In isomeric reactions

A convincing example of selectivity control in isomerization reactions is the formation of cis-2-butene from the isomerization of 1-butene using the catalyst [(C6H5)3P]2NiX2-P(CeH5)3-Zn-SnCl2 a ratio of c/i-2-bu-tene /ra .s-2-butene as high as 98 2 has been observed. Isomerization to the thermodynamically more stable /ra ns-olefin occurs only after conversion of all the 1-butene (98). Further examples of selective olefin isomerization will be discussed in Section IV,D. 

Isomerization reactions are associated with a change in either the connectivity (the constitution) of the molecule or the steric arrangement of atoms or groups in the molecule. However, no change of the empirical formula is involved. Thus in isomerization reactions leading to allenes, the 1,2-diene substructure that is characteristic of this class of compounds has to be formed by one of the following reactions (Scheme 1.1) ... 

While Ru-dppes incorporated mesoporous silica was used in isomerization reactions [75], hydrogenation of various olefins was aeeomplished by the use of Rh-dppes complex... 

In a similar manner, many additions of heteroatom radicals to unsaturated positions have been studied. In many cases, addition reactions of heteroatom radicals to alkenes are reversible and thermodynamically disfavored, but their occurrence is apparent. For example, the rapid addition and elimination of thiyl radicals to unsaturated fatty acid methyl esters results in isomerization reactions from which kinetic parameters can be obtained. Additions of group 14 (IV A) metal-centered... 

Many experiments (see Refs. 154-160) have shown that Kramers theory fails to describe the viscosity dependence of rate in isomerization reactions. This is especially the case where the barrier frequency (< , ) giving the curvature at the barrier top is large. In this case both experimental (see Refs. 154-160) and simulation studies [147, 153] find a rate that decreases with viscosity at a rate much slower than that predicted by Kramers theory. In fact, at high viscosities, it is often found that the rate could be fitted to a form given by... 

It must be concluded, therefore, that the ligands do not become completely detached from the metal ion in isomerization reactions. Comparable results have been observed in the isomerization95 of potassium diaquodioxalatochromium(III) and the racemization96 of optically active potassium tris(oxalato)chromium(III) when no exchange with free ligand in solution occurs. Thus, although it is not practicable to take advantage of the desirable properties of individual isomers of 2 1 chromium and cobalt complexes of tridentate azo compounds because of the facility with which such compounds isomerize in solution, the technically important unsymmetrical 2 1 complexes are capable of practical application because they show little or no tendency to disproportionate in solution. 

Catalytic activity in zeolitic materials is strongly influenced by the type of alkali metal cations, and maximum catalytic activity, e.g, in isomerization reactions, is explained by the formation of an imide species EuNH [305]. Synergetic effects were observed in bimetallic supported Si02 which showed considerable hydrogen uptake during hydrogenation reactions [307]. The formation of Ln-NH2, -NH, -N species seemed to be suppressed in the presence of transition metal powders and precipitation of elemental lanthanides is favored [309]. Lanthanide imides were favored as active species in the Ln/AC-mediated cyclization of ethyne and propyne [310]. 

In isomerization reactions, an alkene is deprotonated to form an allyl anion, which is reprotonated to give the more stable alkene (double-bond migration). The most simple example is the isomerization of 1-butene producing a mixture of cis- and trans-2-butene (Scheme 3). Because the stability of the cis-allyl anion formed as an intermediate is greater than for the trans form, a high cis/trans ratio is observed for base-catalyzed reactions whereas for acid-catalyzed reactions the ratio is close to unity. Thus, the cis/trans ratio of the products has frequently been used as an indication of base-catalyzed reaction mechanisms. The carbanions formed in the course of such superbase reactions are not freely mobile in solution,... 

