Ethene initiation mechanism

C. The Ethene Initiation Mechanism as Investigated by FTIR Spectroscopy... 

Exercise 16-3 The foregoing discussion explicitly refers to addition of polar reagents to carbonyl groups. Therefore an ionic mechanism is implied. Consider whether the same reactivity differences would be expected forethene and methanal intherad/ca/-chain addition of hydrogen bromide to methanal and ethene initiated by peroxides. What about the relative equilibrium constants Show your reasoning. (Review Section 10-7.)... 

Many questions remain about the initiation, propagation, and termination steps of the ethene polymerization mechanism. The most important models proposed to date are the Cossee model, which requires a vacant coordination site on the metal center in the position adjacent to the growing alkyl chain, where ethene is coordinated before insertion into the chain (628), and the Green-Rooney model, which requires the presence of a metal-carbene species and a vacant site where ethene is coordinated prior to insertion (629). 

The aim of this Section is to discuss the experimental methods, problems, and recent improvements in the determination of the exact nature of the precursor species Yi, Y2, Y3, etc. (i.e., the determination of the initiation mechanism for the ethene polymerization). Facing this topic, we must be aware that, besides the problems related to the determination of the Cr(II) structure (vide supra Section VI.A.2), the identification of the species formed during the initial stages of the reaction has been prevented so far for two other reasons (a) only a fraction of the Cr(II) sites are active in the polymerization under the usually adopted conditions (225), so that almost all the characterization techniques give information about the inactive majority Cr sites and (b) the active sites are characterized by a very high polymerization rate (high turnover frequency, TOF). It is thus clear that any experimental efforts devoted to the detection of the precursor and/or intermediate species must solve these two problems (vide infra Section VI.C). 

Two competing chain-transfer mechanisms in copolymerization of CO and ethene catalyzed by Pd11 acetate/dppp complexes were found. One involves termination via an isomerization into the enolate followed by protonation with methanol the rate of this reaction should be independent of the concentration of the protic species. The second chain-transfer mechanism comprises termination via methanolysis of the acylpalladium species, and subsequent initiation by insertion of ethene into the palladium hydride bond.501... 

From what is reported above, it is evident that the CO-ethene copolymerisation and the methoxycarbonylation of ethene are closely related. In principle the mechanisms discussed for the copolymerisation process are valid also for the case when termination occurs after the insertion of just one molecule of each monomer into the species that initiate the catalysis, Pd-OCH3+ or Pd - H+. These species can form as schematized by Eqs. 10-16. The copoly-... 

The first steps involve coordination and cycloaddition to the metal. Insertion of a third molecule of ethene leads to a more instable intermediate, a seven-membered ring, that eliminates the product, 1-hexene. This last reaction can be a (3-hydrogen elimination giving chromium hydride and alkene, followed by a reductive elimination. Alternatively, one alkyl anion can abstract a (3-hydrogen from the other alkyl-chromium bond, giving 1-hexene in one step. We prefer the latter pathway as this offers no possibilities to initiate a classic chain growth mechanism, as was also proposed for titanium [8]. The byproduct observed is a mixture of decenes ( ) and not octenes. The latter would be expected if one more molecule of ethene would insert into the metallocycloheptane intermediate. Decene is formed via insertion of the product hexene into the metallo-cyclopentane intermediate followed by elimination. 

Studies in deuterated water have shown that the hydroxyl proton does not end up in the ethanal formed. The decomposition of the 2-hydroxyethyl is not a simple P-elimination to palladium hydride and vinyl alcohol, which then isomerises to ethanal. Instead, the four protons stemming from ethene are all present in the initial ethanal product [6] (measured at 5 °C in order to suppress deuterium/hydrogen exchange in the product) and most authors have therefore accepted an intramolecular hydride shift as the key-step of the mechanism (see Figure 15.2). There remains some doubt as to how the hydride shift takes place. 

A number of ex situ spectroscopic techniques, multinuclear NMR, IR, EXAFS, UV-vis, have contributed to rationalise the overall mechanism of the copolymerisation as well as specific aspects related to the nature of the unsaturated monomer (ethene, 1-alkenes, vinyl aromatics, cyclic alkenes, allenes). Valuable information on the initiation, propagation and termination steps has been provided by end-group analysis of the polyketone products, by labelling experiments of the catalyst precursors and solvents either with deuterated compounds or with easily identifiable functional groups, by X-ray diffraction analysis of precursors, model compounds and products, and by kinetic and thermodynamic studies of model reactions. The structure of some catalysis resting states and several catalyst deactivation paths have been traced. There is little doubt, however, that the most spectacular mechanistic breakthroughs have been obtained from in situ spectroscopic studies. 

Similar experiments have been carried out with the monosilyl substituted cyclohexane, CsSiH 63. Reactions of the silane with Fe+, Co+ and Ni+ in this case, however, did not lead to dehydrogenation, but instead to mainly ethene elimination. Labeling studies indicated a reaction mechanism as depicted in equation 5. Initially, insertion of the metal... 

Although scheme (138) is the standard mechanism for the radical-catalyzed isomerization of isomeric alkenes, kinetic data for both substitution and isomerization are sparse. Using cis- or frcms-diiodo-ethene and labeled iodine atoms, Noyes et al. (1945) demonstrated that iodine atoms exchanged with predominant retention isomerization was the slower process, the barrier being <4 kcal/mole. Corresponding studies with dibromoethene and bromine atoms indicate a barrier of ca. 3 kcal/mole (Steinmetz and Noyes, 1952) in which bromine-atom departure from and isomerization of the intermediate were competitive. Qualitative selective or stereospecific radical-initiated additions to alkenes have since indicated that radical intermediates probably have stereostability, but the studies cited are definitive. The kinetic analysis provided the essential model for SS in mechanistic schemes such as (138), whether for SE, SH or SN processes. 

More than 140 different alkenes have been identified in the atmosphere [27]. The sources of alkenes are similar to those for the alkanes with combustion of fossil fuel being a major source. The presence of unsaturated bonds makes these compounds much more reactive than the alkanes. The most persistent member of this class of compounds (ethene) has an atmospheric lifetime of the order of a day, while more typically the lifetimes for alkenes are measured in hours. As a result of their short lifetimes the atmospheric concentrations of alkenes are highly variable and decrease dramatically away from their source locations. The mechanisms of atmospheric oxidation of alkenes have recently been reviewed [55]. As with the alkanes the reaction of OH radicals is an important loss mechanism. This reaction proceeds mainly via addition to the unsaturated bond as illustrated for ethene in Fig. 4. In one atmosphere of air at 298 K the dominant atmospheric fate of the alkoxy radical HOCH2CH2O is decomposition via C - C bond scission, while reaction with O2 makes a 20% contribution [56]. The fate of alkoxy radicals resulting from addition of OH to alkenes is generally decomposition via C - C bond scission [8]. Thus, the OH radical initiated oxidation of propene gives acetaldehyde and HCHO, oxida-... 

The ability of a p-carbene to react with an unsaturated hydrocarbon and form an enlarged dimetallocycle encourages speculation over their role in such processes as alkene metathesis and Fischer-Tropsch synthesis. In Scheme 6 a possible mechanism for metathesis initiated by a p-carbene is presented, owing much to other workers (T7,22). Reactions of p-carbenes with alkenes are under investigation in our laboratory. Recently Pettit has observed that the p-methylene complex [Fe2(C0)8(p-CH2)] generates propene when subjected to a pressure of ethene and has also suggested the intermediacy of a three-carbon dimetallocycle (23). 

All these mechanisms start from the common hypotheses that the coordinatively unsaturated Cr(II) site initially adsorbs one, two, or three ethene molecules (left column. Scheme 11) (211,217), via a coordinative d-n bond (which can be all Yi species in the model of Scheme 10). Support for the postulate that the Cr site can coordinate up to three ethene molecules was inferred by Zecchina et al. (217), who showed that Cr(II) is able to adsorb and trimerize acetylene to give benzene. If Yi is the diethene complex, Y2 species can be the metallacyclopropane, the ethylidene, or the ethenylhydride species (first row. Scheme 11). If Yi is the triethene complex, the Y2 species can be the metallacyclopentane, the metallacyclobutane, and so on (second row. Scheme 11). 

The detailed description of all the proposed mechanisms is not the aim of this work (see Reference (i) for more details), but a few concepts are briefly discussed in the following (a) Scheme 11 may be read in two dimensions in the vertical direction, the evolution of the initial species upon addition of one ethene molecule is represented, whereas, in the horizontal direction, all the possible isomeric structures characterized by an average C fCv ratio equal to 2, 3, and 4 are reported, (b) In all the proposed reactions, the metal formally becomes Cr(IV) as it is converted into the active site. This hypothesis is supported by investigations of the interaction of molecular transition metal complexes with ethene (226,227). Furthermore, it has... 

CC = C2H5 ). Initiation breaks the ethane carbon-carbon bond, which is weaker than the carbon-hydrogen bonds. The most plentiful free radical under most conditions of interest is QHj- [30], so that coupling of two of these to butane should be the dominant termination mechanism, probably accompanied to a small extent by disproportionation to ethene and ethane [31,32] ... 

---