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 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– (1), in a noncoordinating solvent such as octane. Complexes 1 and 2 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)]
(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.