Olefin, mechanism variables

Double-bond migrations during hydrogenation of olefins are common and have a number of consequences (93). The extent of migration may be the key to success or failure. It is influenced importantly by the catalyst, substrate, and reaction environment. A consideration of mechanisms of olefin hydrogenation will provide a rationale for the influence of these variables. 

Although the actual reaction mechanism of hydrosilation is not very clear, it is very well established that the important variables include the catalyst type and concentration, structure of the olefinic compound, reaction temperature and the solvent. used 1,4, J). Chloroplatinic acid (H2PtCl6 6 H20) is the most frequently used catalyst, usually in the form of a solution in isopropyl alcohol mixed with a polar solvent, such as diglyme or tetrahydrofuran S2). Other catalysts include rhodium, palladium, ruthenium, nickel and cobalt complexes as well as various organic peroxides, UV and y radiation. The efficiency of the catalyst used usually depends on many factors, including ligands on the platinum, the type and nature of the silane (or siloxane) and the olefinic compound used. For example in the chloroplatinic acid catalyzed hydrosilation of olefinic compounds, the reactivity is often observed to be proportional to the electron density on the alkene. Steric hindrance usually decreases the rate of... 

As is the case with the Wittig and Peterson olefinations, there is more than one point at which the stereoselectivity of the reaction can be determined, depending on the details of the mechanism. Adduct formation can be product determining or reversible. Furthermore, in the reductive mechanism, there is the potential for stereorandomization if radical intermediates are involved. As a result, there is a degree of variability in the stereoselectivity. Fortunately, the modified version using tetrazolyl sulfones usually gives a predominance of the E-isomer. 

A conveniently prepared amorphous silica-supported titanium catalyst exhibits activity similar to that of Ti-substituted zeolites in the epoxidation of terminal linear and bulky alkenes such as cyclohexene (22) <00CC855>. An unusual example of copper-catalyzed epoxidation has also been reported, in which olefins are treated with substoichiometric amounts of soluble Cu(II) compounds in methylene chloride, using MCPBA as a terminal oxidant. Yields are variable, but can be quite high. For example, cis-stilbene 24 was epoxidized in 90% yield. In this case, a mixture of cis- and /rans-epoxides was obtained, suggesting a step-wise radical mechanism <00TL1013>. 

Apparently, in the reaction of olefins with hydrogen on catalysts such as palladium and platinum, both the dissociative and the associative mechanisms operate for isomerization and exchange. However, the dissociative mechanism accompanies those factors which tend to slow the addition or accelerate the removal of hydrogen from either substrate or intermediate. These factors may be any of the independent variables, such as the pressure of hydrogen, the structure of the substrate, or the catalyst (5). 

The dynamic behavior of the model intermediate rhodium-phosphine 99, for the asymmetric hydrogenation of dimethyl itaconate by cationic rhodium complexes, has been studied by variable temperature NMR LSA [167]. The line shape analysis provides rates of exchange and activation parameters in favor of an intermo-lecular process, in agreement with the mechanism already described for bis(pho-sphinite) chelates by Brown and coworkers [168], These authors describe a dynamic behavior where two diastereoisomeric enamide complexes exchange via olefin dissociation, subsequent rotation about the N-C(olefinic) bond and recoordination. These studies provide insight into the electronic and steric factors that affect the activity and stereoselectivity for the asymmetric hydrogenation of amino acid precursors. 

It has been reported occasionally that variable quantities of organo-selenium compounds are produced during the oxidation (3, 7). These compounds have been considered as side reaction products and have received little attention regarding their nature or possible role in the reaction. In this work we have studied the characteristics of these organoselenium compounds, their role in the mechanism of the oxidation of olefins with selenium dioxide, and their catalytic properties in the oxidation of olefins with other oxidants. 

T he epoxidation of olefins using organic hydroperoxides has been studied in detail in this laboratory for a number of years. This general reaction has also recently been reported by other workers (6,7). We now report on the effects of five reaction variables and propose a mechanism for this reaction. The variables are catalyst, solvent, temperature, olefin structure, and hydroperoxide structure. Besides these variables, the effect of oxygen and carbon monoxide, the stereochemistry, and the kinetics were studied. This work allows us to postulate a possible mechanism for the reaction. 

Kranz, K., 2003, Alkylation chemistry. Mechanisms, operating variables, and olefin interactions, http //www.stratco. dupont.com/alk/pdf/alkylation chemistry 2003.pdf (last visited February 2006)... 

Qince the discovery (6) of supported chromium oxide catalysts for polymerization and copolymerization of olefins, many fundamental studies of these systems have been reported. Early studies by Topchiev et al. (18) deal with the effects of catalyst and reaction variables on the over-all kinetics. More recent studies stress the nature of the catalytically active species (1, 2, 9,13, 14,16, 19). Using ESR techniques, evidence is developed which indicates that the active species are Cr ions in tetrahedral environment. Other recent work presents a more detailed look at the reaction kinetics. For example, Yermakov and co-workers (12) provide evidence which suggests that chain termination in the polymerization of ethylene on the catalyst surface takes place predominantly by transfer with monomer, and Clark and Bailey (3, 4) give evidence that chain growth occurs through a Langmuir-Hinshelwood mechanism. 

Following the approach of separating variables, we shall review here the models appropriate to describe the distribution of products over simple FT catalysts only. The products then consist of paraffins and a-olefins with essentially unbranched carbon chains. As paraffins and olefins can easily be interconverted under FT conditions, the information most pertinent to the reaction mechanism has to be derived from the distribution with respect to the chain length. The formation of alcohols is briefly reviewed in Section VI. 

After 30 years, olefin polymerization by a coordinated anionic mechanism continues to receive worldwide attention as evidenced by a voluminous patent and journal literature. Much attention has been directed to catalyst and process optimization and understanding of key reaction variables. The development of high-activity Ziegler-Natta catalysts has spurred a renewed interest in simplified processes requiring no post-treatment of the polymers. Recent announcements by Union Carbide of a low-pressure, fluid bed... 

The kinetics of the olefin metathesis of 2-pentene by (pyri-dine)2Mo(NO)2Cl2 and organoaluminum halides have been measured (56) as first order in the metal and variable order in olefin (seemingly first order at high olefin concentration and up to order 1.7 at low olefin concentration) and were originally interpreted to support the conventional mechanism, but they now also seem in accord with the metal-carbene chain mechanism. 

No attempt was made to optimise the yield of any one product but it was noted that the product composition was highly dependent on the reaction variables. The mechanism is similar to that of the Ritter reaction in which an olefin reacts with a nitrile and a nucleophOe in a sulphuric acid medium but in this case HCN is the nitrile and the products obtained are quite different. Compound (37) is stable in the absence of protic solvents and reacts with HCN or sodium cyanide to give di-t-octylamino-maleonitrile (40). The latter is oxidised by benzoyl peroxide to give di-t-octyliminosuccinonitrile (41). 

In both the hydrogen wag vibrations discussed previously, the motions of the hydrogens are partially balanced by countermotions of the substituents (See Fig. 4.15). This makes the substituents more mechanically involved in the vibrations which makes these vibrations more variable in frequency than those discussed. When rotational isomers are possible in the cw-olefins, the wag band can be broadened. Cyclohexenes absorb at the low frequency end of the cis region. 

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