Exothermicity, olefin insertion

The potential energy profile is smooth without excessive barriers and too stable intermediates which would break the sequence of steps. The rate-determining step is found to be olefin insertion followed by isomerization, supporting the Halpern mechanism. Isomerization of the ethyl hydride complex is an important part of the rate-determining step. These two reactions, exothermic overall, has an overall barrier height of about 20 kcal/mol. The trans ethyl hydride complex, the product of ethylene insertion, may not be a local minimum (per MP2 calculation) and these two steps may well be a combined single step. 

Firstly, the stabilization by chelate formation should increase the exothermicity of olefin insertion. Since part of the stabilization will already be felt in the transition state, it will reduce the barrier for olefin insertion in a palladium-acyl bond. Secondly, chelate formation should oppose or slow down termination by ff-elimi-nation from palladium-alkyl. For -elimination to occur the P-R atom has to approach the palladium ion, but that is opposed by coordination of the carbonyl group (eq. (13)). Inhibition ofy9-H elimination in metallacycles is well known [27]. 

In summary, chain propagation involves alternating reversible carbon monoxide insertion in Pd-alkyl species and irreversible insertion of the olefin in the resulting Pd-acyl intermediates. The overall exothermicity of the polymerization is caused predominantly by the olefin insertion step. Internal coordination of the chain-end s carbonyl group of the intermediate Pd-alkyl species, together with CO/olefin competition, prevents double olefin insertion, and thermodynamics prevent double CO insertions. The architecture of the copolymer thus assists in its own formation, achieving a perfect chemoselectivity to alternating polyketone. 

A digital functional approach has been employed to investigate important steps in the Heck reaction catalyzed by a bis(carbene)Pd complex and one in which the Pd is coordinated by a bidentate carbene-phosphine ligand. The crucial steps of olefin insertion into the palladium-aryl bond and / -hydride elimination were investigated. For the bis(carbene)Pd catalyst, a mechanism was proposed, which proceeds via halide abstraction, to give a cationic species, prior to olefin coordination and insertion. The total insertion/elimination process was found to be exothermic (—8.9 kcal moP ). For the carbene-phosphine ligated system, the vacant site for olefin coordination was provided by phosphine dissociation. The energetics for the total insertion/elimination process was very similar to that of the bis-carbene system. 

Chain propagation of CO/ethylene copolymerization proceeds by a strictly alternating insertion of CO and olefin monomers in the growing chain. It is safe to assume that double CO insertion does not occur for thermodynamic reasons [Ic]. However, the complete absence of double ethylene insertions is remarkable because ethylene insertion in a Pd-alkyl species must be exothermic by about 20 kcal/mol (84 kJ mol). The observation of strict alternation is the more surprising since the same palladium catalysts also efficiently dimerize ethylene to butenes [25]. The perfect alternation is maintained even in the presence of very low concentrations of carbon monoxide. When starting abatch polymerization at a high ethylene/CO ratio, error-free copolymer is produced until all the CO is consumed then the system starts forming butenes (with some catalyst systems at about twice the rate of copolymerization ). 

The essential mechanistic step in olefin polymerization is the insertion of an olefin into the metal-carbon bond of the catalyst leading to an extension of the polymer chain by one monomer unit (Scheme 1). In the simplest case of ethylene this step is exothermic by about 20 kcal/mol, which of course is independent of the catalyst. The catalyst complex is usually a cationic complex (the restriction to early transition metals is no longer valid) with an empty coordination site, which has to be formed from the inactive precursor complex. 

The most intriguing difference is the thermodynamically as well as kinetically facilitated hydrogen addition 11 —> 12 that hampers the last catalytic step, i. e. reductive aldehyde elimination 12 —> 7. In contrast to rhodium, Olefin/CO insertions 8 9/10 —> 11 are thermoneutral/endothermic, whereas Olefm/CO associations 7 8/9 —> 10 are significantly more exothermic. In summary, the total reaction... 

In the vapor phase, there are two additional considerations that are very important in understanding of carbene chemistry. The first point reflects the fact that carbene reactions are normally highly exothermic (about 90kcal mol for insertions or additions). Thus, a product molecule is frequently produced with a large amount of excess internal energy. In the vapor phase without solvent molecules to help dissipate the excess vibrational energy, the molecule may be subject to further reactions. Such reactions are often called hot molecule reactions. Cyclopropanes from cycloaddition reactions are particularly susceptible to hot molecule decomposition to the thermodynamically more stable olefin, since for cyclopropane isomerization is only 64kcal mol . ... 

Ir in oxidation state IV, and reductive elimination occurs with a negligible energy barrier. Based on these calculations, the Ir is never present in oxidation state I, but changes from III to V, which represents an important difference from the analogous Rh catalysts. The rate-determining step is likely either the H2 coordination or the migratory insertion, suggesting zeroth-order kinetics with respect to the olefin, which is consistent with the extremely exothermic coordination to the metal. 

The behavior of Ir alkyl complexes 104a and 104b was studied upon complexation with DIB as labile chelating ligand. These complexes showed catalytic activity in olefin isomerization, that is, for 1-pentene, C=C double bond isomerization occurs over several days at room temperature giving a product distribution of 2% 1-pentene, 14% ds-2-pentene, and 84% trans-Z-pentene. For isobutylene, / -pinene, and styrene, exothermic reactions were obtained with conversion of the monomers to polymerized/oligomerized olefins in all cases. Polymerizations proceed via a cationic mechanism with the complex working as initiator, rather than by a coordination/insertion mechanism, as indicated by the wide polydispersity. ... 

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