N-Heterocyclic carbene (NHC) ligands are intensively used in transition-metal-catalyzed reactions. Their bond-activation reactions, however, are much less understood. We now report unprecedented dearylation reactions of N-aryl substituted NHCs mediated by cobalt(-I) and rhodium(-I) species, which lead to the conversion of N-aryl NHCs into N-aryl imidazolate ligands. The reduction reactions of [(IMes)Co(η2:η2-dvtms)] (IMes = 1,3-dimesitylimidazol-2-ylidene, dvtms = divinyltetramethyldisiloxane) and [Rh(cod)Cl]2/IPr (cod = cycloocta-1,4-diene, IPr = 1,3-di(2’,6’-diisopropylphenyl)imidazol-2-ylidene) with KC8 and 18-crown-6 gave the corresponding N-aryl imidazolate complexes [(18-crown-6)K(μ-C3H2N2Mes-κC2,κN3)Co(η2:η2-dvtms)] (1) and [K(18-crown-6)(THF)][(η2:η2-cod)2Rh2(μ-C3H2N2(Dipp)-κC2,κN3)2(μ-Dipp)2K] (3) that have been characterized by NMR and X-ray diffraction studies. In supporting the possible invovlement of cobalt(-I) and rhodium(-I) species in the dearylation reactions, the three-coordinate formal cobalt(-I) and rhodium(-I) complexes [K(2,2,2-Crypt)][(ICy)Co(η2:η2-dvtms)] (2) and [K(18-crown-6)][(IPr)Rh(η2:η2-dvtms)] (5) have been obtained from the reduction reactions of [(IMes)Co(η2:η2-dvtms)] and [Rh(dvtms)Cl]2/IPr with KC8 and crown ethers. These results suggest that the C-N bond cleavage step of the dearylation reactions is likely achieved by cobalt(-I)/rhodium(-I)-mediated C-N bond oxidative addition, and also imply the possible involvement of similar degradation reaction in N-aryl NHC-transition-metal-catalyzed reactions operating at strong reducing conditions

Cobalt(−I)- and Rhodium(−I)-Mediated Dearylation of N‑Aryl N‑Heterocyclic Carbene Ligands

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