Abstract:
The reaction of electron-rich carbene-precursor olefins containing two imidazolinylidene moieties [(2,4,6- Me3C6H2CH2)NCH2 CH2N(R)C=]2 (2a: R = CH2CH2OMe, 2b R = CH2Mes), bearing at least one 2,4,6-trimethylbenzyl (R = CH2Mes) 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(II) complex 3. Then a further arene displacement reaction takes place to give the chelated η6-mesityl,η1-carbene - ruthenium complexes 4 and 5. An analogous η6-a- rene,η1-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 η6-arene,η1-carbene complex 9. On reaction with AgOTf and the propargylic alcohol HC≡CCPh2OH, 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- [η1CN(CH2C6H2 Me3-2,4,6)CH2CH2N- (CH2CH2OMe)}(C6Me6)] 3 with a monodentate carbene ligand and [RuCl2{η1- CN[CH2(η6-C6H2 Me3-2,4,6)]CH2CH2N- (CH2C6H2Me3-2,4,6)}] 5 with a chelating carbene-arene ligand were characterised by X-ray crystallography.