First ruthenium complexes with a chelating arene carbene ligand as catalytic precursors for alkene metathesis and cycloisomerisation

Electron-rich carbene precursors 2 and 3, containing the imidazolidin-2-yli dene moiety with one (2) and two (3) pendent N-(2,4,6-trimethylbenzyl) grou ps, on reaction with [RuCl2(arene)](2) lead to ruthenium(II) complexes 5 an d 6 containing the chelating 8-electron mixed arene-carbene ligand; the X-r ay diffraction crystal structure of RuCl2{eta (1)- CN[CH2(eta (6)-2,4,6-Me3 C6H2)]CH2CH2N(CH2CH2OMe)} 6, was established. These complexes are precursor s of the unstable ruthenium-allenylidene intermediates 7 and 8, but are act ive catalysts either for selective catalytic alkene metathesis or cycloisom erization, depending on the nature of the 1,6-diene.