η 6 ‐Mesityl, η 1 ‐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‐Me 3 C 6 H 2 CH 2 )NCH 2 CH 2 N(R)C] 2 ( 2a : R=CH 2 CH 2 OMe, 2 b R=CH 2 Mes), bearing at least one 2,4,6‐trimethylbenzyl (R=CH 2 Mes) group on the nitrogen atom, with [RuCl 2 (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 η 6 ‐mesityl, η 1 ‐carbene–ruthenium complexes 4 and 5 . An analogous η 6 ‐arene, η 1 ‐carbene complex with a benzimidazole frame 6 was isolated from an in situ reaction between [RuCl 2 ( p ‐cymene)] 2 , the corresponding benzimidazolium salt and cesium carbonate. On heating, the RuCl 2 (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 η 6 ‐arene, η 1 ‐carbene complex 9 . On reaction with AgOTf and the propargylic alcohol HCCCPh 2 OH, 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 [RuCl 2 { η 1 ‐CN(CH 2 C 6 H 2 Me 3 ‐2,4,6)CH 2 CH 2 N‐ (CH 2 CH 2 OMe)}(C 6 Me 6 )] 3 with a monodentate carbene ligand and [RuCl 2 { η 1 ‐CN[CH 2 ( η 6 ‐C 6 H 2 Me 3 ‐2,4,6)]CH 2 CH 2 N‐(CH 2 C 6 H 2 Me 3 ‐2,4,6)}] 5 with a chelating carbene–arene ligand were characterised by Xray crystallography.