Dowex 50 acid, polymer-supported

A compound prepared and first described as nicotinium dichromate (NDC) by Palomo et al.,379 was later shown by X-ray-crystal analysis380 to be a betainic mixed anhydride of nicotinic and chromic acid (NACAA). Because of its unique structure, it deserves a close scrutiny of its oxidative properties.381 Replacement of the chloride anion in the quaternary ammonium resin, Dowex 1-X8, for the dichromate anion, leads to a polymer supported dichromate, which is able to make selective benzylic oxidations.382 Finally, poly[vinyl(pyridinium dichromate)] (PVPDC), a polymeric analogue of PDC, must be mentioned whose use in the oxidation of alcohols allows for a very easy work-up.383... 

Discussion In principle, acetals are cleaved by acid-catalyzed hydrolysis. In most cases aqueous acetic acid, aqueous trifluoracetic acid, dilute HC1 in THF or DOWEX 50W (H+) resin are used. Thus, treatment of 6 with DOWEX ion exchange resin in methanol rapidly furnishes the corresponding 1,2-diol without any further chromatographic purification steps. Generally, polymer supported reagents benefit from the ease of removal from the reaction mixture just by filtration of the insoluble resin. The resulting diol is acetylated by addition of acetic anhydride and pyridine. Final acetal exchange is achieved by acetic anhydride and catalytic amounts of concentrated sulfuric acid. A mixture (2 1) of anomers is obtained. 

General. Polymer-supported sulfonic acids are reliable catalysts for many acid-mediated reactions. Macroretlcular poly-styrene-based resins such as Amberlyst 15 are preferentially used in organic solvents, whereas Dowex resins work better in aqueous solutions. All resins are insoluble and can be separated from reactions by simple filtration. 

After reaction with polymer-supported acids, like Dowex resin, the new Ru complex 9 can be conveniently immobilized by ion exchange. In this novel strategy for the immobilization of Ru-based metathesis catalysts, the amino group plays a twofold role, being first an active anchor for immobilization (vide supra) and second, after protonation, activating the catalysts (electron donating to electron withdrawing activity switch). 

Table 3.6 and Scheme 3.31, in the model reaction of lauric acid with 3-phenyl-1-propanol, commercially available DOWEX 500W-X2 (H+-form, 9) did not promote esterification. The result indicates that a highly hydrophobic nature of the polymer-supported catalysts is important for activity in the dehydration reaction in water. It was found that resin 9 swelled significantly in water due to its high sulfonic acid content. On the other hand, both 11 and 12 scarcely swelled in water but worked as efficient catalysts. Resin 11 was easily recovered by simple filtration after the esterification was complete, and the catalyst could be continuously reused at least four times without loss of catalytic activity (Scheme 3.32). 

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