Mild, single-pot hydrocarboxylation of gaseous alkanes to carboxylic acids in metal-free and copper-promoted aqueous systems.

A direct, selective and highly efficient method has been developed for the hydrocarboxylation of C(n) gaseous alkanes into C(n+1) carboxylic acids, in aqueous acid-solvent-free medium at low temperatures. The approach is based on a metal-free or copper-promoted reaction of ethane, propane or n-butane with carbon monoxide and water, in a H(2)O/MeCN medium at 50-60 degrees C, in the presence of potassium peroxodisulfate. The effects of various reaction parameters, such as the absence or presence of a copper promoter, solvent composition, temperature, time, CO and alkane pressure, were studied. A free radical mechanism was confirmed by radical trap experiments involving acyl radical formation, oxidation and subsequent hydroxylation by water. Remarkable yields (based on alkane) of carboxylic acids in the 34-41 % range were achieved even in the metal-free systems, although in the presence of a tetracopper(II) triethanolaminate derived promoter they reach superior values of 58 and 87 % for the hydrocarboxylations of propane and n-butane, respectively; in these cases, branched isobutyric and 2-methylbutyric acids were the predominant products. From a green perspective, important features of the present alkane hydrocarboxylations include the exceptional metal-free, mild and acid-solvent-free reaction conditions, the operation in aqueous medium with a rare hydroxylating role of water, with high selectivities, and yields of carboxylic acids. Taken together, these conditions correspond to the mildest and the most efficient method so far reported for the oxidative functionalisation of gaseous alkanes.