A convenient and clean route to transform, in aqueous medium, various alkanes to carboxylic acids via single-pot carboxylation with CO and water, under mild conditions, has been achieved, proceeding efficiently and selectively even without any metal catalyst and any acid additive, at low temperatures; the relevant hydroxylating role of H(2)O and radical mechanisms are disclosed by radical-trap, H(2)(18)O and DFT studies.
Seine biologische Rolle ist noch unklar, dennoch zeichnen sich für den natürlich vorkommenden Vanadiumkomplex Amavadin (1) bereits industrielle Anwendungen ab: Amavadin (und ebenso andere VIV‐ und VV‐Komplexe mit N,O‐ und O,O‐Liganden) können die direkte Umwandlung von Methan in Essigsäure katalysieren – die Reaktion verläuft in Abwesenheit von CO unter sehr milden Bedingungen und mit hohen Ausbeuten (siehe Schema).
Despite current interest in the biological roles of vanadium, its application in catalysis is still an underdeveloped field of research.[1] We have already reported that amavadine, a natural vanadium complex present in some Amanita fungi and whose biological function is still unknown, can exhibit haloperoxidase-and peroxidase-type activities and act as a catalyst for the oxidation of some biological thiols, [2] as well as for the peroxidative halogenation, hydroxylation, and oxofunctionalization of alkanes and aromatic compounds. [3] Following this work, we have been searching for other reactions that could be catalyzed by this and related V complexes, in particular the conversion of methane-the main component of natural gas and the most abundant and least reactive alkane-into functionalized products with added commercial value.[4] Recently attention has focused on the metal-catalyzed transformation of CH 4 and CO into acetic acid, [5][6][7][8][9][10][11] as well as the formation of carbonylated products without requiring the use of noxious CO, such as the conversion of CH 4 into methyl esters [12] and into acetic acid (and methanol).[9] The latter, the reaction of CH 4 and CO 2 catalyzed by NaVO 3 /pyrazine-2-carboxylic acid (in the presence of H 2 O 2 in aqueous solution), occurs in very low yield based on CH 4 (ca. 0.01 %) and at a considerable pressure (50 bar) of this gas, although at low temperature (40 8C).[9]The reaction also proceeds with CO, which apparently is the carbonylating agent even when CO 2 is used (CO is then formed by reduction of CO 2 by methyl and/or hydroxyl radicals).[9] We now report the unprecedented (to our knowledge) conversion of CH 4 into CH 3 COOH in the absence of either CO or CO 2 , in a novel single-pot catalytic reaction [Eq. (1)] under considerably mild conditions and in high yields. Table 1, and typical conditions include conducting the reaction in CF 3 COOH at 80 8C with molar ratios of CH 4 :V catalyst and K 2 S 2 O 8 :V catalyst of 46:1 (corresponding to CH 4 pressure of 5 atm) and 200:1, respectively. Usually the yields were determined after a reaction time of 20 h, but often much shorter times were sufficient to attain close values (see entries 1 and 11 for yields obtained after 2 h; the former is 92 % of that in entry 2 obtained after 20 h). The most active catalysts, which can give yields over 50 % and TONs close to 30, are complex 1 within the type
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