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-ylidene moiety with one (2) and two (3) pendent N-(2,4,6-trimethylbenzyl) groups, on reaction with [RuCl2(arene)]2 lead to ruthenium(II) complexes 5 and 6 containing the chelating 8-electron mixed arene-carbene ligand; the X-ray diffraction crystal structure of RuCl2{?1-CN[CH2(?6- 2,4,6-Me3C6H2)]CH2CH2 N(CH2CH2OMe)} 6, was established. These complexes are precursors of the unstable ruthenium-allenylidene intermediates 7 and 8, but are active catalysts either for selective catalytic alkene metathesis or cycloisomerization, depending on the nature of the 1,6-diene.