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.