Directed ortho-C-H Silylation Coupled with trans-Selective Hydrogenation of Arylalkynes Catalyzed by Ruthenium Complexes of a Xanthene-Based Si,O,Si-Chelate Ligand, "xantsil"

Ruthenium complexes bearing a xanthene-based bis(silyl) chelate ligand, "xantsil" ((9,9-dimethylxanthene-4,5-diyl)bis(dimethylsilyl)), Ru{κ³(Si,O,Si)-xantsil}(CO)(PR₃) (1-Cy: R = Cy (cyclohexyl), 1-Cyp: R = Cyp (cyclopentyl)), were found to catalyze the reactions of internal arylalkynes with tertiary silanes (1-1.3 equiv) at a moderate temperature (room temperature to 70°C) to give (E)-alkenes having an ortho-silylated aryl group, i.e., (E)-R¹C(H)=C(H)(C₆H₃-o-SiR³₃-p-R²). These catalytic reactions involve a unique ortho-selective C-H silylation of an aryl group in arylalkynes accompanied by hydrogenation of their C-C triple bonds (ortho-C-H silylation/hydrogenation). Importantly, in these reactions, the alkynyl moiety of arylalkynes serves as both a directing group and a hydrogen acceptor. The substrate scope of this ortho-C-H silylation/hydrogenation was explored by use of eight combinations of arylalkynes and tertiary silanes. In cases using bulky substrates, the catalytic performance of 1-Cyp with a relatively less bulky phosphine ligand (PCyp₃) was shown to be superior to that of the PCy₃ analogue 1-Cy.