Iridium‐Catalyzed Silylation of Five‐Membered Heteroarenes: High Sterically Derived Selectivity from a Pyridyl‐Imidazoline Ligand

Abstract

The steric effects of substituents on five‐membered rings are less pronounced than those on six‐membered rings because of the difference in bond angles. Thus, the regioselectivities of reactions of five‐membered heteroarenes that occur with selectivities dictated by steric effects, such as the borylation of C−H bonds, have been poor in many cases. We report that the silylation of five‐membered‐ring heteroarenes occurs with high sterically derived regioselectivity when catalyzed by the combination of [Ir(cod)(OMe)]2 (cod=1,5‐cyclooctadiene) and a phenanthroline ligand or a new pyridyl‐imidazoline ligand that further increases the regioselectivity. The silylation reactions with these catalysts produce high yields of heteroarylsilanes from functionalization at the most sterically accessible C−H bonds of these rings under conditions that the borylation of C−H bonds with previously reported catalysts formed mixtures of products or products that are unstable. The heteroarylsilane products undergo cross‐coupling reactions and substitution reactions with ipso selectivity to generate heteroarenes that bear halogen, aryl, and perfluoroalkyl substituents.

Mechanism of the Iridium-Catalyzed Silylation of Aromatic C–H Bonds

“…This origin of selectivity contrasts with that we reported recently for the silylation of five-membered ring heteroarenes. 6 In our study of H/D exchange between the silane and these heteroarenes, the selectivity for H/D exchange at the least hindered C−H bond was lower than the selectivity for silylation at the least sterically hindered C−H bond. These data imply that the high selectivity for reactions of these less hindered substrates results from a slower rate for reductive elimination from a more hindered arylsilane than from a less hindered arylsilane.…”

C–H Bond Silylation of Heteroarenes

“…Meanwhile, the Hartwig group developed another efficient method for the silylation of five-membered heteroarenes using a sterically controlled strategy with the help of either a pyridine-imidazoline ligand ( 43 ) or a phenanthroline ligand ( 44 ) (Scheme 12 ). 23 Because of the acute bond angles in five-membered rings, the distance between the substituents is greater than those in six-membered rings, making the steric influence on the regioselectivity of five-membered heterocycles less pronounced. Mechanistic studies revealed that the rate of formation of C–Si bonds from isomeric heteroaryliridium complexes influences the selectivities of the silylation reactions more than the rates of the cleavage of carbon–hydrogen bonds during complex formation.…”