Parallel Screening of Homogeneous Copper Catalysts for the Oxidation of Benzylic Alcohols with Molecular Oxygen in Aqueous Solutions

Abstract

A simple and efficient parallel screening method to evaluate the catalytic activities of homogeneous copper complexes for the oxidation of benzylic alcohols in aqueous solutions with molecular oxygen is reported. Copper(II) sulfate was treated in situ with 22 nitrogen donor ligands, and the catalytic activities of these combinations were studied at four different pH values with two substrates (benzyl alcohol and 3,4-dimethoxy benzyl alcohol (veratryl alcohol)), resulting in 176 oxidation experiments in the primary screening stage. Copper complexes based on N,N,N',N'-tetramethyl ethylenediamine (TMEDA), 9,10-diaminephenanthrene (DAPHEN), and 1,2-diaminocyclohexane (DACH) were determined to be the most active catalysts. In the second screenings, the influence of reaction conditions on Cu(DACH)-, Cu(TMEDA)-, and Cu(DAPHEN)-catalyzed reactions were investigated in more detail. It was found that highly basic reaction conditions favor the reaction with the exception of Cu(TMEDA), which is active at a lower pH range. Under optimized conditions, Cu(DAPHEN) catalyzes the transformation of veratryl alcohol to the corresponding aldehyde with 100% conversion.

An efficient method to investigate metal?ligand combinations for oxygen bleaching

“…A more detailed description of the parallel reactor setup is reported elsewhere [31]. The metal salt (0.138 mmol, 13.8 mmol/kg), ligand (molar ratio 1:2 to metal, 0.276 mmol, 27.6 mmol/kg) and veratryl alcohol (molar ratio 1:10 to catalyst, 0.2 ml, 1.38 mmol, 138 mmol/kg) were mixed with 10 ml of 0.05 M NaOH in de-ionised water (pH ∼12.6).…”

Ru-catalyzed oxidation of primary alcohols

“…Suitable active metals for both homogeneously and heterogeneously catalyzed oxidations with molecular oxygen include ruthenium and other group VIII metals like palladium [9][10][11][12][13][14][15], cobalt [16][17][18], iron [19], nickel and osmium [20][21][22] and also the groups I, V, VI, and VII metals such as copper [23][24][25][26][27][28][29], vanadium [30,31], molybdenum [32], and manganese [33,34]. Ruthenium catalysts are known to oxidize alcohols and other substrates according to different reaction mechanisms such as hydridometal or oxometal pathways and even radical mechanism, depending on the oxidation states of ruthenium, oxidant and cocatalysts or additives [23][24][25][26][27][28][29][30][31][32][33][34][35]. Various homogeneous Ru catalysts, like RuCl 3 [36], perruthenate [37,38] or Ru complexes [20,39,40] p...…”

An Overview of Selective Oxidation of Alcohols: Catalysts, Oxidants and Reaction Mechanisms

“…Ahmad et al [54] devoted themselves into developing green catalytic systems for alcohol oxidation [3,[55][56][57][58][59][60]. They reported detailed studies on oxidation of benzylic alcohols in water over several copper salts catalysts with H 2 O 2 as an end oxidant.…”