Search

The catalytic properties of in situ formed copper(II) complexes of N,N,N′,N′-tetramethylethylenediamine (TMEDA), rac-1,2-diaminocyclohexane (DACH) and 9,10-diaminophenantrene (DAPHEN) were investigated under optimized reaction conditions in the oxidation of veratryl alcohol (3,4-dimethoxybenzyl alcohol) with molecular oxygen in alkaline aqueous solutions. The kinetics of the reaction, catalytic species and their deactivation were studied, in order to clarify the reasons for low conversions in Cu(TMEDA) and Cu(DACH) catalyzed reactions. The most active catalyst was the scarcely soluble Cu(DAPHEN) with turn over number 5700 in the oxidation of veratryl alcohol. The catalytic precursors in Cu(TMEDA) catalyzed reactions is binuclear hydroxyl bridged complex (LCu-μ-(OH)2-CuL)2+. The main reason for the deactivation of these catalysts according to our results is the reactivity of the ligands under reaction conditions.

Catalytic activities of 189 metal–ligand combinations in aqueous alkaline solution were studied in the oxidation of veratryl alcohol with molecular oxygen. The parallel screening revealed high oxidation activity of copper ions with dimethylglyoxime, 2,2′-bipyridine and 1,10-phenanthroline ligands. Further study with these in situ prepared catalysts indicated that ionic complexes with two ligands are probably the most active. The maximum activities of the three catalysts were achieved at different pH and metal–ligand molar ratio. In 3 h reaction with catalyst–substrate molar ratio 1:1000, the highest turnover numbers obtained were 420 for the Cu-dimethylglyoxime catalyst, 830 for Cu-bipyridine and 760 for Cu-phenanthroline. These most effective Cu-complexes catalysed not only the oxidation of lignin model compounds but also the depolymerisation of the carbohydrate model compound dextran. Results achieved in oxygen delignification of softwood pulp were in accordance with the model compound studies; the catalysts effectively lowered the kappa number, but they also tended to damage the cellulose fibres.

Cobalt salen-type [salen= N , N ′-bis(salicylidene)ethylenediamine] complexes 1 – 6 were studied as catalysts for dioxygen activation in the oxidation of veratryl alcohol in basic aqueous conditions. The complexes Co(salen) ( 1 ), Co(αCH 3 salen) ( 2 ) [αCH 3 salen= N , N ′-bis(αmethylsalicylidene)ethylenediamine], Co(4OHsalen) ( 3 ) [4OHsalen= N , N ′-bis(4-hydroxosalicylidene)ethylenediamine], Co(sulfosalen) ( 4 ) [sulfosalen= N , N ′-bis(5-sulfonatosalicylidene)ethylenediamine], Co(acacen) ( 5 ) [acacen= N , N ′-bis(acetylacetone)ethylenediamine) and Co(N-Me-salpr) ( 6 ) [N-Me-salpr=bis(salicylideniminato-3-propyl)methylamine] were chosen to examine the influence of ligand structure on the catalytic activity. The effect of reaction conditions on the oxidation of veratryl alcohol was studied by varying temperature, pH, time or the nature and amount of the axial base needed to enhance the activity of complexes 1 – 5 . The catalytic behaviour of the studied complexes was shown to be very depended on the applied conditions and distinct differences could be observed among the complexes. In all reactions, veratraldehyde was the only product observed. The unsubstituted complex 1 was the most efficient catalyst in the studied system achieving turnover numbers of up to 28 at 80 °C and pH 12.5.

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.