Alkyl groups, dehydrogenation

An immobilized catalyst concept has been described in a recent patent application (25). The catalyst, used for the dehydrogenation of organic compounds, comprises an organometallic pincer complex bonded to a TUD-1 support. The pincer complex possesses catalytic activity for alkyl group dehydrogenation. This catalyst can be used for various refining and petrochemical processes, such as paraffin dehydrogenation. 

Alkyl groups attached to aromatic rings are oxidized more readily than the ring in alkaline media. Complete oxidation to benzoic acids usually occurs with nonspecific oxidants such as KMnO, but activated tertiary carbon atoms can be oxidized to the corresponding alcohols (R. Stewart, 1965 D. Arndt, 1975). With mercury(ll) acetate, allyiic and benzylic oxidations are aJso possible. It is most widely used in the mild dehydrogenation of tertiary amines to give, enamines or heteroarenes (M. Shamma, 1970 H. Arzoumanian. 1971 A. Friedrich, 1975). 

If both X and Y are alkyl groups, the reference compound is a singly branched hydrocarbon, and it is converted to a doubly branched structural unit in the hypothetical dehydrogenation process hence... 

If alkyl groups having (3-hydrogens are present on platinum cis to an open site, (3-H-elimination will indeed occur, reversibly sometimes, and it can occur both from Pt(II) and Pt(IV) (52,97,213-219). Catalytic dehydrogenation of an alkane using a soluble platinum complex has been reported in an early study on acceptorless thermal dehydrogenation. At 151 °C, cyclooctane was catalytically dehydrogenated (up to 10 turnovers)... 

Dehydrogenation of Alkyl Groups in Molecules Containing Heteroatoms... 

The mechanism of this elimination reaction may be regarded as being similar to that of the dehydrogenation reaction. Double-bond isomerization leads to formation of a species with both double bonds in the ring. This is then metalated and eliminates the alkyl group (A )-... 

The other 60% of the alkyl groups for LAS detergents are made through linear a-olefins. n-Alkanes can be dehydrogenated to a-olefins, which then can undergo a Friedel-Crafts reaction with benzene as described above for the nonlinear olefins. Sulfonation and basification gives the LAS detergent. 

Desaturation of alkyl groups. This novel reaction, which converts a saturated alkyl compound into a substituted alkene and is catalyzed by cytochromes P-450, has been described for the antiepileptic drug, valproic acid (VPA) (2-n-propyl-4-pentanoic acid) (Fig. 4.29). The mechanism proposed involves formation of a carbon-centered free radical, which may form either a hydroxy la ted product (alcohol) or dehydrogenate to the unsaturated compound. The cytochrome P-450-mediated metabolism yields 4-ene-VPA (2-n-propyl-4pentenoic acid), which is oxidized by the mitochondrial p-oxidation enzymes to 2,4-diene-VPA (2-n-propyl-2, 4-pentadienoic acid). This metabolite or its Co A ester irreversibly inhibits enzymes of the p-oxidation system, destroys cytochrome P-450, and may be involved in the hepatotoxicity of the drug. Further metabolism may occur to give 3-keto-4-ene-VPA (2-n-propyl-3-oxo-4-pentenoic acid), which inhibits the enzyme 3-ketoacyl-CoA thiolase, the terminal enzyme of the fatty acid oxidation system. 

Oxidation of aromatic alkyl groups. Aromatic methyl or ethyl groups para or ortho2 to an alkoxy function can be oxidized by DDQ in refluxing methanol to aldehydes or methyl ketones, respectively. Simultaneous dehydrogenation can also be effected. ... 

Very little skeletal rearrangement occurs via pyrolysis, a fact inherent in the failure of free radicals to readily isomerize by hydrogen atom or alkyl group migration. As a result, little branched alkanes are produced. Aromatization through the dehydrogenation of cyclohexanes and condensation to form polynuclear aromatics can take place. Additionally, olefin polymerization also can occur as a secondary process. 

When ethylene is adsorbed on bare nickel at 35° C. or on either bare or hydrogen-covered nickel at 150° C., the intensity of the C—H bands, shown as A of Fig. 3, is small compared with those of the associated chemisorbed ethylene shown in Fig. 2. When the species represented by A is treated with H2 at 35° C., the band intensities increased as is shown in B of Fig. 3. This behavior shows that A is due to a dissociatively chemisorbed ethylene in which the number of hydrogens per carbon is low (7). The species obtained by dissociative chemisorption will be referred to as a surface complex. It is doubtful whether the surface complex has a specific stoichiometric composition. Rather it appears that the carbon-hydrogen ratio will depend on the severity of the dehydrogenation conditions. In some cases it appears that a surface carbide, which has no hydrogens, is obtained. Even in this case the carbons appear to be easily rehydrogenated to adsorbed alkyl groups. 

Several benzologs of furan and thiophene are conveniently formed by procedures of ring closure over dehydrogenation catalysts. o-Ethylphenol is cyclized at 620° over a palladium catalyst to benzofuran (11%). Chromium oxide on alumina at 450° converts o-ethylthiophenol to benzo-thiophene (42%). Alkyl groups in the alpha and beta positions are obtained by suitable variation of structure in the alkyl side chain. For the... 

Surface alkyl chains terminate by the addition or elimination of hydrogen and desorb as paraffins and olefins. Growth and termination of alkyl groups attached to the surface through nonterminal carbons lead to internal olefins and branched hydrocarbons. Nonterminal attachment is not favored on surfaces covered by strongly adsorbed CO reactants as a result, branched products form with low selectivity at typical FT conditions, unless secondary isomerization sites are also present. These chain termination processes resemble elementary steps previously proposed for dehydrogenation-hydrogenation (12) and isomerization (13) reactions of hydrocarbons... 

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