Revisiting the Reaction of IPr with Tritylium: An Alternative Mechanistic Pathway

Despite the high ionization energy of imidazol-2-ylidene, the reactions of N-heterocyclic carbenes (NHCs) with oxidants have been postulated to occur via a transient NHC radical cation intermediate. Herein, we propose the alternative mechanism to explain the outcome of the tritylium reaction with IPr without the intermediacy of unstable [IPr]⋅⁺.

Abstract

Despite a recent proposal on the mechanism of a single-electron transfer (SET) process between tritylium and 2,6-bis(diisopropylphenyl)imidazol-2-ylidene (IPr) based on evidence of transient IPr radical cation intermediate ([IPr]⋅⁺) formation, such oxidation is still contentious because of the high oxidation potential of N-heterocyclic carbenes. Our experimental analysis indicates that the appearance of deep purple color, previously considered to be from transient [IPr]⋅⁺, originates from a zwitterionic intermediate (3 a), not a radical cation. Here, we propose an alternative mechanism for the reaction involving tritylium and IPr. This mechanism is noteworthy for explaining how [NHC−H]⁺ can be generated without the formation of transient [NHC]⋅⁺, which has been frequently proposed as an intermediate for the reaction between NHC and oxidants. These results also show that a transient strong single-electron donor (3 a) could be generated by the alternative mechanism for oxidants using NHCs, which is a more feasible explanation for the reactivity of NHCs with oxidants.

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