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, NHtBu) 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 catalyst precursors have shown that both type 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 th common pincer iridium(I) species [IrH(κ3C,N,N-tBuImCH2PyCH2NEt2)], which may be key to the borrowing hydrogen reaction mechanism.