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.

Combined experimental and computational mechanistic studies of the reactions of unsymmetrical, para -substituted N -aryl imidazolium salts, L2-R 1 ,R 2 , at [MCl 2 Cp*] 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 [Et 4 N]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, [Et 4 N]Cl, can have a dramatic effect on selectivity by changing the mechanism of ligand substitution.