Oxidation of mono-phenols to para-benzoquinones: a comparative study

Abstract

A oxidação de mono-fenóis à para-benzoquinonas é assunto de interesse contínuo devido à existência de inúmeros produtos naturais contendo esta unidade estrutural. As para-benzoquinonas possuem reatividade química importante como agentes oxidantes e como dienófilos na reação de Diels-Alder. Usualmente nós preparamos as para-benzoquinonas pela reação de oxidação dos respectivos mono-fenóis com oxigênio molecular e catalisada por [Co II (salen)]. Porém, foi necessário estudar estas oxidações utilizando-se outros oxidantes. Nós apresentamos aqui nossos resultados sobre esta importante reação de oxidação com uma variedade de oxidantes, utilizando onze mono-fenóis como substratos. Os oxidantes utilizados foram cobalto, níquel, cobre e vanádio derivados com alguns ligantes do tipo salen. Também foram estudados peróxido de hidrogênio, OXONE®, dimetil dioxirano e ácido iodoxibenzóico.The oxidation of mono-phenols to para-benzoquinones is of continuing interest due to the existence of numerous natural products containing this structural unit. The chemical reactivity of para-benzoquinones is also noteworthy, as oxidants and dienophiles in the Diels-Alder reaction. We have used for quite some time now, molecular oxygen and catalysis with [Co II (salen)] as the oxidation procedure, but felt the need for other oxidants and conditions to be of use with different phenol substrates. We now present our results on this important oxidation with a variety of oxidants, using eleven mono-phenols as substrates. The oxidants tested are cobalt, nickel, copper and vanadyl metals, with a selection of different salen type ligands. Completing this study we also investigated the use of hydrogen peroxide, OXONE®, dimethyl dioxirane and iodoxybenzoic acid. Keywords: oxidation, alkyl-substituted phenols, para-benzoquinones IntroductionNatural products frequently occur containing the parabenzoquinone sub-structural unit within the global structure, as can be exemplified by vitamins K 1 and K 2 , co-enzyme Q (ubiquinone), 1 and in many terpenes and alkaloids. 2 These natural products 3-13 are associated with important biological properties, such as cardiovascular, anti-tumour, antibacterial, anti-germination and anti-protozoan among others. There is no doubt that the para-benzoquinone nucleus is an important charge transfer receptor and easily undergoes nucleophilic addition reactions. 1 The redox relation with para-hydroquinones (para-quinols) is of importance in biological activity, and also makes many simple para-benzoquinones excellent oxidizing agents. 1 Synthetically, the para-benzoquinone structural unit is an excellent dienophile for the Diels-Alder reaction, among other relevant reactions. 1 The usual and obvious manner to prepare parabenzoquinones 1,14,15 is from the corresponding parahydroquinones, which is however rather limited due to the lack of a wide range of commercially or synthetically available substrates. As a second choice, the simple monophenol is an ideal substrate due to the ready availability of an extremely wide range of...

Larvicidal activity of para-Benzoquinones

“…The ethereal solution was dried over anhydrous MgSO 4 and the solvent evaporated. The residue was usually purified by sublimation (or by flash column chromatography when necessary, using as eluent a mixture of 9:1 hexane/ethyl acetate) (Dockal et al 1985;Uliana et al 2008).…”

The Diels-Alder reaction of para-benzoquinones and simple dienes under catalysis with FeCl3 on Aerosil® silica

“…Simple mortar and pestle mixture of a cheap grade of FeCl 3 and Aerosil ® silica produces the catalyst. All the reactions were tested first without FeCl 3 , or the silica, or the catalyst mixture, and then with 4% and 10% mol equivs.…”

Tyrosinase Inhibitor and Antioxidant Activity of Seaweed Powder from Fresh and Dried Sargassum plagyophyllum

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