Eliminative Nucleophilic Aromatic Substitution of Hydrogen (SNArH): An Alternative Mechanism Explaining Reactivity of N-heterocyclic Carbene (NHC) toward Tritylium

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 a deep purple color, previously considered to be from transient [IPr]●+, originates from a zwitterionic intermediate (3a), not a radical cation. Here, we propose an alterna-tive mechanism for the reaction involving tritylium and IPr: the eliminative nucleophilic aromatic substitution of hydrogen (SNArH). This mechanism is noteworthy in two respects: (1) for showing the first example of SNArH without any vicarious leaving group and (2) for explaining how [NHC–H]+ can be generated without the formation of tran-sient [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 (3a) could be generated by the eliminative SNArH mechanism for oxidants using NHCs, which is a more feasible explanation for the reactivity of NHCs with oxidants.