Rearrangements of hydrocarbons

Isomerization (rearrangement) of hydrocarbons is of substantial practical importance. Straight-chain alkanes obtained from petroleum... 

Offer an explanation for the facility of the reaction, as compared to the vinylcyclopropane rearrangement of hydrocarbons, which requires a temperature above 200°C. Consider concerted reaction pathways which would account for the observed stereospecificity of the reaction. 

Rearrangements of Monofunctional Cyclopropylmethyl Compounds 3.2.1.1. Rearrangements of Hydrocarbons and Solvolysis of Vinyl Halides... 

Random rearrangements of hydrocarbons were noted by the early mass spectrometrists in the petroleum industry. For example,... 

Even after the poisoning effectiveness of an amine is optimized by redistribution treatments, a considerable fraction of the amine can remain coordinatively bound to Lewis acid sites that are inactive for skeletal rearrangements of hydrocarbons. However, advantage can be taken of the fact that the relative extent of reaction of an amine with Lewis and... 

In these sections of our chapter, we emphasize research advances in the area of surface acidity of specific solids that have occurred during the period from 1970 to the fall of 1976. As stated earlier, the class of solids with which we are chiefly concerned are metal oxides that catalyze skeletal rearrangements of hydrocarbons via carbonium ion intermediates. However, we have included reviews of silica gel and alumina, which are relatively inactive, because the properties of these solids form a useful frame of reference. The initial sections (Sections III.A-III.D) deal predominantly with amorphous catalysts the final sections (Sections III.E and III.F), with crystalline catalysts. 

The principle of the Lewis acid catalyzed rearrangements of hydrocarbons is well documented 4,81. Lewis acids react with a promotor deliberately added or present as an impurity in the reaction mixture to form carbonium ions which initiate intermolecular hydride transfers involving the hydrocarbon. These hydride transfers appear to be fairly unselective processes. While the expected tertiary > secondary > primary selectivity order is observed, the differences are significantly reduced relative to typical carbonium ion reactions. Possibly this is due to a hydride transfer mechanism which involves a pentaco-ordinate carbon transition state in which charge development on carbon would be minimized 38dh... 

First, as discussed earlier in connection with the aluminum halide catalyzed rearrangements of hydrocarbons (Section II. A. 2), intermolecular hydride transfer reactions appear to be fairly unselective processes. Apparently, charge development in the transition states of these reactions is minimized a penta-coordinate carbon intermediate may be involved. As a result, the strong preference for the bridgehead positions exhibited by most ionic substitution reactions is partially overcome. 

Surprisingly, gold can also catalyze skeletal rearrangements of hydrocarbons for instance, the isomerization of 2,2-dimethylbutane to n-hexane has been achieved by Schmid with the aid of AU55 clusters on titanium dioxide [4c, 6] and the aromatization of the dispirocycle 1 to tetrahydronaphthalene 2 was achieved by de Meijere et al. in a reactor with gold surface at 100 °C in a few seconds (Scheme 1) [7]. 

Sinfelt has greatly contributed to the catalyses of bimetallic nanoparticles [18]. His group has thoroughly studied inorganic oxide-supported bimetallic nanoparticles for catalyses and analyzed their microstructures by an EXAFS technique [19-22]. Nuzzo and co-workers have also studied the structural characterization of carbon-supported Pt/Ru bimetallic nanoparticles by using physical techniques, such as EXAFS, XANES, STEM, and EDX [23-25]. These supported bimetallic nanoparticles have already been used as effective catalysts for the hydrogenation of olefins and carbon-skeleton rearrangement of hydrocarbons. The alloy structure can be carefully examined to understand their catalytic properties. Catalysis of supported nanoparticles has been studied for many years and is practically important but is not considered further here. 

The skeletal rearrangements of hydrocarbons represent an important class of reactions catalyzed by metal surfaces, which have few counterparts in homogeneous catalysis. Although two reviews dealing with the subject (7, 2) have recently been published, this review seemed worthwhile on account... 

Within each section, rearrangements of hydrocarbons are treated before those of compounds containing oxygen, nitrogen, halogen, or sulfur. 

Figure 4. Calculated transition state structures for degenerate Cope rearrangements of hydrocarbon 41 via transition state 43 and 2,3-epoxyoxepin (18) via transition state 46 (see ref. 34).

Clusters, alloys and poisoning. An overview Molecular organometallic chemistry on surfaces reactivity of metal carbonyls on metal oxides Mechanisms of skeletal rearrangements of hydrocarbons on metals elementary steps Surface-bound metal hydrocarbyls. Organometallic connections between heterogeneous and homogeneous catalysis... 

The reactions include hydrogenolysis or hydrocracking (rupture of C—C bonds), isomerization, dehydrocyclization and aromatization of the C U2n + 2 molecules " . They are observed with varying selectivities, which depend on the experimental conditions, e.g. temperature, pressure and the type of catalyst. A multitude of supported (Al203,Si02) or unsupported catalysts (e.g. monometallic transition metals inter-metallic compounds , bimetallic alloys, etc.) are applied to induce skeletal rearrangements of hydrocarbons. The latter, however, are the most important catalysts used in industrial practice, which allow reforming processes at low pressure and at low temperatures. 

The nature of the reactive intermediates involved in hydrocracking or in skeletal rearrangements of hydrocarbons is still under active discussion. Many arguments are in favour of metalla-carbyne or metalla-carbene like complexes . However, several experimental results may only be explained by proposing metalla-cyclobutanes or cr-alkyl-metal species as adsorbed reactive intermediates ", and they depend strongly on the nature of the active sites (e.g. large or small size particles, single crystal metal surfaces ). 

As might have been anticipated on the basis of the Cope rearrangement of hydrocarbons, which was extended to alcohols and enols earlier in this chapter (e.g., see Scheme 8.84), alkyl and aryl ethers also enjoy the ability to rearrange (a Claisen rearrangement, vide supra. Scheme 8.43 see Claisen and Tietze, 1925, 1926) to, respectively, isomeric ethers (Scheme 8.99) or ketones (aldehydes) (Scheme 8.100) and phenols (Scheme 8.101). 

Rearrangements. AICI3 catalyzed rearrangement of hydrocarbon derivatives to adamantanes has been well documented (eq 35). Other rearrangements have been used in triquinane synthesis (eq 36). ... 

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