Aromatization

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Aromatization is a chemical reaction in which an aromatic system is formed from a single nonaromatic precursor. Typically aromatization is achieved by dehydrogenation of existing cyclic compounds, illustrated by the conversion of cyclohexane into benzene. Aromatization includes the formation of heterocyclic systems.[1]

The conversion of methylcyclohexane to toluene is a classic aromatization reaction. This platinum (Pt)-catalyzed process is practiced on scale in the production of gasoline from petroleum.

Industrial practice

Although not practiced under the name, aromatization is a cornerstone of

naphthenes into aromatics. The process, which is catalyzed by platinum, is exemplified in the conversion methylcyclohexane (a naphthene) into toluene (an aromatic).[2] Dehydrocyclization converts paraffins (acyclic hydrocarbons) into aromatics.[3] A related aromatization process includes dehydroisomerization of methylcyclopentane
to benzene:

Biochemical processes

Aromatase inhibitors like exemestane (which forms a permanent and deactivating bond with the aromatase enzyme)[6] and anastrozole and letrozole (which compete for the enzyme)[7] have been shown to be more effective than anti-estrogen medications such as tamoxifen likely because they prevent the formation of estradiol.[5]

Aromatization pathways

Oxidative dehydrogenation

For cyclohexane, cyclohexene, and cyclohexadiene, dehydrogenation is the conceptually simplest pathway for aromatization. The activation barrier decreases with the degree of unsaturation. Thus, cyclohexadienes are especially prone to aromatization. Formally, dehydrogenation is a redox process. Dehydrogenative aromatization is the reverse of arene hydrogenation. As such, hydrogenation catalysts are effective for the reverse reaction. Platinum-catalyzed dehydrogenations of cyclohexanes and related feedstocks are the largest scale applications of this reaction (see above).[1]

methyl migration.[8] In the process, DDQ is itself reduced into an aromatic hydroquinone
product.

Sulfur and selenium are traditionally used in aromatization, the leaving group being hydrogen sulfide.[9]

Soluble transition metal complexes can induce oxidative aromatization concomitant with complexation.

ruthenium trichloride.[10]

Oxidative dehydrogenation of dihydropyridine results in aromatization, giving pyridine.[11]

Dehydration

240pxSemmler-Wolff synthesis of aniline

Non-aromatic rings can be aromatized in many ways.

Semmler-Wolff reaction
of
2-cyclohexenone oxime to aniline under acidic conditions.[12]

Tautomerization

1,4-Dioxotetralin and its aromatized tautomer 1,4-naphthalenediol coexist in equal abundance in solution.

The isomerization of cyclohexadienones gives the aromatic tautomer phenol.[13][14] Isomerization of 1,4-naphthalenediol at 200 °C produces a 2:1 mixture with its keto form, 1,4-dioxotetralin.[15]

Hydride and proton abstraction

Classically, aromatization reactions involve changing the C:H ratio of a substrate. When applied to cyclopentadiene, proton removal gives the aromatic conjugate base cyclopentadienyl anion, isolable as sodium cyclopentadienide:[16]

2 Na + 2 C5H6 → 2 NaC5H5 + H2

Aromatization can entail removal of hydride. Tropylium, C
7H+
7 arises by the aromatization reaction of cycloheptatriene with hydride acceptors.

C
7H
8 + Br
2 → C
7H+
7 + Br−
 + HBr
Ciamician-Dennstedt rearrangement of a pyrrole to a pyridine. The first step involves dearomatization. The second step involves aromatization.

From acyclic precursors

The aromatization of acyclic precursors is rarer in organic synthesis, although it is a significant component of the BTX production in refineries.

Among acyclic precursors, alkynes are relatively prone to aromatizations since they are partially dehydrogenated. The

1,3-cyclohexadiene to produce benzene and tetralin
at 37 °C, the reaction being highly favorable owing to the formation of two new aromatic rings:

Scheme 1. Bergman cyclization
Scheme 1. Bergman cyclization

See also

References

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    ISBN 978-0-471-72091-1
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  3. doi:10.1080/01614949208020306
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  5. ^
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  6. ISBN 9783852001814
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  10. ISBN 9780470132524
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  11. ISBN 3527306730
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  12. doi:10.1021/ja01140a048
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  13. ISBN 9780198503460
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  14. doi:10.1139/cjc-77-5-6-605
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  15. PMID 16304647
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  16. ISBN 9780471199571
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  17. PMID 23600723
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