituted aryl rings toward C–H activation to be monitored. At 348 K in dichloroethane C–H activation of the aryl with the more electron-withdrawing substituents is generally favored. DFT calculations show similar barriers for proton transfer and dissociative HOAc/Cl– ligand substitution, with proton transfer favoring electron-donating substituents, and ligand substitution favoring electron-withdrawing substituents. Microkinetic simulations reproduce the experimental preference implying that the ligand substitution step dominates selectivity. For several substrates, notably L2-F,OMe and L2-F,H, running the C–H activation reactions at 298 K in the presence of added [Et4N]Cl reverses the selectivity. The greater availability of chloride in solution makes an alternative dissociative interchange ligand substitution mechanism accessible, leaving proton transfer as selectivity determining and so favoring electron-donating substituents. Our results highlight the potential importance of the ligand substitution step in the interpretation of substituent effects and demonstrate how a simple additive, [Et4N]Cl, can have a dramatic effect on selectivity by changing the mechanism of ligand substitution.

History

Author affiliation

Department of Chemistry, University of Leicester

Version

  • AM (Accepted Manuscript)

Published in

Journal of Organic Chemistry

Volume

87

Issue

2

Pagination

1445 - 1456

Publisher

American Chemical Society (ACS)

issn

0022-3263

eissn

1520-6904

Copyright date

2022

Available date

2024-04-16

Spatial coverage

United States

Language

eng