Cyclometallation of Aryl-Substituted Phosphinines through C-H-Bond Activation: A Mechanistic Investigation

A series of 2,4,6-triarylphosphinines were prepared and investigated in the base-assisted cyclometalation reaction using [Cp*IrCl2]2 (Cp*= 1,2,3,4,5-pentamethylcyclopentadienyl) as the metal precursor. Insight in the mechanism of the C H bond activation of phosphinines as well as in the regioselectivity of the reaction was obtained by time-dependent 31P{1H} NMR spectroscopy. At room temperature, 2,4,6- triarylphosphinines instantaneously open the Ir-dimer and coordinate in an h1-fashion to the metal center. Upon heating, a dissociation step towards free ligand and an Ir-acetate species is observed and proven to be a first-order reaction with an activation energy of DEA=56.6 kJmol 1 found for 2,4,6-triphenylphosphinine. Electron-donating substituents on the ortho-phenyl groups of the phosphorus heterocycle facilitate the subsequent cyclometalation reaction, indicating an electrophilic C H activation mechanism. The cyclometalation reaction turned out to be very sensitive to steric effects as even small substituents can have a large effect on the regioselectivity of the reaction. The cyclometalated products were characterized by means of NMR spectroscopy and in several cases by single-crystal X-ray diffraction. Based on the observed trends during the mechanistic investigation, a concerted base-assisted metalation–deprotonation (CMD) mechanism, which i