Thermodynamics, palladium catalytic reactions

Alternating insertions. The reaction proceeds via a perfectly alternating sequence of carbon monoxide and alkene insertions in palladium-carbon bonds (Figure 12.1). Several workers have shown the successive, stepwise insertion of alkenes and CO in an alternating fashion. In catalytic studies this was demonstrated by Sen, Nozaki, and Drent etc. In particular the work of Brookhart [15,22] and Vrieze/van Leeuwen [12,13,14,20,23,32] is relevant for stepwise mechanistic studies. The analysis of final polymers shows that also in the final product a perfect alternation is obtained. It is surprising that in spite of the thermodynamic advantage of alkene insertion versus CO insertion nevertheless exactly 50% of CO is built in. 

The catalytic reduction of copper by hydrogen has been investigated on platinum, rhodium, ruthenium, and palladium. Thermodynamics show that Cu2+ in aqueous solution can be reduced by molecular hydrogen at room temperature. However, a copper solution under a hydrogen flow is perfectly stable because of the slow kinetics of the reaction. Metals such as platinum, palladium, and rhodium, which are able to activate hydrogen, can catalyze the reduction of copper. 

Only a small minority of organometallic reactions have cleared the hurdle to become catalytic reality in other words, catalyst reactivation under process conditions is a relatively rare case. As a matter of fact, the famous Wacker/Hoechst ethylene oxidation achieved verification as an industrial process only because the problem of palladium reactivation, Pd° Pd", could be solved (cf. Section 2.4.1). Academic research has payed relatively little attention to this pivotal aspect of catalysis. However, a number of useful metal-mediated reactions wind up in thermodynamically stable bonding situations which are difficult to reactivate. Examples are the early transition metals when they extrude oxygen from ketones to form C-C-coupled products and stable metal oxides cf. the McMurry (Ti) and the Kagan (Sm) coupling reactions. Only co-reactants of similar oxophilicity (and price ) are suitable to establish catalytic cycles (cf. Section 3.2.12). In difficult cases, electrochemical procedures should receive more attention because expensive chemicals could thus be avoided. Without going into details here, it is the basic, often inorganic, chemistry of a catalytic metal, its redox and coordination chemistry, that warrant detailed study to help achieve catalytic versions. 

Hydride elimination is the step of the Mizoroki-Heck reaction yielding the product (Figure 3.1, step 4). For this process to occur, the insertion complex must be able to rotate to a position where a /3-hydrogen is aligned syn to the palladium(ll) centre. The elimination will then result in formation of a reconstituted alkene and a palladium hydride species. The j8-H-ehmination is reversible (see Figures 3.5 and 3.6) and the preferred formation of the thermodynamically more stable tram-products is thus explained [12]. There is today no precise knowledge of how palladium(ll) is reduced back to catalytically... 

The Mizoroki-Heck reaction is usually performed in polar solvents, and salt additives such as tetrabutylammonium chloride have been shown to activate and stabihze the catalytically active palladium species [19]. Furthermore, the reactions in ionic hquids perform differently in terms of thermodynamic and kinetic properties of the reaction system. Additionally, ionic liquids allow a facile recovery of catalyst and substrates, as well as an easy product separation. Here, another beneficial effect might be used by combination of solvent mixtures for example, of ionic liquids and SCFs. SCFs and ionic liquids have a mixing gap which allows working in two-phase systems, and results in a straightforward phase separation [20]. 

Studies on the origin of die regioselectivity of these reactions revealed that attack by amines occurred at the more-substituted position, but isomerization of the kinetic branched product to the thermodynamic linear product occurred faster than the catalytic process. As a result, the linear isomer was observed as the final reaction product. However, isomerization of the products formed by reactions of aziridines, hydroxylamines, and hydrazone deriviatives was slower than the catalytic substitution process, and these different relative rates allowed isolation of the branched substitution products. The isomerization process presumably occurs by protonation of the amine to form an ammonium salt that undergoes oxidative addition to palladium, as was observed in the initial allyUc substitution processes that involved allylic ammonium salts as electrophile. Thus, addition of a strong, non-nucleophihc base to the reactions of amine nucleophiles allowed isolation of the branched kinetic product. ... 

The success of this triple catalytic system relies on a highly selective kinetic control. From a thermodynamic point of view, there are 10 possible redox reactions that could occur in this system. However, the energy barrier for six of these (O2 + diene, O2 + Pd(0), etc.) are too high, and only the kinetically favored redox reactions shown in Scheme 11.14 occur. A likely explanation for this kinetic control is that the barrier is significantly lowered by coordination. Thus, the diene coordinates to Pd(II), BQ coordinates to Pd(0), HQ coordinates to (ML,), and Oj coordinates to ML ,. In a related system for aerobic oxidation, a heteropolyacid was employed in place of the metal macrocyclic complex (ML ,) as oxygen activator and electron transfer mediator [72]. Recent immobilization of the macrocyclic complex in ZeoHte-Y, led to eflBcient reoxidation of the HQ in the palladium-catalyzed 1,4-diacetoxylation [73]. 

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