Kinetic Analysis and Sequencing of Si-H and C-H Bond Activation Reactions: Direct Silylation of Arenes Catalyzed by an Iridium-Polyhydride

摘要

The saturated trihydride IrH_3{κ~3-P,O,P-[xant(P~iPr_2)_2]} (1; xant(P~iPr_2)_2 = 9,9-dimethyl-4,5-bis(diisopropylphosphino)xanthene) coordinates the Si-H bond of triethylsilane, 1,1,1,3,5,5,5-heptamethyltrisiloxane, and triphenylsilane to give the σ-complexes IrH_3(η~2-H-SiR_3){κ~2-cis-P,P-[xant(P~iPr_2)_2]}, which evolve to the dihydride-silyi derivatives IrH_2(SiR_3){κ~3-P,O,P-[xant(P~iPr_2)_2]} (SiR_3 = SiEt_3 (2), SiMe(OSiMe_3)_2 (3), SiPh_3 (4)) by means of the oxidative addition of the coordinated bond and the subsequent reductive elimination of H2. Complexes 2-4 activate a C-H bond of symmetrically and asymmetrically substituted arenes to form silylated arenes and to regenerate 1. This sequence of reactions defines a cycle for the catalytic direct C-H silylation of arenes. Stoichiometric isotopic experiments and the kinetic analysis of the transformations demonstrate that the C-H bond rupture is the rate-determining step of the catalysis. As a consequence, the selectivity of the silylation of substituted arenes is generally governed by ligand-substrate steric interactions.