Iridium-Catalyzed C–H Borylation of Heterocycles Using an Overlooked 1,10-Phenanthroline Ligand: Reinventing the Catalytic Activity by Understanding the Solvent-Assisted Neutral to Cationic Switch

The preformed catalyst [Ir(Cl)(COD)(1,10-phenanthroline)] ( 2; COD = cyclooctadiene) was found to be highly effective in a model reaction for the borylation of N-Boc-indole at the 3-position with B 2 pin 2 (pin = pinacolato) as the borylating agent to give consistently 99% yield with 0.5 mol % catalyst loading. The corresponding in situ formed catalyst from [Ir(Cl)(COD)] 2 and 1,10-phenanthroline provided very inconsistent results for the same reaction (0–94% conversion). We propose this to be due to the competing formation of a catalytically inactive cationic complex, [Ir(COD)(1,10-phenanthroline)] + Cl – ( 1 ), in a noncoordinating solvent such as octane. Complexes 1 and 2 were characterized using solid-state NMR ( 13 C and 35 Cl) in conjunction with XPS to be cationic and neutral, respectively. The X-ray crystal structure of a pentavalent neutral Ir complex, [Ir(Cl)(COD)(2,2′-bipyridine)] ( 3 ), was also obtained for comparison purposes. Using catalyst 2, the total synthesis of Meridianin G was accomplished in 87% overall isolated yield in a one-pot, three-step process.