Chiral NCN Pincer Iridium(III) Complexes with Bis(imidazolinyl)phenyl Ligands: Synthesis and Application in Enantioselective C-H Functionalization of Indoles with α-Aryl-α-diazoacetates

A series of chiral NCN pincer iridium(III) complexes 2a-h with bis(imidazolinyl)phenyl ligands were synthesized via the central aryl C2-H bond activation of the 1,3-bis(2'-imidazolinyl)benzene ligands. The incorporation of a 5-tert-butyl group into the central aryl ring of the ligands was found to markedly improve the efficiency of the desired C2-metalation, leading to an obvious enhancement in the yields of the Ir(III) complexes. Consequently, complexes with a tert-butyl group on the central aryl ring were obtained in 34-47% yields, whereas those without the group were produced in only 13-16% yields. All of the new complexes have been characterized by elemental analysis and H-1 and C-13{H-1} NMR spectroscopy. In addition, the molecular structures of complexes 2c, 2d, and 2g' have been determined by X-ray single-crystal diffraction. 2c and 2d are, indeed, the anticipated six-coordinate pincer Ir(III) complexes. In contrast, 2g' is a coordinatively unsaturated five-coordinate pincer Ir(III) complex. The Ir(III) complexes were used as the catalysts for the asymmetric C-H insertion reaction of alpha-aryl-alpha-diazoacetates with N-protected indoles. With a catalyst loading of 3 mol % and in the presence of 6 mol % of NaBArF, a variety of optically active indole derivatives bearing chiral functional groups at the C3 position were obtained in good yields with moderate to good enantioselectivities (up to 86% ee).