Isomerization skeletal

Alcohol Substitution. In the early period of normal thiol production, the normal alcohols were utilized as feedstocks. The use of a strong acid catalyst results in the formation of a significant amount of secondary thiol, along with other isomers resulting from skeletal isomerization of the starting material. This process has largely been replaced by uv-initiation because of the higher relative cost of alcohol vs alkene feedstock. 

An extremely wide variety of catalysts, Lewis acids, Brmnsted acids, metal oxides, molecular sieves, dispersed sodium and potassium, and light, are effective (Table 5). Generally, acidic catalysts are required for skeletal isomerization and reaction is accompanied by polymerization, cracking, and hydrogen transfer, typical of carbenium ion iatermediates. Double-bond shift is accompHshed with high selectivity by the basic and metallic catalysts. 

Slow double-bond shifts and little skeletal isomerization H-transfer is minor and nonselective for tertiary olefins only small amounts of aromatics formed from aliphatics at 932°F (500°C)... 

Rapid double-bond shifts, extensive skeletal isomerization, H-transfer is major and selective for tertiary olefins large amounts of aromatics formed from aliphatics at 932°F (50t) O... 

Dimtrogen tetroxide is the most versatile of the nitrosating reagents and, in addition, it is readily available. The nitro-soamide method of deamination gives far superior yields and much less skeletal isomerization than the nitrous acid method (which is essentially limited to aqueous media), and it leads to a greater retention of optical activity than the triazene method3... 

Sandelin, F., Salmi, T., and Murzin, D. (2006) Dynamic modelling of catalyst deactivation in fixed bed reactors skeletal isomerization of 1-pentene on ferrierite. Ind. Eng. Chem. Res., 45, 558-566. 

In the case of n-butene isomerization it was demonstrated (Figure 2) that the ideal micro-pore topology led to retardation of the C8 dimer intermediate and that the catalyst based on the ferrierite structure was close to optimal in this respect [1). For selective isodewaxing a one-dimensional pore structure which constrained the skeletal isomerization transition state and thereby minimized multiple branching such as the SAPO-11 structure was found to meet these criteria. Clearly, these are ideal systems in which to apply computational chemistry where the reactant and product molecules are relatively simple and the micro-porous structures are ordered and known in detail. 

Hosoi et al. reported that Pt/S042--Zr02 persists a high activity for a long period in alkane skeletal isomerization when the reaction is carried out in the... 

The complexity of skeletal isomerization of decalin is depicted in Figures 8A and 8B. It shows that methylbicyclo[4.3.0]nonanes (MBCN1) are the... 

The (=Si03)3TiH surface complex was active in the skeletal isomerization of alkanes at temperatures as low as 50°C and this remarkable activity is very promising because this catalyst has the possibility to isomerize light hydrocarbons under very mild conditions.261... 

It was assumed that C—C bond cleavage passes through an elementary step of p-alkyl transfer. The mechanism of hydroisomerization passes also by a p-alkyl transfer step, but in this case the P-H elimination-olefin reinsertion occurs rapidly and a skeletal isomerization also occurs. 

In addition to this skeletal isomerization reaction, Anderson and Avery (24) showed that in a suitable isotopically labeled hydrocarbon, a reaction leading to positional isomerization occurred. Thus, with n-butane-l-13C as the reactant, the isomerization products were 2-(methyl-13C) propane, and 7i-butane-2-13C ... 

The isomerization reactions in the skeleton of the reactant prior to aromatization clearly involve the basic processes which we have already discussed in some detail. In passing we may note that conversion to aromatic is so favorable at any temperature (say) >350°C that this would be, of itself, sufficient reason for an adsorbed cyclo-Ce intermediate to be of negligible importance compared to cyclo-C5 as a pathway for skeletal isomerization at these temperatures. 

A wide range of nonacidic metal oxides have been examined as catalysts for aromatization and skeletal isomerization. From a mechanistic point of view, chromium oxide catalysts have been, by far, the most thoroughly studied. Reactions over chromium oxide have been carried out either over the pure oxide, or over a catalyst consisting of chromium oxide supported on a carrier, usually alumina. Depending on its history, the alumina can have an acidic function, so that the catalyst as a whole then has a duel function character. However, in this section, we propose only briefly to outline, for comparison with the metal catalyzed reactions described in previous sections, those reactions where the acidic catalyst function is negligible. 

Reactions over chromium oxide catalysts are often carried out without the addition of hydrogen to the reaction mixture, since this addition tends to reduce the catalytic activity. Thus, since chromium oxide is highly active for dehydrogenation, under the usual reaction conditions (temperature >500°C) extensive olefin formation occurs. In the following discussion we shall, in the main, be concerned only with skeletally distinguished products. Information about reaction pathways has been obtained by a study of the reaction product distribution from unlabeled (e.g. 89, 3, 118, 184-186, 38, 187) as well as from 14C-labeled reactants (89, 87, 88, 91-95, 98, 188, 189). The main mechanistic conclusions may be summarized. Although some skeletal isomerization occurs, chromium oxide catalysts are, on the whole, less efficient for skeletal isomerization than are platinum catalysts. Cyclic C5 products are of never more than very minor impor-... 

A mixture of EtsSiH/TFA in dichloromethane reduces 3-methyl-5-a-cholest-2-ene to give the pure equatorial methyl isomeric product, 3/3-methyl-5o -cholestane, in 66% yield (Eq. 79).126 On the other hand, attempts to reduce cholest-5-ene using the same technique yield neither 5a-cholestane nor 5/3-cholestane, but instead an isomeric mixture of rearranged olefins. This result is presumably because of the inability of hydride attack to compete with carbocation skeletal isomerization and elimination.126... 

Skeletal Isomerization, -Elimination, and Ring Expansion Reactions... 

The rearrangement of platinacyclobutanes to alkene complexes or ylide complexes is shown to involve an initial 1,3-hydride shift (a-elimina-tion), which may be preceded by skeletal isomerization. This isomerization can be used as a model for the bond shift mechanism of isomerization of alkanes by platinum metal, while the a-elimination also suggests a possible new mechanism for alkene polymerisation. New platinacyclobutanes with -CH2 0SC>2Me substituents undergo solvolysis with ring expansion to platinacyclopentane derivatives, the first examples of metallacyclobutane to metallacyclopentane ring expansion. The mechanism, which may also involve preliminary skeletal isomerization, has been elucidated by use of isotopic labelling and kinetic studies. 

This skeletal isomerization is strongly retarded by the presence of free pyridine and is not observed when the monodentate pyridine ligands are replaced by bidentate ligands such as 2,2 -bipyridyl 07). This behavior is explained by the need to dissociate a pyridine ligand before the skeletal isomerization can occur, and can be useful in mechanistic investigations (vide infra). 

Our first observations related to the particular skeletal isomerization of Scheme I were obtained in a study of steric effects of ligands on the stability of platinacyclobutanes (9). Three products could be obtained as shown in equation 2. 

III) at -10°C. The mechanism shown in Scheme II was therefore suggested, involving skeletal isomerization and a-elimination from the platinacyclobutane. 

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