Шаблон:Short descriptionAromatization 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]
Although not practiced under the name, aromatization is a cornerstone of oil refining. One of the major reforming reactions is the dehydrogenation 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:
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]
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. α-Phellandrene (2-methyl-5-iso-propyl-1,3-cyclohexadiene) is oxidised to p-iso-propyltoluene with the reduction of ruthenium trichloride.[10]
Oxidative dehydrogenation of dihydropyridine results in aromatization, giving pyridine.[11]
Файл:Tetrahydronaphthalenedione.png1,4-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]
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 Bergman cyclization is converts an enediyne to a dehydrobenzene intermediate diradical, which abstracts hydrogen to aromatize.[17] The enediyne moiety can be included within an existing ring, allowing access to a bicyclic system under mild conditions as a consequence of the ring strain in the reactant. Cyclodeca-3-en-1,5-diyne reacts with 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:
