Alkene metathesis dissociative mechanisms

Based on the insight that a dissociative mechanism plays the major role along the metathesis pathway [11], these catalysts have been designed such that only one bulky phosphine, one chloride and one cumulenylidene ligand are attached to a Ru(II) center. Because arene ligands are known to be labile on such a metal fragment, they will easily liberate free coordination sites ( ) for the interaction with the alkene substrate. Although the precise mode of action of such allenyli-... 

Typical alkene metathesis precatalysts take the form displayed in Figure 2.3, consisting of a ruthenium(II) center, a carbene with substituent R, two anionic ligands X (typically chloride), a nondissociating l and L (typically a trialkylphosphine or NHC), and a dissociating ligand, which is most often either a phosphine or a chelating alkoxyarene. While the nature of X, L,, and R all influence the initiation rate and mechanism, it is the nature of L and X that detemiine the catalytic activity of the active species itself complexes G2, M2, and GH2 all produce the same active species, albeit via different mechanisms and at different rates. 

The initiation event is a key step in the alkene metathesis mechanism, and considerable effort has been expended in attempts to understand it (vide supra). Much of the interest in this step was driven by what was seen as the counterintuitive order of initiation rates for G1 and G2 the NHC present in G2 would be expected to accelerate dissociation of the phosphine due to the trans effect, yet it was found that G2 initiated orders of magnitude... 

It is generally accepted that the Ru-catalyzed alkene metathesis reaction proceeds via a dissociative mechanism, which is initiated by the dissociation of a phosphine ligand from RuX2(PR3)L(=CHR) to form a 14-electron species ( B ). A i in this sense, catalyst initiation involves the dissociation of PCyj for both I and n. However,... 

Usefid reviews address alkyl zirconocene catalysts for the pol)mierisation of silanes to poly silanes by a a-bond metathesis mechanism, chiral titanates as promoters in aldol reactions, and MeTiCl3 as a reagent for chelate-controlled carbonyl addition reactions. 5 xhe reactions of terminally functionalized alkenes with zirconocene hydrides are reviewed. Thermochemical studies show that while the bond dissociation enthalpies of Zr—C6H13 and Zr— CgHjj in zirconocene systems are comparable, the insertion of cyclohexene into the Zr—bond is more exothermic than the insertion of hexene. 

Early work had suggested mechanisms for metathesis reactions catalyzed by G1 in which the alkene might coordinate first, followed by phosphine dissociation, or where both phosphine Hgands remain coordinated throughout the catalytic cycle. However, it was later estabHshed that intermediates in the catalytic cycle were monophosphine ruthenium complexes, and not bisphosphine species Lloyd-Jones has previously discussed and summarized some of these early studies. 

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