C–H Activation of Pyridines by Boryl Pincer Complexes: Elucidation of Boryl-Directed C–H Oxidative Addition to Ir and Discovery of Transition Metal-Assisted Reductive Elimination from Boron at Rh

Experimental and theoretical techniques were used to
investigate
the mechanism of pyridine C–H activation by diarylboryl/bis(phosphine)
PBP pincer complexes of Ir. The critical intermediate (PBP)IrCO (4) contains a three-coordinate, Ir-bound boron that retains
Lewis acidity in the perpendicular direction. Coordination of pyridine
to this boron center in 4 leads to fast insertion of
Ir into the 2-CH bond of pyridine, providing a different topology
of direction than the conventional directed C–H activation
where both the directing group coordination and C–H activation
happen at the same metal center. Beyond this critical sequence, the
system possesses significant complexity in terms of possible isomers
and pathways, which have been thoroughly explored. Kinetic and thermodynamic
preferences for the activation of differently substituted pyridines
were also investigated. In experimental work, the key intermediate 4 is accessed via elimination of benzene from a phenyl/hydride
containing precursor (PBPhP)IrHCO (3). Density
functional theory (DFT) investigations of the mechanism of benzene
loss from 3 revealed the possibility of a genuinely new
type of mechanism, whereby the Ph–H bond is made in a concerted
process that is best described as C–H reductive elimination
from boron, assisted by the transition metal (TMARE). For Ir, this
pathway was predicted to be competitive with the more conventional
pathways involving C–H reductive elimination from Ir, but still
higher in energy barrier. However, for the Rh analog 3-Rh, TMARE was calculated to be the preferred pathway for benzene loss
and this prediction was experimentally corroborated through the study
of reaction rates and the kinetic isotope effect.