Adamantyl-substituted N-heterocyclic carbene ligands in second-generation Grubbs-type metathesis catalysts

The N-heterocyclic carbene (NHC) ligands H(2)iAd (H(2)IAd = 1,3-di(1-adamantyl)-4,5-dihydroimidazol-2-ylidene) and H(2)IAdMes (H(2)IAdMes = 1-(1-adamantyl)-3-mesityl-4,5-dihydroimidazol-2-ylidine) were prepared and treated with [RuCl2(=CHPh)(PCy3)(2)] (1). While H(2)IAd failed to react with 1, H(2)IAdMes readily produced [RuCl2(=CHPh)(H(2)IAdMes)(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.