Alkene metathesis phosphine dissociation

Fig. 17. Mechanistic studies of alkene metathesis provided methods to increase the rates of reaction. Copper (I) chloride and hydrochloric acid have been used to encourage dissociation of the phosphine ligands...

We note that there are NMR-based kinetic studies on zirconocene-catalyzed pro-pene polymerization [32], Rh-catalyzed asymmetric hydrogenation of olefins [33], titanocene-catalyzed hydroboration of alkenes and alkynes [34], Pd-catalyzed olefin polymerizations [35], ethylene and CO copolymerization [36] and phosphine dissociation from a Ru-carbene metathesis catalyst [37], just to mention a few. 

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. 

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. 

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... 

Density Functional Theory Computational Study of Phosphine Ligand Dissociation versus Hemilability in a Grubbs-Type Precatalyst Containing a Bidentate Ligand during Alkene Metathesis... 

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,... 

M. Jordaan and H. C. M. Vosloo. A DPT computational study of phosphine ligand dissociation versus hemilability in a Grubbs-type precatalyst containing a bidentate ligand during alkene metathesis. Mol. Simul. 34, 2008, 10-15. 

Another piece of mechanistic evidence was reported by Snapper et al. [14], who describe a ruthenium catalyst caught in action . During studies on ring opening metathesis, these authors were able to isolate and characterize carbene 5 in which a tethered alkene group has replaced one of the phosphines originally present in Id. Control experiments have shown that compound 5 by itself is catalytically active, thus making sure that it is a true intermediate of a dissociative pathway rather than a dead-end product of a metathetic process. 

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-... 

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