Adamantyl-Substituted <i>N</i>-Heterocyclic Carbene Ligands in Second-Generation Grubbs-Type Metathesis Catalysts

The N-heterocyclic carbene (NHC) ligands H2IAd (H2IAd = 1,3-di(1-adamantyl)-4,5-dihydroimidazol-2-ylidene) and H2IAdMes (H2IAdMes = 1-(1-adamantyl)-3-mesityl-4,5-dihydroimidazol-2-ylidine) were prepared and treated with [RuCl2(CHPh)(PCy3)2] (1). While H2IAd failed to react with 1, H2IAdMes readily produced [RuCl2(CHPh)(H2IAdMes)(PCy3)] (7). Only a single isomer of 7 was formed, this being that with the mesityl ring situated above the benzylidene moiety, as confirmed by an X-ray structure. Complex 7 was found to be only a very poor olefin metathesis catalyst, likely a consequence of the excessive steric crowding imparted by the 1-adamantyl moiety towards the position trans to the benzylidene group.