When maximum concentration of enzyme is attained in the fermenter, the broth may be filtered, the solid washed thoroughly with water, and the enzyme-containing cell-mass used directly in isomerization reactions. Takasaki and Kamibayashi described a heat-treatment system that was applied to a Streptomyces fermentation-broth before filtration, and that fixed the isomerase within the cell structures by inactivating lytic enzymes that would otherwise cause the isomerase enzyme to be leached out of the cell when added to substrate in the isomerization reactors.44 The cell-fixed isomerase can be used in stirred-tank or fixed-bed reactors through which the D-glucose solution to be isomerized may be passed continuously while the enzyme that is fixed within the cell material is retained in the reactor. 

In addition to its utility for structure elucidation, and in tautomerism studies, H NMR has found applications for analysis in kinetic studies, in conformational studies (e.g. 1-arylimidazoles and reduced imidazoles), for the determination of product ratios in isomerization reactions, and in reaction mechanism studies. Thus the technique was able to show that deuterium exchange at the 2-position of imidazoles can occur either in the presence or absence of added base (Section 4.07.1.6.2). 

Related to their similar pore diameter and pore structure, unsurprisingly the Henry adsorption constants for linear alkanes are very close to each other on zeolite ZSM-22 and ZSM-23 (Table I). Somewhat higher constants are obtained for 2- and 3-methylbranched alkanes on ZSM-23 compared to zeolite ZSM-22. The adsorption constants of linear alkanes are obviously hi er than branched alkanes on the two cases. The separation power of a zeolite between a linear and a branched hydrocarbon may be given by the separation factor (a), which is the ratio of Henry consteints of linear and branched molecules at a certain temperature, a values at 523 K are given for both zeolites in Table 1. For comparison, values for ZSM-5 are also included, which is one of the most popular shape selective catalyst used in isomerization reactions. From this table it can be seen that both ZSM-22 and ZSM-23 have higher separation constants compared to ZSM-5. The zeolites can be listed in the following order with respect to their separation capacity between linear and 2- and 3-methylbranched alkanes ZSM-22 > ZSM-23 > ZSM-5. In narrow pore structures such as zeolites ZSM-22 and ZSM-23 it is very probable that linear alkanes with smaller kinetic diameters have more access to the available adsorption sites compared to the more bulky branched molecules. This may be regarded as the first... 

In this scheme a full positive charge of -FI is assumed. In fact, the polarity of the chemical bonds will be lower. The same is true, however, for the classical carbenium ion model. It was shown by Kazansky 316) that surfece protons in zeolites are, in reality, O—H bonds with a finite dipole moment likewise the carbenium ions assumed in isomerization reactions, etc., in acid zeolites are merely transition states of alkoxy groups with a finite dipole moment of the O—C bond. In the same manner the ions in the above reaction scheme should be read as limiting structures of a complex with nonnegligible polarity. 

Isomerization Reactions. In petroleum production, the conversion of normal hydrocarbon chains to branched chains is important, since the latter has a higher octane number. Acid-promoted AI2O3 is a catalyst used in such isomerization reactions. Although this and other acid catalysts are used in isomerization reactions, it has been found that the conversion of normal paraffins to isoparaffins is easiest when both acid sites and hydrogenation sites are present, such ais in the catalyst Pt on AI2O3. 

Allylic or propargylic metal compounds are often intermediates in isomerization reactions of unsaturated systems (see Section 4.5.5) the product distribution on protonation depends very much on the substitution and the conditions of hydrolysis and most often mixtures of isomers are obtained. In contrast double bond shift generally occurs on protodesilylation of allylic silanes by acids, a reaction which takes place stereospecifically anti. Only when intramolecular delivery of the proton in a chair-like transition state is possible may a different kind of stereoselectivity be effective and good 1,3- or 1,4-induction in hydrogen transfer be observed (Scheme 83). ° ... 

In order to give answer to these questions, this study presents data on the effect of WOx loading, calcination temperature, and preparation methodology on the characteristics of the W0x-Ce02 catalysts. If, indeed, the oxidative behavior is the one that promotes the isomerization, then the replacement of Zr02 with Ce02 should improve the performances of this catalyst in isomerization reactions. 

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