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Cyclic mechanism 1,5-dehydrocyclization

The cyclic mechanism (Scheme 7), which involves dehydrocyclization to an adsorbed cyclopentane intermediate C, followed by ring cleavage and desorption of the products, and is responsible for the isomerization of larger molecules on dispersed platinum-alumina catalysts (52, 55). [Pg.4]

The first approach to the cyclic mechanism of isomerization was the finding that the interconversion of n-hexane and methylpentanes takes place under the conditions where the nonselective mechanism of hydrogenolysis (Mechanism A) is the only one operating that is, on 0.2% Pt/AljOj (32). The identical product distributions in isomerization of hexanes and hydrogenolysis of methylcyclopentane suggested that both reactions involve a common intermediate with a methylcyclopentane structure. It was then proposed that the species responsible for dehydrocyclization of hexanes are a,j8, -triadsorbed species involving a single metal atom (55) (Scheme 40). [Pg.35]

Since 1-5 ring closure provides a route for the skeletal isomerization of alkanes, isomerization of substituted benzenes by a cyclic mechanism should also be possible. That was verified by Shephard and Rooney (95), who found that, on 0.5% Pt/Al2O3, interconversion of o-ethyltoluene and n-propyl-benzene accompanied dehydrocyclization to indane (Scheme 80). In these... [Pg.66]

Having characterized the three hydrogenolysis mechanisms by their precursor species dicarbenes (Scheme 34), 7c-adsorbed olefins (Scheme-36), and metallocyclobutanes (Schemes 38 and 39), the knowledge of the overall mechanism of cyclic type isomerization requires the identification of the precursor species in 1-5 dehydrocyclization, the reverse reaction of hydrogenolysis of cyclopentanes. [Pg.35]

On the other hand, the selective dehydrocyclization, which does not allow the formation of secondary-primary C-C bonds, must involve only two methylic carbon atoms in the 1 and 5 positions. Although the reverse reaction (selective hydrogenolysis of methylcyclopentane) could be observed on platinum catalysts of low dispersion at 220°C (86), the selective dehydrocyclization of methylpentanes on these catalysts is detectable only at higher temperatures (280°-300°C), where it competes with another process, ascribed to Mechanism C (33). Fortunately, it was found recently that iridium supported on AI2O3 or SiOj selectively catalyzes at 150°C the cyclic type interconversion of 2-methyl- and 3-methylpentanes (88). n-Hexane under the same conditions yields only cracked products (702) (Scheme 52). Similarly,... [Pg.42]


See other pages where Cyclic mechanism 1,5-dehydrocyclization is mentioned: [Pg.44]    [Pg.184]    [Pg.125]    [Pg.150]    [Pg.85]    [Pg.85]    [Pg.116]    [Pg.271]    [Pg.115]    [Pg.8]    [Pg.266]   
See also in sourсe #XX -- [ Pg.35 , Pg.36 , Pg.37 , Pg.38 , Pg.39 , Pg.40 , Pg.41 , Pg.42 ]




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Cyclic mechanism

Dehydrocyclization

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