Polysaccharides catalytic oxidation

Oxidation of polysaccharides is a far more attractive route to polycarboxylates, potentially cleaner and less cosdy than esterification. Selectivity at the 2,3-secondary hydroxyls and the 6-primary is possible. Total biodegradation with acceptable property balance has not yet been achieved. For the most part, oxidations have been with hypochlorite—periodate under alkaline conditions. In the 1990s, catalytic oxidation has appeared as a possibiUty, and chemical oxidations have also been developed that are specific for the 6-hydroxyl oxidation. 

Catalytic oxidation ia the presence of metals is claimed as both nonspecific and specific for the 6-hydoxyl depending on the metals used and the conditions employed for the oxidation. Nonspecific oxidation is achieved with silver or copper and oxygen (243), and noble metals with bismuth and oxygen (244). Specific oxidation is claimed with platinum at pH 6—10 ia water ia the presence of oxygen (245). Related patents to water-soluble carboxylated derivatives of starch are Hoechst s on the oxidation of ethoxylated starch and another on the oxidation of sucrose to a tricarboxyhc acid. AH the oxidations are specific to primary hydroxyls and are with a platinum catalyst at pH near neutraUty ia the presence of oxygen (246,247). Polysaccharides as raw materials ia the detergent iadustry have been reviewed (248). 

Catalytic oxidation promotes the regioselective oxidation at C-6 primary hydroxyl groups of polysaccharides. With water-soluble and stable nitroxyl radical... 

Table II lists derivatives of D-glucuronic acid which have been obtained by catalytic oxidation. A ready preparation of 3-0-methyl-D-glucuronic acid is by the oxidation of l,2-0-isopropylidene-3-0-methyl-D-gluco-furanose and subsequent hydrolysis of the product." 4-0-Methyl-D-glucuronic acid, a compound of considerable interest because of its occurrence as a fundamental unit in hemicelluloses" and polysaccharides," cannot be obtained by the simple catalytic oxidation of methyl 4-0-methyl-D-glucopyranoside. f...

Aspinall and Cairncross subjected an arabinoxylan from rye flour to catalytic oxidation, thus converting the hydroxymethyl group in the L-arabinofuranose residues into a carboxyl group. The aldobiouronic acid obtained on partial hydrolysis of the oxidized polysaccharide was characterized as being 3-0-[(a or /3)-L-arabinofuranosyluronic acid]-D-xylose (40), showing again that alternative (38) applies. 

In the recent decade, catalytic oxidation of carbohydrates transposed to polysaccharides using highly reglo-selectlve and stable nitroxyde radical (2,2,6,6-tetramethylpiperidine-l-oxyl radical (TEMPO)) has become one of the most promising procedures to convert polysaccharides into the corresponding polyuronic acids. The method is veiy suitable for... 

Fine and specialty chemicals can be obtained from renewable resonrces via multi-step catalytic conversion from platform molecules obtained by fermentation. An alternative method decreasing the processing cost is to carry out one-pot catalytic conversion to final product without intermediate product recovery. This latter option is illustrated by an iimovative oxidation method developed in our laboratory to oxidize native polysaccharides to obtain valuable hydrophilic end-products useful for various technical applications. 

An advantageous variant of the oxidation of polysaccharides by nitrogen oxides in 85% phosphoric acid consists in the use of sodium nitrate, instead of sodium nitrite, as the stoichiometric oxidant and a catalytic amount of sodium nitrite to decrease the induction time.123 The advantage of this variant may be seen by taking into account the stoichiometry of the overall reactions ... 

In contrast to enzymatic or metal-catalyzed oxidation, the TEMPO-oxidation process is highly effective in the conversion of high molecular weight polysaccharides. Other advantages of this catalytic process are the high reaction rate and yield, the high selectivity, and little degradation of the polysaccharides [12]. 

Matsumura [185-188] has oxidized a wide range of polysaccharides, starch, xyloses, amyloses, pectins, and the like with hypochlorite/periodate. The products are either biodegradable at low oxidation levels or functional at high oxidation levels the balance has not yet been established for commercial success. Other than Matsumura, van Bekkum and co-workers, at Delft University, has been the major player in the search to control the hypochlorite/periodate liquid-phase oxidations of starches [189-191]. He has been searching for catalytic processes to speed up the oxidation with hypochlorite. Hypobromite is a more active oxidant than hypochlorite but more expensive, however, it may be generated in situ from the cheap hypochlorite and bromide ion in one solution [191, 192]. This is shown in Scheme 16. 

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