Base-promoted selective aryl C-Br and C-I bond cleavage by iridium(III) porphyrin: Reduction of Ir~(III)-OH to Ir~(II) for metalloradical ipso substitution of aryl-halogen bonds

摘要

Base-promoted selective aryl carbon-bromine and carbon-iodine bond (Ar-X, X = Br, I) cleavage by iridium(III) porphyrin carbonyl chloride (Ir ~(III)(ttp)(CO)Cl) was achieved in the presence of base (K _2CO_3, NaOH) to give iridium(III) porphyrin aryls (Ir ~(III)(ttp)Ar). Mechanistic studies revealed that the base undergoes ligand substitution with Ir(ttp)(CO)Cl to yield an iridium(III) hydroxo species (Ir~(III)(ttp)OH). The hydroxo ligand most likely reduces the Ir(III) center to give iridium(II) porphyrin dimer ([Ir~(II)(ttp)]_2) and H_2O_2. In a competitive pathway, [Ir II(ttp)]_2 disproportionates in the presence of base and residual water to give an iridium(III) hydride (Ir~(III)(ttp)H) and Ir(ttp)OH. In a productive process, [Ir(ttp)]_2 undergoes Ir II(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.