Supercritical carbon dioxide, a new medium for aerobic alcohol oxidations catalysed by copper-TEMPO

The copper catalysed aerobic oxidation of selected alcohol substrates in supercritical carbon dioxide (scCO₂), employing a range of simple copper(II) catalyst compounds, is here described. The copper acetate complex of polydimethylsiloxane (PDMS) functionalised pyridine (1), compound 2, has previously been synthesised and characterised by us and its solubility in scCO₂ demonstrated. Due to this solubility we anticipated that the selective aerobic oxidation of alcohols to aldehydes could be homogeneously catalysed by this compound in scCO₂ in combination with the co-catalyst 2,2,6,6-tetramethylpiperidin-1-yloxy free radical (TEMPO). Our initial results showed that complete oxidation of 4-nitrobenzyl alcohol was achieved within 4 h of reaction. However, the activities of analogous copper derivatives containing simpler pyridine substituents, [Cu(AcO)₂(py)]₂ and [Cu(AcO)₂(4VP)]₂ (4VP = 4-vinylpyridine), were shown to be similar, in spite of their negligible solubility in scCO₂. When we repeated the reactions in highly non-polar hexane rather than scCO₂ similar observations were made. In both cases, as 2 is soluble and the pyridine analogues are not, a much higher reaction rate was anticipated for 2 as it is the only compound capable of homogeneous catalysis. However, in some cases slightly better activities were observed for [Cu(AcO)₂(py)]₂ rather than for the PDMS functionalised analogue, 2. Thus, despite poor catalyst solubility typically being very inhibitory in this type of catalytic process, in this system solubilisation of the catalyst is not necessary. In continuation the activity of silica supported copper complexes was therefore investigated. Employing such catalysts the 4-nitrobenzyl and benzyl alcohol substrates were completely oxidised to the corresponding aldehydes in scCO₂, this time employing lower catalyst loadings. Other types of alcohol substrate showed more limited conversions however. To conclude, alcohol oxidation in the non-conventional green solvent scCO₂, with the benign terminal oxidant, dioxygen, and simple, cheap, easily prepared metal catalyst compounds was demonstrated. This is the first copper-TEMPO catalysed alcohol oxidation system in scCO₂ to be described.