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

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.