Scope and mechanistic studies of intramolecular aliphatic C-H bond activation of N-heterocyclic carbene iridium complexes

The series Cp*Ir(NHC) (Cp* = eta(5)-pentamethylcyclopentadienyl; NHC = N-heterocyclic carbene) complexes, Cp*Ir(IEt)Cl-2 (1a), Cp*Ir(IPr)Cl-2 (1b), and Cp*Ir(IBu)Cl-2 (1c) (IEt = 1,3-diethylimidazol-2-ylidene; IPr = 1,3-di-n-propylimidazol-2-ylidene; IBu = 1,3-di-n-butylimidazol-2-ylidene) have been prepared by the carbene-transfer method using silver salts. The reactions of 1a-c with 1 equiv of i-PrONa in isopropyl alcohol give chloro hydrido complexes Cp*Ir(NHC)(H)(Cl) (2a-c). The reaction of 1a with 2 equiv of i-PrONa in isopropyl alcohol results in the intramolecular C-H activation of the ethyl group in the NHC ligand to give Cp*Ir(IEt')(H) (3), while the similar reactions of 1b and 1c give dihydrido complexes Cp*Ir(IPr)(H)(2) (4b) and Cp*Ir(IBu)(H)(2) (4c) as main products, respectively. These reactions proceed via alkoxo species, [Cp*Ir(NHC)(Oi-Pr)]Cl and Cp*Ir(NHC)(Oi-Pr)(H), as the key intermediates. Derivation of 3 into the chloro complex Cp*Ir(IEt')(Cl) (5) and cationic complexes [Cp*Ir(IEt')(L)]OTf [L = acetonitrile (6); pyridine (7)] is also described.