Cyclometalation 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 CH bond activation of phosphinines as well as in the regioselectivity of the reaction was obtained by time-dependent P-31{H-1}NMR spectroscopy. At room temperature, 2,4,6-triarylphosphinines instantaneously open the Ir-dimer and coordinate in an (1)-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 E-A=56.6kJmol(-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 CH 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 is electrophilic in nature, is proposed.