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

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(eta(2):eta(2)-dvtms)] (IMes = 1,3-dimesitylimidazol-2-ylidene, dvtms = divinyltetramethyldisiloxane) and [Rh(cod)Cl](2)/IPr (cod = cycloocta-1,5-diene, IPr = 1,3-bis(2',6'-diisopropylphenyl)imidazol-2-ylidene) with KC8 and 18-crown-6 gave the corresponding N-aryl imidazolate complexes [(18-crown-6)K(mu-C(3)H(2)N(2)Mes-kappa C-2,kappa N-3)Co(eta(2):eta(2)-dvtms)] (1) and [K(18-crown-6) (THF)] [(eta(2):eta(2)-cod)(2)Rh-2(mu-C3H2N2(Dipp)-kappa C-2,kappa N-3)(2)(mu-Dipp)(2)K] (3), which have been characterized by NMR and X-ray diffraction studies. In supporting the possible involvement 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(eta(2):eta(2)-dvtms)] (2) and [K(18-crown-6)][(IPr)Rh(eta(2):eta(2) dvtms)] (5) have been obtained from the reduction reactions of [(ICy)Co(eta(2):eta(2)-dvtms)] (ICy = 1,3-dicyclohexylimidazol-2-ylidene) 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 reactions in N-aryl NHC-transition-metal-catalyzed reactions operating under strong reducing conditions.