Aromatization of para-menthenic terpenes by aerobic oxidative dehydrogenation catalyzed by p-benzoquinone

The aerobic dehydrogenation of cheaply available para-menthenic terpenes, i.e., gamma-terpinene, alpha-terpinene, limonene, and terpinolene, to commercially more valuable p-cymene has been developed. The reaction is catalyzed by p-benzoquinone and occurs in acetic acid solutions under relatively mild conditions (80-100 degrees C, 5-10 atm). In the presence of Cu(OAc)(2) as co-catalyst which accelerates the re-oxidation of p-hydroquinone to p-benzoquinone by molecular oxygen, the aerobic dehydrogenation can be performed even under atmospheric pressure. The reaction with gamma-terpinene, the most reactive isomer, gives p-cymene in near quantitative yield and shows high turnover numbers (up to 1000). A solvent scope for this reaction can be extended to more friendly non-acidic alcoholic or amidic solvents. a-Terpinene also undergoes aromatization under similar conditions giving p-cymene in excellent yield (95%). On the other hand, the dehydrogenation of limonene and terpinolene, the substrates containing an exocyclic double bond, seems to be preceded by the double bond isomerization; so that these reactions require the presence of acid co-catalysts and give p-cymene in lower yields (ca. 60%). (C) 2008 Elsevier B.V. All rights reserved.