Oxidação aeróbica de olefinas catalisada por sistemas baseados em paládio e/ou em benzoquinona

Abstract

This work describes the development of the catalytic systems based on palladium and/or benzoquinone for the selective oxidation of olefins with molecular oxygen and the optimization of reaction conditions to attain the best yield for the desired products. The aerobic dehydrogenation of cheap and easily available p-menthenic terpenes, e.g., g-terpinene, a-terpinene, limonene, and terpinolene, to the commercially more valuable p-cymenehas been developed. The reaction is catalyzed by benzoquinone and occurs in acetic acid solutions under relatively mild conditions (80-100 °C, 5-10 atm). In the presence of Cu(OAc)2 as co-catalyst which accelerates the reoxidation of hydroquinone to benzoquinone by molecular oxygen, the aerobic dehydrogenation can be performed even under atmospheric pressure. Thereaction with g-terpinene, the most reactive isomer, gives p-cymene in near quantitative yield andshows high turnover numbers (up to 1000). The 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 thepresence of acid co-catalysts and give p-cymene in lower yields (ca. 60 %). Palladium catalyzed aerobic oxidation of styrene and 2-vinylnaphthalene in DMA/H2O or DMF/H2O solutions under mild conditions has been developed, in which PdCl2 is used in the absence of co-catalysts or special stabilizing ligands as the sole and recyclable catalyst. Corresponding methyl ketones have been obtained in good to excellent yields with low catalyst loadings (0.2-5 mol %) and high turnover numbers (up to ca. 1000 to Pd). This simple and efficientcatalytic method represents ecologically benign and economically attractive synthetic pathway to the industrially important compounds used in the manufacture of various polymers and drugs. The allylic oxidation of limonene and a-terpineol with molecular oxygen catalyzed by the combination of palladium(II) complex and benzoquinone in acetic acid solutions under relatively mild conditions (70-90 °C; 1-10 atm) has been developed. The palladium/benzoquinone catalytic system is used in the absence of auxiliary redox-active co-catalysts. Benzoquinone is directly recycled by molecular oxygen under superatmospheric pressures, which prevents the deactivation of the catalyst by precipitation of inactive palladium metal. A good control of regioselectivity has been achieved through the appropriate choice of ligands on palladium. Corresponding allylic acetates have been obtained in good to excellent yields (96 % for limonene and 80 % for a-terpineol). All oxygenated terpenic derivatives obtained in the present work have a pleasant scent with flower or fruit tinge and could be useful as components of synthetic perfumes in addition to their potential applications in pharmaceutical industry. The catalytic system can be generally applied to the allylic oxidation of simple cycloalkenes with molecular oxygen, e.g., cyclohexene gives corresponding allylic acetate in a nearly quantitative yield