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

Abstract

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 solidstate 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.

Intermolecular CH Bond Activation by a Cationic Iridium(III) Dichloride Phenanthroline Complex

“…Much of the previous work has utilized Cp* ligands on iridium with only a few examples of other ligand systems 9. 12, 15, 16 Here we report the first example of bimolecular CH activation by a cationic iridium(III) dichloride phenanthroline complex in the gas phase. This is a 14‐electron species with a tetracoordinate iridium.…”

Iridium‐Catalysed C−H Borylation of Heteroarenes: Balancing Steric and Electronic Regiocontrol

“…The NBpin substrate 148 is prepared in situ in a similar manner to NBpin pyrrole 97 (Section 4.1.1; Scheme 16 d). [78,96,103,123] Interestingly, the corresponding reaction with B 2 eg 2 (eg = ethylene glycolato) affords the aborylated product via a process involving an electrostatic outer-sphere interaction between the NBeg group and the ancillary ligand (Scheme 16 e). [124] If borylation in the heterocyclic ring is sterically inhibited, then borylation in the carbocyclic ring occurs, often with sterically controlled regioselectivity.…”

An Ylide Transformation of Rhodium(I) Carbene: Enantioselective Three‐Component Reaction through Trapping of Rhodium(I)‐Associated Ammonium Ylides by β‐Nitroacrylates

“…In transition state V, an intimate ion pair with chloride is obtained, probably due to the solvent-assisted neutral-to-cationic switch. [21] To gain further insight into the reaction mechanism, a solution of the reaction mixture was analyzed by HPLC-MS to monitor the progress of the reaction. The molecular ion peak of the chiral rhodium(I) complex I dimer ([Rh(L3)Cl] 2 ) was found to be 793.2 (calculated [M+H] + for I is 793.04) during the entire course of the reaction.…”