A hexahydro-s-indacene based NHC ligand for olefin metathesis catalysts

The reaction of amino hexahydro-s-indacene with glyoxal, reduction of the diimine to the diamine and cyclization with HC(OEt)3 yields the new 1,3-bis(1,2,3,5,6,7-hexahydro-s-indacen-4-yl)-4,5-dihydro-1H-imidazol-3-ium chloride 3· HCl with conformationally restricted alkyl groups ortho to the heterocyclic substituent. Reaction of 3· HCl with Ag2O gave the respective silver NHC complex (yield 78%), which was used to synthesize the respective (3)IrCl(cod) (yield 86%), whose reaction with CO gave (3)IrCl(CO)26 (yield 98%). The Grubbs II type complex [(3)RuCl2(PCy3)(3-phenyl-indenylid-1-ene)] was synthesized (yield 68%) and converted into [(3)RuCl2(py)(3-phenyl-indenylid-1-ene)] (yield 78%) in pyridine solvent. With this complex the respective Grubbs–Hoveyda species (yield 68%) and two bisNHC complexes [(3)(NHCewg)RuCl2(3-phenyl-indenylid-1-ene)] (yield 87%, 89%) were obtained. The stereoelectronic properties of the new NHC ligand were determined employing IR spectroscopy, cyclic voltammetry and buried-volume analysis based on the crystal structure of complex 6. The activity of the ruthenium complexes in RCM reactions of sterically hindered substrates was tested.

Graphical abstractA new hexahydro-s-indacene based NHC ligand used for the synthesis of ruthenium-based olefin metathesis catalysts for sterically demanding RCM reactions is reported.Figure optionsDownload full-size imageDownload as PowerPoint slideHighlights► Synthesis of a new NHC ligand. ► Synthesis of new metal NHC complexes with Ag, Ir and Ru. ► Stereoelectronic properties of new NHC ligand. ► Olefin metathesis activity in RCM reactions of sterically demanding substrates.