Insertion ethene

The direct attack of the front-oxygen peroxo center yields the lowest activation barrier for all species considered. Due to repulsion of ethene from the complexes we failed [61] to localize intermediates with the olefin precoordinated to the metal center, proposed as a necessary first step of the epoxidation reaction via the insertion mechanism. Recently, Deubel et al. were able to find a local minimum corresponding to ethene coordinated to the complex MoO(02)2 OPH3 however, the formation of such an intermediate from isolated reagents was calculated to be endothermic [63, 64], The activation barriers for ethene insertion into an M-0 bond leading to the five-membered metallacycle intermediate are at least 5 kcal/mol higher than those of a direct front-side attack [61]. Moreover, the metallacycle intermediate leads to an aldehyde instead of an epoxide [63]. Based on these calculated data, the insertion mechanism of ethene epoxidation by d° TM peroxides can be ruled out. 

The process of chain growing involves alternating reversible CO insertions and irreversible ethene insertions [13,15,67]. As already mentioned, consecutive insertion of two molecules of CO does not occur for thermodynamic reasons and consecutive insertion of two molecules of ethene does not occur because the insertion of CO is much faster than that of ethene [11-13,67]. It was subsequently proposed that coordination through the oxygen atom of a carbonyl group of the growing chain with formation of /3- and y-chelates... 

For a series of model acyl of the type [Pd(COMe)(C2H4)(P - P)]+ it has been found that the insertion of ethene into the Pd-acyl bond with formation of a /3-chelate (Eq. 20) is irreversible and that the energy barrier is ca. 12 kcal/mol [52,55,56]. From thermodynamic and kinetic data, Schultz et al. calculated that the insertion of ethene into a Pd-alkyl bond (double ethene insertion) could occur every ca. 105 CO insertions into the same bond [52], which accounts for the strict alternating chain growing. 

The /3-agostic ethyl complex is similar to a transition state intermediate in the step of the ethene insertion into the Pd - H bond. This might be also relevant to the high catalytic activity of this catalyst. 

Industrially this diene is made the same way as ethylidenenorbomene from butadiene and ethene, but now isomerisation to 2,4-hexadiene should be prevented as the polymerisation should concern the terminal alkene only. In both systems nickel or titanium hydride species react with the more reactive diene first, then undergo ethene insertion followed by (3-hydride elimination. Both diene products are useful as the diene component in EPDM rubbers (ethene, propene, diene). The nickel hydride chemistry with butadiene represents one of the early examples of organometallic reactions studied in great detail [22] (Figure 9.14). 

In spite of the difficulties mentioned above, Brookhart and co-workers succeeded in measuring the barrier for ethene insertion into (dppp)PdC(0)CH3+ at 160 K, starting from the ethene adduct, generated at still lower temperatures, in the absence of CO. The barrier measured (AG ) amounted to only 51.4 kJ/mol, i.e. the reaction is faster than the insertion of CO in an ionic alkylpalladium complex. The barrier of insertion of ethene into a palladium methyl species or palladium ethyl species was higher, at 67 kJ/mol at 233 K. As for the CO insertion described above, these values concern the barriers in preformed ethene adducts at higher temperatures the overall barrier will be higher, because alkene coordination will be disfavoured by entropy and competition with CO and solvent. Formation of CO adducts will also be less favourable at higher temperatures. 

Quantitative data for the difference in complexation of ethene and CO to hydrocarbylpalladium(dppp)+ were reported by Brookhart and co-workers [15,33], The equilibrium between CO and ethene coordination amounts to about 104 at 25 °C. Multiplied by the concentrations of the two gases and the two individual rate constants for the insertion they calculated that the ratio of CO insertion versus ethene insertion is about 105 in an alkyl-palladium intermediate under Curtin-Hammett conditions, that is to say fast exchange of coordinated CO and ethene ligands compared to insertion reactions. Figure 12.9 summarises this. 

The hydride cycle starts with palladium hydride and a fast migratory insertion of ethene. Insertion of CO is also a last reaction, enhanced by the preferential coordination of CO with respect to ethene. The product is formed by reaction of propanoylpalladium with methanol which coordinates to palladium cis to the acyl group. A palladium zero complex may form that regenerates palladium hydride via an oxidative addition with a proton. 

The factors that control the strictly alternating copolymer chain with no detectable errors (e. g., microstructures involving double insertion of ethene) have been the object of detailed studies since the discovery of the first Pd" catalysts for the alternating alkene/CO copolymerisation [11]. Sen was the first to demonstrate that double carbonylation is thermodynamically unfavorable and to suggest that the higher binding affinity of Pd" for CO relative to ethene inhibits multiple ethene insertions, even in the presence of very low concentrations of CO [12]. Therefore, once a palladium alkyl is formed, CO coordination ensures that the next monomer will be a CO molecule to generate the acyl complex. 

On the basis of the PM-RAIRS evidence, Drent has proposed a catalytic cycle (Scheme 7.8) where ethene insertion in the propagation step is CO-assisted and the substitution of the chelating ketone in c by ethene would proceed in two consecutive steps associative substitution of the chelating ketone by CO (c —> d —> e), followed by associative substitution of CO by ethene (e —> f —> i). The disruption of the chelate structure of c would be more facile for CO than for ethene for steric reasons (end-on vs. side-on approach). 

In contrast to theoretical results reported by Morokuma [29] and Ziegler [30], as well as previous studies with Pd"-phen model compounds [26], the lowest experimental energy barrier was found for the migratory insertion of the acyl (ethene) complex (Eq. (10)). The relative rates of alkyl to CO and alkyl to ethene migratory insertion reactions allowed one to estimate that sequential ethene insertions occur once for every ca. 10 insertions of CO into the Pd-alkyl bond [18]. 

Displacement of the chelate carbonyl from palladium by ethene has never been observed in model studies, which accounts for the virtual absence of double ethene insertions in actual copolymerisation reactions. Indeed, (5-chelate opening is actually brought about by CO to generate a six-membered metallacycle (y-chelate), while p-chelates of catalytically active systems generally react with CO to yield carbonyl acyl complexes, even at very low temperature. For the systems investigated by Bianchini [5e, f], the activation barriers for the conversion of the P-chelates... 

Again, approximately half of this effect will already be felt at the transition states for insertion and chain transfer. Thus, internal olefins are inserted less easily, oc-branched alkyls are more reactive towards insertion, and secondary alkyls tend to isomerize to primary alkyls. The abnormally low reactivity of Al—Me bonds towards olefin insertion must probably be explained in this way. The high reactivity of (f-Bu)3Al towards ethene insertion, despite its significant steric hindrance, might in part be due to the same effect. 

Figure 6.6 a Propagation cycle in the synthesis of polyethylene according to the Cossee mechanism b calculated energy/reaction coordinate diagram for the ethene insertion step. 

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