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.