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

Abstract

The copper catalysed aerobic oxidation of selected alcohol substrates in supercritical carbon dioxide (scCO(2)), 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 scCO(2) demonstrated. Due to this solubility we anticipated that the selective aerobic oxidation of alcohols to aldehydes could be homogeneously catalysed by this compound in scCO(2) 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 scCO(2). When we repeated the reactions in highly non-polar hexane rather than scCO(2) 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 scCO(2), 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 scCO(2), 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 scCO(2) to be described.

Synthesis of UiO-66-NH2 derived heterogeneous copper (II) catalyst and study of its application in the selective aerobic oxidation of alcohols

“…However, despite the high activity achieved by these homogeneous copper catalysts, they suffered from inherent problems in separation, deactivation and recycling [15]. To overcome these problems, the oxidation of alcohols using heterogeneous copper catalyst has been explored [16][17][18]. There are several issues, such as low catalytic efficiency, copper leaching, that need to be solved to make heterogeneous copper catalysis more practical [19].…”

Synthesis, characterization and structure of nickel and copper compounds containing ligands derived from keto-enehydrazines and their catalytic application for aerobic oxidation of alcohols

“…1 There are many known means of achieving this conversion but due to environmental concerns more sustainable processes are continually sought. 8 In the development of Cu/TEMPO-catalysed aerobic alcohol oxidations, the role of different copper sources, 8b the use of new ligands, [9][10][11] new nitroxide species or TEMPO variants, 12 the employment of non-conventional solvents, 13,14 heterogeneous media 15,16 or new reaction conditions, 17 and the reaction mechanism, 8 are permanently investigated with the aim of improving this oxidation process. 2 In particular, copper compounds, in conjunction with the co-catalyst TEMPO (2,2,6,6-tetramethylpiperidine-N-oxyl radical), form an appropriate and efficient system.…”

“…13 Following our interest in this area, we looked for novel bidentate N,O-ligands for the preparation of new copper complexes. 13 Following our interest in this area, we looked for novel bidentate N,O-ligands for the preparation of new copper complexes.…”

Room temperature aerobic oxidation of alcohols using CuBr2 with TEMPO and a tetradentate polymer based pyridyl-imine ligand

“…More recently, others have prepared pyridine terminated siloxane-derived ligands for use in a variety of applications including oxidation catalysis [45][46][47][48][49][50]. We have extended this approach to prepare a bis(pyridyl-imine) terminated siloxane ligand and herein, we report the first use of a series of tetradentate N-donor ligands in the aerobic oxidation of alcohols.…”