Kinetics Phosphine dissociation

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. 

The rate constants for phosphine dissociation were determined independently. The eg complex reacted faster, k 0.204 s 1 (298K), whereas the Cp2 complex had k = 0.0013 s. These precise kinetic data suggest that, contrary to previous suggestions, the eg complexes may not be better Lewis acids than their Cp2 counterparts. 

One of the mechanistic steps most often encountered and inferred from kinetic data is ligand dissociation, which leads to the generation of a catalytically active intermediate. If ligand is added to such a catalytic system, the rate of the reaction decreases. Examples of this in homogeneous catalytic reactions are many CO dissociation in cobalt-catalyzed hydroformylation, phosphine dissociation in RhCl(PPh3)3-catalyzed hydrogenation, Cl dissociation in the Wacker process, etc. The actual rate expressions of most of these processes are described in subsequent chapters. 

Wagener ADMET kinetics (2a) and (4a) Reaction rates using (4a) were much more temperature dependant than those using (2a). Indicating that phosphine dissociation is an energy barrier for (4a) processes. 

At first fhe unprecedented activity of fhe second-generation catalysts was thought to be a result of faster phosphine dissociation [95]. This result indicated that fhe phosphine dissociation kinetics as well as fhe metathesis activity of the resulting 14-electron intermediate must be considered to rationalize the activity of these catalysts. Interestingly, returning to PPhs is valuable for fhe NHC complexes, in fhat fhe phosphine exchange rate for complex 11 is almost fhat of complex 6 [101]. 

The kinetic data from phosphine dissociation studies and reaction with ethyl vinyl ether fit the classical dissociative exchange catalysis model very well [101]. Examination of this model leads to the conclusion that there are two main factors controlling the activity of these catalysts the rate of phosphine dissociation and the metathesis activity of the phosphine-dissociated species (Scheme 6.26). 

The phosphine dissociation rates ( d) of 1 and 2 do not track with their olefin metathesis activities, since 2 is significantly more active than 1 for both polymerization [5] and ring closing metathesis reactions. Instead, correlates directly with the initiation rates of the two catalysts. The kinetics of initiation was investigated by monitoring the reaction of 1 or 2 with ethyl vinyl ether using NMR and/or UV-vis spectroscopy [10]. For both catalysts, this reaction showed saturation kinetics, and the values of the initiation rate constant k ) at saturation were identical (within error) to the values of / d at the same temperature. These results implicate a dissociative olefin metathesis mechanism as depicted in Scheme 2. In this mechanism, phosphine dissociation to form a 14-electron intermediate L(Cl)2Ru=CHPh (3) is followed by trapping with olefinic substrate. 

Thus, the reductive eliminations from tranx-bis(triphenylphosphine) amido-aryl complexes 86 showed first-order kinetics demonstrating that the reductive elimination takes place from monomeric species (Scheme 1.54). The dependence of the reaction rate on the concentration of added PPhj is compatible with two competing mechanisms, one involving C-N bond formation to a ds-16-electron species 87 formed by isomerization of the trans derivative. The other mechanism involves initial reversible phosphine dissociation to give a 14-electron threebond formation (Scheme 1.54). Dimeric monophosphine complexes follow a dissociative pathway to give threereductive elimination. The formation of the 14-electron intermediates can be reversible or irreversible depending on the type of amine. 

With regard to the mechanisms, several theoretical studies indicate that for the A systems, the formation of the metalacycle has a low energy barrier. However, for B and C, initial dissociation of the PCy, must occur before coordination of the olefin. Grubbs and co-workers " have described the various kinetic scenarios that can result when phosphine dissociation is slow or when its recoordination is competitive with binding of the olefin. 

NMR kinetic experiments using and nuclides have been routinely employed to measure the initiation rate of precatalysts bearing phosphine ligands. Precatalyst quenching experiments with EVE can be monitored by NMR spectroscopy over time, which provides a measure of initiation rate when phosphine dissociation is rate-determining (i.e. second-generation complexes) (see section 2 for a fuUer discussion of the initiation rate of key metathesis precatalysts). 

The kinetics of the ADMET reaction is not amenable to study by many traditional means, as these polymerizations are mostly conducted in bulk. The most effective way to measure the kinetics of the polymerization is to monitor the volume of evolved ethylene. This technique has been used to probe the difference in activity between [Mo] 2 and [Ru]l for ADMET polymerization of 1,9-decadiene [37]. At 26 °C in bulk monomer, [Mo] 2 promotes ADMET polymerization of 1,9-decadiene at a rate approximately 24 times that of [Ru]l. Additionally, [Mo] 2 polymerizes 1,5-hexadiene 1.7 times faster than 1,9-decadiene, while [Ru]l only cyclodimerizes 1,5-hexadiene to 1,5-cyclooctadiene. Monomers with coordinating functionality, specifically ethers and sulfides, were also investigated. Predictably, these monomers did not undergo polymerization as rapidly as hydrocarbon monomers however, this difference was dramatically more pronounced with [Ru]l than with [Mo]2. In fact, the dialkenyl sulfide monomers that were studied completely shut down the polymerization with [Ru]l, whereas the catalytic activity of [Mo]2 was only slightly lowered. This reduction in polymerization rate is most likely due to coordination of the heteroatom to the vacant coordination site of [Ru] 1, following phosphine dissociation. This reversible coordination of heteroatoms to the ruthenium complex likely occurs both intramolecularly and intermolecularly. Conversely, the steric bulk of the ligands in [Mo] 2 makes it less likely to intramolecularly form a coordinate complex, despite molybdenum being far more electrophilic than ruthenium. 

The catalysts used in hydroformylation are typically organometallic complexes. Cobalt-based catalysts dominated hydroformylation until 1970s thereafter rhodium-based catalysts were commerciahzed. Synthesized aldehydes are typical intermediates for chemical industry [5]. A typical hydroformylation catalyst is modified with a ligand, e.g., tiiphenylphoshine. In recent years, a lot of effort has been put on the ligand chemistry in order to find new ligands for tailored processes [7-9]. In the present study, phosphine-based rhodium catalysts were used for hydroformylation of 1-butene. Despite intensive research on hydroformylation in the last 50 years, both the reaction mechanisms and kinetics are not in the most cases clear. Both associative and dissociative mechanisms have been proposed [5-6]. The discrepancies in mechanistic speculations have also led to a variety of rate equations for hydroformylation processes. 

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