| Abstract |
Synthetic amavadin Ca[V\ON[CH(CH3)COO](2)\(2)] and its models Ca[V\ON(CH2COO)(2)\(2)] and [VO\N(CH2)CH2O)(3)\], in the presence of K2S2O8 in trifluoroacetic acid (TFA), exhibit remarkable catalytic activity for the one-pot carboxylation of ethane to propionic and acetic acids with the former as the main product (overall yields up to 93\%, catalyst turnover numbers (TONs) up to 2.0 x 10(4)). ne simpler V complexes [VO(CF3SO3)(2)], [VO(acac)(2)] and VOSO4 are less active. The effects of various factors, namely, C2H6 and CO pressures, time, temperature, and amounts of catalyst, TFA and K2S2O8, have been investigated, and this allowed optimisation of the process and control of selectivity. C-13-labelling experiments indicated that the formation of acetic acid follows two pathways, the dominant one via oxidation of ethane with preservation of the C-C bond, and the other via rupture of this bond and carbonylation of the methyl group by CO; the C-C bond is retained in the formation of propionic acid upon carbonylation of ethane. The reactions proceed via both C- and O-centred radicals, as shown by experiments with radical traps. On the basis of detailed DFT calculations, plausible reaction mechanisms are discussed. The carboxylation of ethane in the presence of CO follows the sequential formation of C2H5,center dot C2H5CO center dot, C2H5COO(center dot) and C2H5COOH. The C2H5COO center dot radical is easily formed on reaction of C2H5CO center dot with a peroxo V catalyst via a V\eta(1)-OOC(O)C2H5\ intermediate. In the absence of CO, carboxylation proceeds by reaction of C2H5 center dot with TFA. For the oxidation of ethane to acetic acid, either with preservation or cleavage of the C-C bond, metal-assisted and purely organic pathways are also proposed and discussed. |
