Application of NHC-Based Iridium Pincer Complexes in β-Alkylation of Alcohols and N-Alkylation of Amines: Mechanistic Studies on Precatalyst Activation

The iridium(I) complexes [IrBr(cod)(κC-^tBuImCH₂PyCH₂NRR’)] (NRR’ = NEt₂, NH^tBu) have been prepared by reaction of the corresponding functionalized imidazolium salt with the appropriate dinuclear compound [Ir(µ-OR)(cod)]₂ (R = OMe, OEt). These compounds react with H₂(g) (5 bar) to afford the pincer iridium(III) dihydrido complexes [IrBrH₂(κ³C,N,N’-^tBuImCH₂PyCH₂NRR’)] in good yields. The complexes [IrBr(cod)(κC-^tBuImCH₂PyCH₂NRR’)] efficiently catalyzed the β-alkylation of a series of secondary alcohols and the N-alkylation of a range of aniline derivatives with primary alcohols, with good selectivities for the β-alkylated alcohol and monoalkylated secondary amine products, respectively at low catalyst loading typically 0.1 mol% and sub-stoichiometric amount of base in toluene at 383 K. The pincer iridium(III) dihydrido complexes show a catalytic performance similar to that of the iridium(I) complexes in model alkylation reactions. Mechanistic studies on the activation of the catalytic precursors have shown that both types of complexes have the ability to activate benzyl alcohol through the dearomatization of the pyridine ring by selective deprotonation of the methylene linker between the pyridine and the imidazole-2-ylidene fragment. DFT calculations suggest that activation of both catalytic precursors could lead to the common pincer iridium(I) hydrido species [IrH(κ³C,N,N-^tBuImCH₂PyCH₂NEt₂)], which may be key to the borrowing hydrogen reaction mechanism.