η6-mesityl,η1-imidazolinylidene-carbene-ruthenium(II) complexes:: Catalytic activity of their allenylidene derivatives in alkene metathesis and cycloisomerisation reactions

The reaction of electron-rich carbene-precursor olefins containing two imidazolinylidene moieties [(2,4,6-Me3C6H2CH2)NCH2CH2N(R)C=](2) (2a: R = CH2CH2OMe, 2b R = CH(2)Mes), bearing at least one 2,4,6-trimethylbenzyl (R = CH(2)Mes) group on the nitrogen atom, with [RuCl2(arene)](2) (arene = p-cymene, hexamethylbenzene) selectively leads to two types of complexes. The cleavage of the chloride bridges occurs first to yield the expected (carbene) (arene)ruthenium(H) complex 3. Then a further arene displacement reaction takes place to give the chelated eta(6)-mesityl,eta'-carbene-ruthenium complexes 4 and 5. An analogous eta(6)-arene-eta(1)-carbene complex with a benzimidazole frame 6 was isolated from an in situ reaction between [RuCl2(p-cymene)](2), the corresponding benzimidazolium salt and cesium carbonate. On heating, the RuCl2(imidazolinylidene) (p-cymene) complex 8, with p-methoxybenzyl pendent groups attached to the N atoms, leads to intramolecular p-cymene displacement and to the chelated eta(6)-arene,eta(1)-carbene complex 9. On reaction with AgOTf and the propargylic alcohol HC=CCPh20H, compounds 4-6 were transformed into the corresponding ruthenium allenylidene intermediates (4-->10, 5-->11, 6-->12). The in situ generated intermediates 10-12 were found to be active and selective catalysts for ring-closing metathesis (RCM) or cycloisomerisation reactions depending on the nature of the 1,6-dienes. Two complexes [RuCl2-{eta(1)-CN(CH2C6H2Me3-2,4,6)CH2CH2N- (CH2CH2OMe)}(C6Me6)] 3 with a monodentate carbene ligand and [RuCl2-{eta(1)-CN[CH2(eta(6)-C6H2Me3-2,4,6)]CH2CH2N-(CH2C6H2Me3-2,4,6)}] 5 with a chelating carbene-arene ligand were characterised by X-ray crystallography.