We report a new system for the silylation of aryl C-H
bonds. The combination of [Ir(cod)(OMe)]<sub>2</sub> and 2,9-Me<sub>2</sub>-phenanthroline
(2,9-Me<sub>2</sub>phen) catalyzes the silylation of arenes at lower
temperatures and with faster rates than those reported previously, when the
hydrogen byproduct is removed, and with high functional group tolerance and
regioselectivity. Inhibition of reactions by the H<sub>2</sub> byproduct is
shown to limit the silylation of aryl C-H bonds in the presence of the most
active catalysts, thereby masking their high activity. Analysis of initial
rates uncovered the high reactivity of the catalyst containing the sterically hindered
2,9-Me<sub>2</sub>phen ligand but accompanying rapid inhibition by hydrogen.
With this catalyst, under a flow of nitrogen to remove hydrogen, electron-rich
arenes, including those containing sensitive functional groups, undergo
silylation in high yield for the first time, and arenes that underwent
silylation with prior catalysts react over much shorter times with lower
catalyst loadings. The synthetic value of this methodology is demonstrated by
the preparation of key intermediates in the synthesis of medicinally important
compounds in concise sequences comprising silylation and functionalization.
Mechanistic studies demonstrate that the cleavage of the aryl C-H bond is
reversible and that the higher rates observed with the 2,9-Me<sub>2</sub>phen
ligand is due to a more thermodynamically favorable oxidative addition of aryl
C-H bonds.
