Ethylene, coordinated

From the results discussed so far, it is evident that only CH2 groups have been observed in the very early stages of the ethylene polymerization reaction. Of course, this could be due to formation of metallacycles, but can be also a consequence of the high TOP which makes the observation of the first species troublesome. To better focalize the problem it is useful to present a concise review of the models proposed in the literature for ethylene coordination, initiation, and propagation reactions. 

The most famous mechanism, namely Cossets mechanism, in which the alkene inserts itself directly into the metal-carbon bond (Eq. 5), has been proposed, based on the kinetic study [134-136], This mechanism involves the intermediacy of ethylene coordinated to a metal-alkyl center and the following insertion of ethylene into the metal-carbon bond via a four-centered transition state. The olefin coordination to such a catalytically active metal center in this intermediate must be weak so that the olefin can readily insert itself into the M-C bond without forming any meta-stable intermediate. Similar alkyl-olefin complexes such as Cp2NbR( /2-ethylene) have been easily isolated and found not to be the active catalyst precursor of polymerization [31-33, 137]. In support of this, theoretical calculations recently showed the presence of a weakly ethylene-coordinated intermediate (vide infra) [12,13]. The stereochemistry of ethylene insertion was definitely shown to be cis by the evidence that the polymerization of cis- and trans-dideutero-ethylene afforded stereoselectively deuterated polyethylenes [138]. 

Fig. 12 Calculated structure of an ethylene-coordinated cationic methyl species derived from FI catalyst 1...

The sterically encumbered R2 substituents give steric protection to the oxygen-donors that are attached to the metal centers from coordination with Lewis acids such as MAO, or from another molecule of the catalytically active cationic species, which are supposed to be highly electrophilic. The coordination increases steric congestion near the polymerization center, which at least hampers ethylene coordination to the metal. Even worse, it may cause catalyst decay by, for instance, loss of the ligand. 

Large R2 substituents induce effective ion-separation between the cationic active species and an anionic cocatalyst, which allows more space for ethylene coordination to the metal and for its insertion into the carbon-metal bond. In addition, electronically, the ion separation increases the electrophilicity of the catalytically active species and hence enhances the reactivity toward ethylene. 

The essential steps in the nickel-catalyzed 1 1 codimerization reaction, which involve hydride addition to butadiene and ethylene coordination to the metal atom, were first proposed by Kealy, Miller, and Barney (35) and were later demonstrated by Tolman (40) using a model complex. Tolman prepared the complex H—Ni+L PFe [L = (EtO)3P] and showed that, after prior dissociation to form H—NiL3, it can react with butadiene to form a 7r-crotyl complex 19. 

Ethylene coordinates the expression of genes responsible for enhanced respiratory metabolism, chlorophyll degradation, carotenoid synthesis, conversion of starch to sugars, increased activity of cell wall-degrading enzymes, aroma volatile production, and so on. All these events stimulate a series of biochemical, physiological, and structural changes making fruits mature and attractive to the consumer. 

Ligand dissociation energies without ethylene coordination are given in parentheses. 

Noteworthy evidence for a Langmuir—Hinshelwood mechanism is provided by Force and Bell [114,116]. Apart from demonstrating that ethylene does not adsorb on silver completely covered with oxygen, the authors show by IR analysis at 220° C that the adsorbed complex has a structure which is analogous to ethylene, coordinated with isolated silver ions, viz. 

The more basic and less hindered pyridines undergo nucleophilic attack at an ethylene coordinated to platinum(II). Pyridine substitution reactions at platinum also occur, and in the presence of excess ethylene, alkene replacement is observed.72 ... 

Ethylene coordinates too strongly to the ruthenium center to act as a hydrogen acceptor in this system, but slightly larger olefins do show activity. However, reaction selectivity is reduced compared with f-butyl ethylene. Thus, the use of cis and trans 2-pentene as hydrogen acceptor leads to products consisting of 90% carbosilane and 10% hydrosilylated olefin, and 1-hexene yields a carbosilane/hydrosilylation ratio of 4 6. In comparison, no hydrosilylation products are observed for dehydrocoupling in the presence of cyclohexene, but carbosilane formation is accompanied by disproportionation to benzene and cyclohexane as a side reaction. 

