Olefin, mechanism relations

Fig. 2.2 Possible mechanisms of cis-trans (Z-E) isomerization of olefins and related compounds via heterolytic cleavage of the C=C bond photoisomerization (path A).

Aspects of mechanism relating to the order of attack of olefin and hydrogen have received considerable attention in consideration of the reduction with (PPh3)3RhCl and cationic triphenylphosphine analogs. 

The number of branches in HDPE resins is low, at most 5 to 10 branches per 1000 carbon atoms in the chain. Even ethylene homopolymers produced with some transition-metal based catalysts are slightly branched they contain 0.5—3 branches per 1000 carbon atoms. Most of these branches are short, methyl, ethyl, and -butyl (6—8), and their presence is often related to traces of a-olefins in ethylene. The branching degree is one of the important stmctural features of HDPE. Along with molecular weight, it influences most physical and mechanical properties of HDPE resins. 

The reactions proceed via carbenium ions in a chain mechanism, initiated by the reaction between an olefin and an acid to C-C -C, which then reacts with iso-butane to give C-C C)-C. This carbenium ion is the central species in propagation steps to alkylated products such as 2,2-dimethylpentane and related products (Fig. 9.14). 

The reductive elimination of a variety of )3-substituted sulfones for the preparation of di-and tri-substituted olefins (e.g. 75 to 76) and the use of allyl sulfones as synthetic equivalents of the allyl dianion CH=CH—CHj , has prompted considerable interest in the [1,3]rearrangements of allylic sulfones ". Kocienski has thus reported that while epoxidation of allylic sulfone 74 with MCPBA in CH2CI2 at room temperature afforded the expected product 75, epoxidation in the presence of two equivalents of NaHCOj afforded the isomeric j ,y-epoxysulfone 77. Similar results were obtained with other a-mono- or di-substituted sulfones. On the other hand, the reaction of y-substituted allylic sulfones results in the isomerization of the double bond, only. The following addition-elimination free radical chain mechanism has been suggested (equations 45, 46). In a closely related and simultaneously published investigation, Whitham and coworkers reported the 1,3-rearrangement of a number of acyclic and cyclic allylic p-tolyl sulfones on treatment with either benzoyl peroxide in CCI4 under reflux or with... 

The synthetic method leading to Nb-alkylidenes and Nb-alkylidynes was particularly successful, due to a quite remarkable difference in the reaction rate of 29 with ketones or aldehydes, vs the subsequent reaction of the alkylidene with ketones and aldehydes (see Scheme 37). The former reaction takes a few minutes at -40°C, while the latter one occurs in hours at room temperature.88 The reaction between 178 and benzaldehyde led to triphenylethylene and the niobyl derivative 184. Due to the difference in reaction rates between a and b in Scheme 37, it was found that the sequential addition of two different ketones or aldehydes to a THF solution of 29 produced a nonsymmetric olefin in a stepwise McMurry-type reaction.84 This is exemplified in the coupling shown in reaction c (Scheme 37). The proposed reaction pathway does not involve the intermediacy of a pinacolato ligand and therefore differs from the mechanism of the McMurry reaction and related reductive couplings at activated metal sites.89... 

Use of less sterically hindered examples of 5 in combination with MAO allows for active catalysts for the linear (head-to-head) dimerisation of a-olefins such as 1-butene, 1-hexene, 1-decene and Chevron Phillips C20-24 a-olefin mixture (Scheme 4) [47], The mechanism for dimerisation is thought to involve an initial 1,2-insertion into an iron-hydride bond followed by a 2,1-insertion of the second alkene and then chain transfer to give the dimers. Structurally related cobalt systems have also been shown to promote dimerisation albeit with lower activities [62], Oligomerisation of the a-olefms propene, 1-butene and 1-hexene has additionally been achieved with the CF3-containing iron and cobalt systems 5j and 6j yielding highly linear dimers [23],... 

Although the mechanism proposed for the ZSM-5/methanol system adequately explains the production of the primary C2-C5 products, it is not clear how these are converted into the final gasoline product or indeed why this product should be so rich in aromatics. Production of olefins from methanol over zeolite catalysts has previously been described (110, 112) however, the ZSM-5 system appears to be unique with respect to both product selectivity and catalyst stability. Mobil now has some 140 patents relating to the preparation and use of ZSM-5 zeolites and has stated that "given a favorable economic and political climate a commercial unit could be in operation by the early 1980 s (101). 

Brookhart s group has reported a related rhodium-catalyzed olefin insertion. To gain insight into the mechanism of this process, labeling studies were carried out under conditions where no coupling product was observed ( H NMR study at 80 °C). Deuterium loss occurred in both meta- and para-sites of the aromatic group, and deuterium incorporation was observed in the olefin (Equation (95)).89... 

Zavada et ai (1973) have related the trans/cis olefin ratio in the product to competing syn- and anti-mechanisms for elimination. The transition state for syn-elimination [ 176] is stabilized by co-ordinative interactions with the cation. 

The three theoretical studies presented above give somehow different conclusions regarding the detailed reaction mechanisms. The difference is apparently related to the different models used and different theoretical approaches employed in the calculations. In any case, the emerging overall picture is that the reaction mechanism of olefin hydroborations must be complicated. We would like to see more experimental work done in the future so that the theoretical results can be tested. In particular, it would be nice to evaluate experimentally the utility of the suggestion made by Ziegler and co-workers in the choice of the phosphine ligands in order to produce more pure product. 

When thiocarbonyl derivatives are treated with an excess of electrophilic carbene complex, alkenes are usually obtained [1333-1336], The reaction is believed to proceed by the mechanism sketched in Figure 4.18, closely related to the thiocarbonyl olefination reaction developed by Eschenmoser [1337], Few examples have been reported in which stable thiiranes could be isolated [1338], The intermediate thiocarbonyl ylides can also undergo reactions similar to those of carhonyl ylides, e.g. 1,3-dipolar cycloadditions or 1,3-oxathiole formation [1338], Illustrative examples of these reactions are given in Table 4.22. 

Apparently, the exchange patterns can be explained qualitatively by reference to either structure for the adsorbed olefin, the eclipsed 1,2-diadsorbed alkane or the olefin tt complex. This argument should, of course, refer to the transition state for the formation of chemisorbed olefin from monoadsorbed alkane, the critical step in the a,)3 exchange mechanism however the revised argument would be much the same. Nevertheless we are provided with two alternative descriptions of the chemisorbed alkene under conditions closely related to those employed in hydrogenation studies. 

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