Olefin structures hydrogenation

Computation allows one to circumvent nature s reluctance to offer the dihydride to direct detection. The first papers using molecular mechanics to study asymmetric hydrogenation appeared in the late 80 s [53-55], However, molecular mechanics is not the ideal technique for any reaction that involves bond-breaking or bond-forming, such as all catalytic reactions, and only a limited amount of reliable information was obtained from these early studies. An MP2/QC/5IXT) study of (PH3)2Rh(olefin) structures was published in... 

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 iridium complexes used as precatalysts are air-stable and easy to handle. A further attractive feature is the modular nature of the HetPHOX chiral ligands, which makes it possible to tailor the catalyst structure for a specific substrate. Unfunctionalized and functionalized olefins were hydrogenated with good to excellent enantioselectivity using these iridium complexes and it was possible to obtain also high yield and good enantioselectivity (99% yield, 72% ee.) in the hydrogenation of ( )-phenyl-(l-phenylethylidene)-amine. ... 

Marko (91) studied the effect of olefin structure on the ratio of hydroformylation to hydrogenation products, and concluded that the ratio declined with increasing branching of the olefin. This is not surprising in view of the known decrease in rate of hydroformylation with increased... 

The di-f-butylhydroxylamine and di-/-butylhydroxylamine ethers probably result from DTBN scavenging of radicals produced by hydrogen atom abstraction from the olefins by excited 3-ethoxyisoindolenone (50). The observed destruction of DTBN as a function of olefin structure is consistent with this mechanism. Based upon allyl radical stability and the statistical factor, excited 3-ethoxyisoindolenone should abstract hydrogen atoms more rapidly from tetraroethylethylene than from ds-2-butene. 

Because the rate of alkene hydrogenation generally decreases as the number and size of the substituents on the double bond increase, the least hindered double bond should be hydrogenated preferentially in the competitive hydrogenation of olefin mixtures. This selectivity, however, is not always observed either because of diffusion constraints or the presence of concurrent double-bond isomerization. Isomerization modifies the olefin structure, which changes the alkene reactivity and makes reaction selectivity difficult to attain. 

Hydrogenation of a olefines, as well as internal and cyclic olefines, is possible. However the catalyst is very sensitive to the olefin structure and, for example, tri-substituted ethylenic derivatives are hardly reduced. 

A third possible route to the molecular hydrogen to a w-olefin structure (B) is permitted (22) ... 

Oil industry has a long history of application of NMR spectroscopy for characterization of crude oils, products and oil fractions. The methodology has been mainly ID proton- or carbon-detected experiments. Quantitative NMR and NMR experiments have been used in estimation of aromatic, olefin, naphtene and paraffin proportions in the samples. ° A more detailed characterization has been obtained using various ID carbon-detected experiments, like GASPE, CSE, QUAT and DEPT to obtain quantitative CH sub-spectra. " The goal of characterization of the oil fractions and quantification of certain structural features has been to find correlation between these features and the product properties (e.g. viscosity index, pour point). Due to environmental concerns oil companies are nowadays more interested in development of lubricant base oils that have low aromatic and olefin contents. Hydrogenation of unsaturated components also improves the stability of the base oils, which is an important property for the end-product. Quantitative analysis of a saturated oil fraction with NMR is a major challenge. When the oil fraction contains only aliphatic compounds, the spectrum width that contains the resonances narrows to ca. 1 ppm in the NMR spectrum and ca. 50 ppm in NMR spectrum. This causes excessive... 

The chemistry, structure, and reaction mechanism of these catalysts have been studied and compared. These studies reveal important similarities and differences in the action of these oxides. Olefins are hydrogenated in the presence of either catalyst at atmospheric pressure. Hydrogenation is accompanied by isomerization. Aromatics can be partially hydrogenated in the presence of vanadia only at superatmospheric pressure. Chromia is inactive under these conditions. 

There are further subtle infiuences of structure on the strength of allylic C—H bonds. Oxidation reactions in which there is initial removal of allylic hydrogen proceed with probabilities governed in part by the relative strength of this bond. Detailed experimental data are beginning to appear, as noted later, on the more subtle effects of olefin structure, including substituent effects, steric effects, and charge effects. Theoretical treatments of such effects are not far advanced indeed, these will... 

To establish the dependence of reaction selectivity on phosphine and olefin structure, four olefins and four phosphine ligands at a phosphine/rhodium ratio of 2 or 3 were selected. Figure 35 depicts the cyclohexene hydrogenation rate as a function of P/Rh ratio in the range 1.3-3.5. It can be seen that the maximum rate is reached at a P/Rh ratio in the range 2-2.3, and that reaction selectivity for the different olefins depends little on their structure. 

Rates of olefin hydrogenation also depend on olefin structures, as seen in Table 5. Steric inhibition is indicated by the much slower rate of... 

Table 5. Olefin Structure Dependence of Hydrogen Rates Using RhCKPPhsJa...

Apart from pre-vulcanized latex where the rubber molecules have been chemically crosslinked by sulfur, the chemical nature of the rubber molecules of the other latices described above remain chemically intact during and after the process. There are several other latexes available on the market in which the rubber molecules of the latex have been chemically modified. The chemical reactivity of the rubber molecules arises from the olefinic structure of the cis-1,4-isoprene unit within the molecule, which can undergo rapid reactions with, for example, halogens, ozone and hydrogen chloride. Some of these will be described in this section. They are prepared to serve niche applications. 

The double bonds of NBR have been selectively hydrogenated for the same purpose, i.e. in order to improve the resistance of the vulcanisates to ageing in oils and hot air. Pyridine-cobalt complexes and complexes of rhodium, ruthenium, and iridium " have been described as hydrogenation catalysts. In some cases the hydrogenation is incomplete. The main obstacle to homogeneous catalytic hydrogenation of olefinic structures is the difficulty in obtaining adequate selectivity, but there are special catalysts based on transition metals which are almost entirely satisfactory in this respect. 

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