Various nucleophiles can attack coordinated alkenes. Typically the attack of OH anion on ethylene coordinated to Pd(II) as shown by 71 takes place in the Wacker process to afford acetaldehyde (72) [4], Also COD (73), coordinated to PdCl2, was shown to be attacked by carbon nucleophiles such as malonate to give 74. This reaction is the first example of carbopalladation of alkenes [5],... 

Coordinatively unsaturated 6.1 represents the surface Ti4+ responsible for catalyzing the polymerization reaction. By reaction with the Et3Al (or Et2AlCl), a Ti-Et bond, as shown for 6.2, is formed. Ethylene coordination at the vacant... 

Butadiene and ethylene are codimerized with a soluble rhodium-phosphine complex as the catalyst. Very little has been reported on the mechanistic evidence for this reaction. However, a catalytic cycle as shown in Fig. 7.9 involving a rhodium hydride seems likely. Reducing rhodium trichloride with ethanol in the presence of a tertiary phosphine generates the hydride complex 7.32. The 1,4-hydride attack on the coordinated butadiene gives an rf-allyl complex. This is shown by the conversion of 7.33 to 7.34. Ethylene coordination to 7.34 produces 7.35. 

In order to test this hypothesis, calculations were carried out by various techniques for various models of asynchronous (biradicaloid) and synchronous transition states. The results of these calculations are shown in Figs. 2 and 3s. For the Diels-Alder reaction, the procedure consisted of optimizing the butadiene-ethylene coordinates by MINDO/3 for certain fixed values of r i i and t 2 (Pig- 2). The resulting geometries were than used as input for calculations by the other techniques. For fulminic acid-ethylene, the Poppinger transition state was chosen as the mid-point, and rcc and rco were varied, while the fulminic acid and acetylene fragments were held fixed. Operationally, this variation was carried out by a variation in a (Fig. 3). 

The initiation step proceeds by ethylene coordination on Cr(II) with formation of d-7T complexes. 

Henrici-Olive and Olive [205] assume complex II to be octahedral with the metal atoms connected by two Al—Cl—Ti links, with one vacancy (suitable for ethylene coordination), and a reactive ethyl group in the cis position... 

The electron-releasing phosphine promotes oxidative addition of the bromo derivative to Pd(0) and, because of its bulkiness, readily generates free coordination sites by dissociation. Ethylene coordination and insertion then occur, followed by reductive elimination, triethylamine acting as a base to neutralize hydrogen bromide. As in most cases of transition metal-catalyzed reactions the fine details of the mechanism are still under investigation. Thus recent studies by Amatore s group suggest that the palladium(O) species formed by reduction of palladium acetate is an anionic acetato complex. 

Ethylene coordination to 3b, the Brown s intermediate, gives 4a which is higher in energy than 4 by 6 kcal/mol. Olefin insertion of 4a can lead to 5c or 5d. Since 5c is much more stable than 5d, olefin insertion giving 5c would take place exclusively. The instability of 5d with an apical Cl is similar to that of 3d and 3e discussed above. 

In refs. 2-1.128) jjj terms of the CNDO method, the coordination of ethylene on a monometallic AC with various alkyl groups has been analyzed CHj (complex A, reaction (24)), trans-C- Hj (complex C, reaction (25, 26)), and cw-CjHj (complex F, reaction (27)- 29)). In all cases, the initial and final states of the AC in the process of ethylene coordination are optimized by the minimum of the overall energy. This process includes the construction of potential surfaces in two independent coordinates, according to the change of the position of alkyl and ethylene. Several possibilities of ethylene entering into the coordination sphere of Ti(III) ion have been calculated. 

In the case of the active center, containing the transalkyl group (complex C, reaction (25), (26)) there is close probability of ethylene coordination to the both coordination sites. Therefore, such center is non-stereospecific. 

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