Platinum cyclopentane aromatization

With platinum and palladium supported on acidic alumina, cyclopentanes are important intermediates of aromatization (44, 123-124). For example, n-heptane gave about 2-3 times more aromatic product than 2,4-dimethyl-pentane, whereas the formation of C5 cyclic products was about the same from both alkanes. Alkylcyclopentanes aromatized at a reasonable rate (123a). 

Dehydrocyclization of straight-chain or branched acyclic alkanes—the reverse of the hydrogenolysis of cyclopentanes—takes place on the same platinum-charcoal catalyst under similar conditions. This reaction is often accompanied by aromatization (20-27) (Scheme 4). 

Aromatization of substituted cyclopentanes occurs on various group VIII metals, especially palladium (28, 29). However, platinum remains the best metallic catalyst for this reaction. 

On platinum films, gemdisubstituted cyclopentanes initially produce much larger amounts of aromatics than methyl- and ethylcyclopentanes. Relative rates of aromatization are given in Scheme 63. This is readily explained by either methyl stabilization of the n-adsorbed olefin in the carbene-olefin species, or by the greater propensity for a,y,-bonding, that is, metallocyclobutane formation, across a quaternary center. 

The formation of wetn-labeled toluene can be explained neither by direct 1-6 ring closure, nor by cyclic-type isomerization of n-heptane to 3-methylhexane followed by 1-6 ring closure of the latter (94). We suggest that the abnormal aromatization process responsible for the formation of meta-labeled toluene is initiated by a dicarbene as in the nonselective mechanism A (see Section IV, Scheme 47). Aromatization is not influenced by the dispersion of the platinum on the support (758), so that it may be assumed that aromatization involves a single metal atom. Isomerization of the dicarbenes (7) to the dicarbenes (8) via rt-adsorbed cyclopentanes, followed by isomerization to the suitable carbene-olefin species (9), would result in 1-6 ring closure and aromatization (Scheme 69). 

Higher homologs of cyclopentane are converted more readily, but it is important to note that there is a fundamental difference between cyclohexane reactions and those of all other hydrocarbon types. The aromatization of cyclohexane is largely a function of the platinum component. The acid function will, in fact, promote side reactions unless conditions are controlled carefully. In the case of methylcyclopentanes (or paraffins) the extent of conversion is a function of the catalyst halogen content (i.e., the acidity) and depends on the platinum content only up to a very low critical level of platinum. 

After a preliminary study of the properties of a number of platinum and nickel catalysts deposited on various carriers (activated carbon, chromia, alumina, molybdena, etc.) a platinum-alumina catalyst was selected for the present investigation with a 0.5 % Pt content. Cyclopentane (238.2 g) was passed over the catalyst at 0.43 hr. i space velocity, 20 atmospheres hydrogen pressure, and a temperature of 460 . As a result 184.3 grams of liquid product were obtained containing 9 % by volume of aromatics including benzene (81.9%), toluene, and p-xylene, and also w-pentane,... 

The considerable amoimts of aromatic hydrocarbons and methylcyclopentane that were found in the products are especially noteworthy. This new fact shows that under the conditions selected cyclopentane imdergoes a number of interesting reactions, an important part in which is played by methylene radicals arising from disintegration of the pentamethylene ring under the influence of the platinum-alumina catalyst ... 

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