Aromatization

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.

Although not practiced under the name, aromatization is a cornerstone of oil refining. One of the major reforming reactions is the dehydrogenation of paraffins and naphthenes into aromatics.

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

As of alkanes, they first dehydrogenate to olefins, then form rings at the place of the double bond, becoming cycloalkanes, and finally gradually lose hydrogen to become aromatic hydrocarbons.

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).

Aromatases are enzymes that aromatize rings within steroids. The specific conversions are testosterone to estradiol and androstenedione to estrone. Each of these aromatizations involves the oxidation of the C-19 methyl group to allow for the elimination of formic acid concomitant with aromatization. Such conversions are relevant to estrogen tumorogenesis in the development of breast cancer and ovarian cancer in postmenopausal women and gynecomastia in men. Aromatase inhibitors like exemestane (which forms a permanent and deactivating bond with the aromatase enzyme) and anastrozole and letrozole (which compete for the enzyme) have been shown to be more effective than anti-estrogen medications such as tamoxifen likely because they prevent the formation of estradiol.

Although practiced on a very small scale compared to the petrochemical routes, diverse methods have been developed for fine chemical syntheses.

2,3-Dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) is often the reagent of choice. DDQ and an acid catalyst has been used to synthesise a steroid with a phenanthrene core by oxidation accompanied by a double methyl migration. In the process, DDQ is itself reduced into an aromatic hydroquinone product.