C–H Activation\nof Pyridines by Boryl Pincer\nComplexes: Elucidation of Boryl-Directed C–H Oxidative Addition\nto Ir and Discovery of Transition Metal-Assisted Reductive Elimination\nfrom Boron at Rh

Experimental and theoretical techniques were used to\ninvestigate\nthe mechanism of pyridine C–H activation by diarylboryl/bis(phosphine)\nPBP pincer complexes of Ir. The critical intermediate (PBP)IrCO (<b>4</b>) contains a three-coordinate, Ir-bound boron that retains\nLewis acidity in the perpendicular direction. Coordination of pyridine\nto this boron center in <b>4</b> leads to fast insertion of\nIr into the 2-CH bond of pyridine, providing a different topology\nof direction than the conventional directed C–H activation\nwhere both the directing group coordination and C–H activation\nhappen at the same metal center. Beyond this critical sequence, the\nsystem possesses significant complexity in terms of possible isomers\nand pathways, which have been thoroughly explored. Kinetic and thermodynamic\npreferences for the activation of differently substituted pyridines\nwere also investigated. In experimental work, the key intermediate <b>4</b> is accessed via elimination of benzene from a phenyl/hydride\ncontaining precursor (PB<sup>Ph</sup>P)IrHCO (<b>3</b>). Density\nfunctional theory (DFT) investigations of the mechanism of benzene\nloss from <b>3</b> revealed the possibility of a genuinely new\ntype of mechanism, whereby the Ph–H bond is made in a concerted\nprocess that is best described as C–H reductive elimination\nfrom boron, assisted by the transition metal (TMARE). For Ir, this\npathway was predicted to be competitive with the more conventional\npathways involving C–H reductive elimination from Ir, but still\nhigher in energy barrier. However, for the Rh analog <b>3-Rh</b>, TMARE was calculated to be the preferred pathway for benzene loss\nand this prediction was experimentally corroborated through the study\nof reaction rates and the kinetic isotope effect.