η6‐Mesityl,η1‐Imidazolinylidene–Carbene–Ruthenium(II) Complexes: Catalytic Activity of their Allenylidene Derivatives in Alkene Metathesis and Cycloisomerisation Reactions

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Abstract
The reaction of electron-rich carbene-precursor olefins containing two imidazolinylidene moieties [(2,4,6-Me3C6H2CH2)NCH2CH2N(R)C]2 (2a: R=CH2CH2OMe, 2 b 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-arene,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{η1-CN(CH2C6H2Me3-2,4,6)CH2CH2N- (CH2CH2OMe)}(C6Me6)] 3 with a monodentate carbene ligand and [RuCl2{η1-CN[CH2(η6-C6H2Me3-2,4,6)]CH2CH2N-(CH2C6H2Me3-2,4,6)}] 5 with a chelating carbene–arene ligand were characterised by Xray crystallography.