Exploring bis(cyclometalated) ruthenium(II) complexes as active catalyst precursors: Room-temperature alkene-alkyne coupling for 1,3-diene synthesis

Described is the development of a new class of bis(cyclometalated) ruthenium(II) catalyst precursors for C-C coupling reactions between alkene and alkyne substrates. The complex [(cod)Ru(3-methallyl)₂] reacts with benzophenone imine or benzophenone in a 1:2 ratio to form bis(cyclometalated) ruthenium(II) complexes (1). The imine-ligated complex 1a promoted room-temperature coupling between acrylic esters and amides with internal alkynes to form 1,3-diene products. A proposed catalytic cycle involves C-C bond formation by oxidative cyclization, β-hydride elimination, and C-H bond reductive elimination. This Ru^II/Ru^IV pathway is consistent with the observed catalytic reactivity of 1a for mild tail-to-tail methyl acrylate dimerization and for cyclobutene formation by [2+2] norbornene/alkyne cycloaddition.