Cyclohexane, ring aromatization

The second aromatization reaction is the dehydrocyclization of paraffins to aromatics. For example, if n-hexane represents this reaction, the first step would be to dehydrogenate the hexane molecule over the platinum surface, giving 1-hexene (2- or 3-hexenes are also possible isomers, but cyclization to a cyclohexane ring may occur through a different mechanism). Cyclohexane then dehydrogenates to benzene. 

The twice aromatic analog 71 of the propellane 60 containing cyclohexane rings, 5) (in which there are, of course, no bridgehead hydrogens), has been obtained from the tetrabromide and 1,2-benzenedithiol. Again no dispiran is formed 20. ... 

If R is aromatic, it is reduced to a cyclohexane ring unless degassed catalyst... 

Finally, also in connection with functional groups, there is a set of heuristic principles" which refer to the stability of aromatic and conjugated systems, as well as to the relationship between more or less unsaturated cyclohexane rings and benzene rings ("HPs-6"). For instance, through a series of well known reactions, it is possible to establish a relationship between p-methylanisole and cyclohexane derivatives with different levels of unsaturation (Scheme 4.13) ... 

The first example illustrates how a 1,4-dehydroaromatic system with cyclohexane ring having two double bonds may be also disconnected according to a retro-Diels-Alder to give a diene and an acetylene as the dienophile [25]. The second example makes clear that even an aromatic double bond may be -in some instances-involved in a retrosynthetic pericyclic disconnection [26]. In the synthetic direction, the polycyclisation involves a conrotatory electrocyclic cyclobutene ring opening, (16 15) followed by an intramolecular Diels-Alder addition (see Scheme... 

HP-6 Reference to aromatic systems stability, orientation rules and their relationship with more or less unsaturated cyclohexane rings and decalin systems. 

Note that a cyclohexane system will be forced into a similar half-chair conformation by fusing a planar aromatic ring onto a cyclohexane ring (a tetrahydronaphthalene system). 

The ability of a catalyst to promote isomerization plays two roles in reforming it increases the amount of branched chain paraffins in the product and it converts naphthene hydrocarbons with cyclopentane rings into cyclohexane ring naphthenes which are necessary for the formation of aromatics by dehydrogenation. 

The octane number improvement obtained by isomerization of paraffin hydrocarbons is not great since the amounts of the more highly branched paraffins formed at equilibrium are small at the temperatures employed in catalytic reforming (5). Naphthene isomerization, on the other hand, plays a more important role in reforming. In most naphthas about 50% of the naphthene hydrocarbons are of the cyclopentane type (4) so that in order to obtain the maximum aromatic formation, isomerization of these rings to cyclohexane rings must be promoted by the catalyst. 

Our synthesis started with ethyl 5-methyl-4-isoxazole carboxylate (50), prepared from ethyl acetoacetate and DMF dimethyl acetal (Scheme 5.4).14 Ester 50 was reduced with LiAlH4 and the resulting alcohol was oxidized to afford aldehyde 51. Enone 52 was obtained from aldehyde 51 using conditions developed by McCurry and Singh.15 The next step was the aromatization of the cyclohexane ring of 52 to produce the aromatic "A" ring of the monomer. Treatment of enone 52 with iodine in the presence of sodium ethoxide produced phenol 53.16... 

The cyclohexane ring is obtained either via hydrogenation of the aromatic ring or by cyclisation. 

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