Base-Promoted Selective Aryl C–Cl Cleavage by Iridium(III) Porphyrins via a Metalloradical Ipso Addition–Elimination Mechanism

Abstract

Base-promoted aryl carbon–chlorine bond (Ar–Cl) cleavage by iridium(III) porphyrin carbonyl chloride (IrIII(ttp)(CO)Cl; ttp = 5,10,15,20-tetrakis(p-tolyl)porphyrinato dianion) was achieved in the presence of K2CO3 to give iridium(III) porphyrin aryls (IrIII(ttp)Ar). Mechanistic studies revealed that K2CO3 promotes the reduction of Ir(ttp)(CO)Cl to give the iridium(II) porphyrin dimer intermediate [IrII(ttp)]2. [Ir(ttp)]2 is the source of IrII(ttp) metalloradical, which cleaves Ar–Cl to give Ir(ttp)Ar and a chlorine radical (Cl•) via radical ipso substitution in an addition–elimination pathway. Cl• reacts with [Ir(ttp)]2 to yield Ir(ttp)Cl for subsequent base-promoted reduction and Ir(ttp) for radical chain propagation. Additionally, the base-promoted Ar–Cl cleavage of chlorobenzene (PhCl) by Ir(ttp)(CO)Cl gives both Ir(ttp)Ph and 1,4-bis-iridium(III)-porphyrin benzene, IrIII(ttp)(p-C6H4)IrIII(ttp). The reactive Cl• can simultaneously react with PhCl via homolytic aromatic substitution to give 1,4-dichlorobenzene, which further undergoes double Ar–Cl cleavage to form Ir(ttp)(p-C6H4)Ir(ttp).