Initial Results with Phosphine Ligands

Third generation initiators are based on the NHC system of second generation initiators, but do not contain any phosphine ligand. Instead, one or two pyridine ligands are weakly bound to the ruthenium centre (c/. Fig. 3.28, complexes 73 and 74c). Pyridine dissociates very easily and hardly competes with the olefin for the coordination site. As a result, complete initiation and fast propagation are enabled, therefore living polymerisation is rendered possible. 

Remarkably, the catalytic cycle is not controlled by the presence of phosphine ligands, but it is controlled by the organo group Y at the cobalt the neutral ligand L is displaced by the substrates in the initial step. Oxidative addition of two acetylenes results in a cobaltacycle that reacts with the nitrile to give the pyridine derivative with regeneration of the active [YCo] species. 

The initial study in this area employed the meso-1,3-dimethylallyl ligand.432 The chloride dimer was generated from 2-pentene, and a variety of optically active phosphine ligands were added to form the chiral bisphosphine complexes. Reaction of these allyl complexes with sodiodiethyl malonate resulted in optical yields in the range of 2-29% (equation 351). 

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