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)] (<b>2</b>; COD = cyclooctadiene) was found to be highly effective in a model reaction for the borylation of N-Boc-indole at the 3-position with B₂pin₂ (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^– (<b>1</b>), in a noncoordinating solvent such as octane. Complexes <b>1</b> and <b>2</b> were characterized using solid-state NMR (¹³C and ³⁵Cl) 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)] (<b>3</b>), was also obtained for comparison purposes. Using catalyst <b>2</b>, the total synthesis of <i>Meridianin G</i> was accomplished in 87% overall isolated yield in a one-pot, three-step process