Base-Promoted Selective Aryl C–Br and C–I Bond Cleavage by Iridium(III) Porphyrin: Reduction of IrIII–OH to IrII for Metalloradical Ipso Substitution of Aryl–Halogen Bonds

Abstract

Base-promoted selective aryl carbon–bromine and carbon–iodine bond (Ar–X, X = Br, I) cleavage by iridium(III) porphyrin carbonyl chloride (IrIII(ttp)(CO)Cl) was achieved in the presence of base (K2CO3, NaOH) to give iridium(III) porphyrin aryls (IrIII(ttp)Ar). Mechanistic studies revealed that the base undergoes ligand substitution with Ir(ttp)(CO)Cl to yield an iridium(III) hydroxo species (IrIII(ttp)OH). The hydroxo ligand most likely reduces the Ir(III) center to give iridium(II) porphyrin dimer ([IrII(ttp)]2) and H2O2. In a competitive pathway, [IrII(ttp)]2 disproportionates in the presence of base and residual water to give an iridium(III) hydride (IrIII(ttp)H) and Ir(ttp)OH. In a productive process, [Ir(ttp)]2 undergoes IrII(ttp) metalloradical-mediated ipso substitution of Ar–X via an addition–elimination pathway to form Ir(ttp)Ar and Ir(ttp)X. Ir(ttp)X is recycled by reacting with base to regenerate [Ir(ttp)]2 for subsequent Ar–X cleavage.