Dimerization of terminal arylalkynes in aqueous medium by ruthenium and acid promoted (RAP) catalysis: acetate-assisted (sp)C-(sp2)C bond formation

The hexamethylbenzene ruthenium(II) dimer [{RuCl(¼-Cl)(·6-C6Me6)}]2 (5 mol%), tested among a series of ruthenium(II) and ruthenium(IV) complexes, represents an efficient precatalyst source for the dimerization of terminal aryl alkynes ArCaCH (Ar = C6H5, 3,4,5-(OMe)3C6H2, 4-MeOC6H4, 2-MeOC6H4, 4-MeC6H4, 2,4,5-Me3C6H2, 4-BrC6H4, 4-ClC6H4, 4-FC6H4, 4-HC(=O)C6H4, 4-CH2=CHC6H4, 3-NCC6H4, 4-O2NC6H4, 4-EtO2C-(CH2)3OC6H4, 4-HO(CH2CH2O)3C6H4, 3-HO(CH2CH2O)3-C6H4) in acetic acid/water mixture (1:1, v/v). The reactions proceed for 24 h at r.t. under heterogeneous conditions and afford the dimeric enyne derivatives (E)-Ar-CH=CH-CaC-Ar in high yields and stereoselectivity. The preformed acetato complex [RuCl(·6-C6Me6)(º2-OAc)] catalyzes the dimerization of phenylacetylene under analogous conditions, with rapid substrate conversion. The presence of cosolvents of acetic acid different from water reduces dramatically efficiency and selectivity of the reaction. The aqueous medium facilitates the activation stage of the precatalyst by assisting the splitting of the ruthenium dimer. The addition or generation in situ of acetate salts result in shorter reactions times (0.5-3 h) and excellent yields, due to the rapid formation of active acetato complexes. Circumstantial evidence suggests that the À-bound alkyne molecule is activated by intramolecular proton abstraction. This is currently the most efficient, E-selective and wide-scope catalytic system for the alkyne dimerization reaction in protic aqueous media. Keywords: Terminal alkynes; Dimerization; Conjugated enynes; Ruthenium carboxylate catalysts; Bifunctional catalysis