Arie Cornelis Besemer

Highly selective nitroxyl radical-mediated oxidation of primary alcohol groups in water-soluble glucans

With catalytic amounts of 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO) and hypochlorite/bromide as the regenerating oxidant in water, primary alcohol groups in glucans and derivatives thereof were rapidly and completely oxidised. For pyranosides, selectivity was higher than 95% and no side products could be detected with 1H and 13C NMR or with high-performance anion-exchange chromatography (HPAEC). The optimum pH for the reaction was between 10 and 11. The oxidation was found to be first order in TEMPO and Br-. The oxidation method can be applied to determine the amount of primary alcohol groups in water-soluble glucans; for pullulan, a proportion of 70% and for dextran, a proportion of 3% primary alcohol groups was found.

TEMPO-Mediated Oxidation of Polysaccharides: Survey of Methods and Applications

This review deals with TEMPO as a catalyst in oxidation of alcohol functions in polysaccharides. Synthesis of TEMPO and derivatives and the mechanism of the oxidative cycle in which TEMPO is involved in oxidation of alcohols are discussed. Results of oxidation of various polysaccharides with respect to yield, and introduction of the functional groups (aldehyde and/or carboxylate) are presented. Most of the primary oxidants are not ideal, as they produce large amounts of salts, e.g., sodium chloride from sodium hypochlorite. Results and perspectives are given to change the salt-based oxidative systems for much cleaner oxygen or hydrogen peroxide/enzyme-based TEMPO systems. Moreover, several immobilized TEMPO systems have been developed.

Selective oxidation of primary alcohols mediated by nitroxyl radical in aqueous solution. Kinetics and mechanism

The kinetics of the TEMPO-mediated oxidation of methyl α-D- glucopyranoside to sodium methyl α-D-glucopyranosiduronate were studied. An intermediate was found which was identified as the hydrated aldehyde. This was oxidised in the same manner as the alcohol, with pseudo first order rate constants ratio k(obs,ald)/k(obs,alc) = 7. The reaction mechanism is discussed with emphasis on steric factors and compared to literature data. Two different reaction pathways are postulated; under basic reaction conditions via a cyclic transition state 3 and under acid reaction conditions through an acyclic transition state 4.

Bromide-free TEMPO-mediated oxidation of primary alcohol groups in starch and methyl α-d-glucopyranoside

TEMPO (2,2,6,6-tetramethylpiperidine-1-oxyl)-mediated oxidation of potato starch and methyl alpha-D-glucopyranoside (MGP) was performed in the absence of sodium bromide (NaBr) as co-catalyst, solely using sodium hypochlorite (NaOCl) as the primary oxidant. The low reaction rate associated with a bromide-free process was increased by performing the oxidation at increased temperatures. The reaction proceeded stoichiometrically and with high selectivity and with only minor depolymerisation, provided that temperature and pH were kept < or = 20 degrees C and < 9.0, respectively. At 20 degrees C and pH 8.5, the reaction rate was comparable to that of a corresponding oxidation catalysed by NaBr at 2 degrees C. Consequently, this is a simple approach to raise the TEMPO/NaOCl reaction rate under bromide-free conditions while still maintaining good product properties. At higher oxidation temperatures (> or = 25 degrees C) and under more alkaline conditions (pH > or = 9.0) degradation of the starch skeleton occurred. Simultaneously, side-reactions of the nitrosonium ion lowered the yield of the oxidation. Despite the absence of the NaBr catalyst, the reaction rate-controlling step was found to be the oxidation of the primary hydroxyl groups with the nitrosonium ion. The reaction was first-order in MGP and in TEMPO.

TEMPO-derivatives as catalysts in the oxidation of primary alcohol groups in carbohydrates

Abstract Primary hydroxyl groups in aqueous starch, pullulan and methyl α- d -glucopyranoside were oxidised to the corresponding carboxylic acid functionalities by TEMPO-(4-X)-derivatives using sodium hypochlorite as the primary oxidant. All the combinations of substrates and nitroxyl radicals resulted in stoichiometric conversions, and the selectivity for oxidation of primary hydroxyls was high. Some depolymerisation occurred throughout the oxidation, especially when 4-acetoxy and 4-mesyl-TEMPO were used. The pH window of the activity of the inexpensive 4-acetamido-TEMPO was found to be substantially lower from that of the other tested TEMPO-derivatives; thus allowing milder reaction conditions. At pH 8, the rate of oxidation was ca. two times higher when 4-acetamido-TEMPO was used compared to the other catalysts.

Selective oxidation of carbohydrates by 4-AcNH-TEMPO/peracid systems

Abstract Starch, amylopectin, inulin, pullulan and methyl α- d -glucopyranoside (Me α-Glc p ) were oxidised by 4-acetamido-2,2,6,6-tetramethylpiperidine-1-oxyl (4-AcNH-TEMPO) as the mediator and peracetic acid or monoperoxysulfate (Oxone ® ) as the regenerating oxidant. The conversion of primary alcohol groups to the corresponding carboxyl groups proceeded with high yield and selectivity, provided that sodium bromide was added as co-catalyst. The mass molecular distributions of the oxidised polysaccharides indicated that no major depolymerisation occurred during oxidation. Oxone appeared to be the most efficient oxidant as the reaction rate was 25 times higher than that of peracetic acid in the oxidation of Me α-Glc p . On the other hand, oxone produces a larger amount of waste as by-product than peracetic acid.

Autocatalytic oxidation of primary hydroxyl functions in glucans with nitrogen oxides

Abstract The selective oxidation of the primary hydroxyl groups in the glucans cellulose, amylose and pullulan with nitrogen oxides has been studied. The polymers were dissolved in 85% phosphoric acid and sodium nitrate was used as the stoichiometric oxidant. A catalytic amount of sodium nitrite was added to reduce the induction time. With this reaction system, where the oxidising nitrogen oxides are formed in situ, the primary hydroxyl groups could be completely oxidised (> 95%) to carboxylic acids. Undesired ketones due to secondary hydroxyl group oxidation were subsequently reduced with sodium borohydride. Especially for the α-glucans, this oxidation-reduction sequence of secondary hydroxyl functions apparently gave epimerisation. Degradation of the polymers was slow provided the oxidation was performed at 4 °C. Thus, pullulan with 〈M w 〉 ≈ 170 kg/mol yielded a polyuronate with 〈M w 〉 ≈ 100 kg/mol. A study of this reaction system with β-cyclodextrin as the substrate clearly showed that the reaction was autocatalytic.