Oxidation of Polycyclic Aromatic Hydrocarbons with Hydrogen Peroxide in the Presence of Transition Metal Mono-Substituted Keggin-Type Polyoxometalates

The oxidation of anthracene, naphthalene, 1-ethylnaphthalene, and 2-ethylnaphthalene with environmentally benign hydrogen peroxide has been studied in the presence of the tetrabutylammonium salts of iron and manganese mono-substituted Keggin-type polyoxometalates [XW(11)M(H(2)O)O(39)](n-) (X=P, Si, or B, M=Mn(III) or Fe(III)). The reactions were performed under homogeneous conditions at 80 degrees C, using acetonitrile as the solvent. For all the catalysts studied, anthracene was selectively oxidized to 9,10-anthraquinone at 100% conversion in the presence of [BW(11)Mn(H(2)O)O(39)](6-). The oxidation of 1-ethylnaphthalene and 2-ethylnaphthalene occurs mainly at the alkyl substituent, along with the formation of phthalic anhydrides, which resulted from aromatic ring oxidation, with iron substituted anions. Naphthalene could not be oxidized with the conditions used. To understand the reaction pathways, the oxidation of several products was performed in the presence of [PW(11)Fe(H(2)O)O(39)](4-). Under the conditions used, the oxidation of vinylnaphthalene derivatives gave rise to mainly carbon-carbon double-bond cleavage, affording the corresponding naphthaldehydes in high yields (> 67%). Possible reaction pathways are presented and the role of different catalysts is discussed.