Effect of Ligand Structure on the Electron Density and Activity of Iridium Catalysts for the Borylation of Alkanes

An in-depth study of iridium catalysts
for the borylation of alkyl
C–H bonds is reported. Although the borylation of aryl C–H
bonds can be catalyzed by iridium complexes containing phen or bpy
ligands at mild temperatures and with limiting arene, the borylation
of alkyl C–H bonds remains underdeveloped. We prepared a library
of phenanthrolines that contain varying substitution patterns. The
corresponding phen–Ir trisboryl carbon monoxide complexes were
synthesized to determine the electron-donating ability of these ligands,
and the initial rates for the borylation of the C–H bonds in
THF and diethoxyethane β to oxygen catalyzed by Ir complexes
containing these ligands were measured. For some subsets of these
ligands, the donor ability correlated positively with the rate of
C–H borylation catalyzed by the complexes containing ligands
within a given subset. However, across subsets, ligands possessing
similar donor properties to one another form catalysts for the borylation
of alkyl C–H bonds with widely varying activity. This phenomenon
was investigated computationally, and it was discovered that the stabilizing
interactions between the phenanthroline ligand and the boryl ligands
attached to Ir in the transition state for C–H oxidative addition
could account for the differences in the activity of the catalysts
that possess similar electron densities at Ir. The effect of these
interactions on the borylation of secondary alkyl C–H bonds
is larger than it is on the borylation of primary alkyl C–H
bonds.