Reversible Insertion of Ir into Arene Ring C-C Bonds with Improved Regioselectivity at a Higher Reaction Temperature.

Regioselective metal insertion into aromatic C-C bonds is a long-standing problem critical for development of new arene functionalizations and cleaner conversion of fossil fuel into value-added chemicals. We report reversible insertion of iridium into the aromatic C-C bonds of η4-bound methyl arenes to give eight-membered diiridium metallacycles with yields up to 99%. While at 50-100 °C the reaction yields a mixture of isomers corresponding to iridium insertion in both unsubstituted and Me-substituted ring C-C bonds, at 150 °C a single isomer dominates. Kinetic and DFT studies suggest that at 150 °C insertion of iridium is reversible, allowing equilibration of the metallacycle products via a diiridium arene sandwich complex. The selectivity of metal insertion is determined by the relative stabilities of isomeric metallacycles governed by steric repulsion between methyl groups of the hydrocarbon chain of the cleaved arene and the Cp* ligands.