Silylation of Aromatic Carbon-Hydrogen Bonds

5 Silylation of Aromatic Carbon-Hydrogen Bonds 359 SiPha 

The direct silylation of arenes through C—H bond activation provides an attractive route for the synthesis of useful aromatic compounds [64]. Vaska s complex was the first of the iridium catalysts to be reported for activation of the C—H bond in benzene by Si—H of pentamethyldisiloxane to yield phenylsubstituted siloxane [65]. However, a very attractive method for the aromatic C—H silylation with disilanes has been recently reported by the groups of Ishiyama and Miyaura [66-68]. 

The reaction of tetrafluorodi-sec-butyldisUane of many arenes catalyzed by Ir(I) complexes generated from [ Ir(p-OMe)(cod) 2] and 2,9-diisopropyl-l,10-phenan-troline (dipphen) results in the formation of the relevant corresponding arylfluo-rosilanes in high yields and with excellent regioselectivities (Equation 14.8). 

The aromatic silylation of five-membered heteroarenes under the same conditions (catalyst, temperature, solvent) also proceeded in regioselective fashion. Both, thiophene and furane derivatives are exclusively silylated at the a-position, but 1-triisopropylsily 1-pyrrole and -indole each produce selectively ]3-silyl products (Equations 14.9 and 14.10). 

Fluorosilylsubstituted aryl derivatives were found to be useful reagents for carbon-carbon bond formation via palladium-catalyzed cross-coupling with aryl halides in the presence of fluoride anions as Si—C bond activator in dimethylformamide (DMF), as well as rhodium-catalyzed 1,4-addition to a, 3-unsaturated ketones in the presence of a fluoride anion source (Equation 14.11) [66, 69, 70], 

---