Polymer-supported, acid anions

In summary, we have established a powerful novel concept of polymer-supported acyl anion equivalents that have been demonstrated to enable addition of a large variety of readily accessible activated carboxylic acids. Diversity in all relevant positions of the norstatine core structure (Figure 3.6.IB) could be achieved smoothly. 

Figure 7.33. HPLC analysis of nuclei acids (pBR322-DNA HAE-III digest) using gradient weak-anion exchange chromatography on a column packed with 2.5-pm nonporous polymer support (weak anion exchange) with UV detection at 260nm. Reprinted with permission from reference 40.

Polymer-supported catalysts often have lower activities than the soluble catalysts because of the intraparticle diffusion resistance. In this case the immobilization of the complexes on colloidal polymers can increase the catalytic activity. Catalysts bound to polymer latexes were used in oxidation reactions, such as the Cu-catalyzed oxidation of ascorbic acid,12 the Co-catalyzed oxidation of tetralin,13 and the CoPc-catalyzed oxidation of butylphenol14 and thiols.1516 Mn(III)-porphyrin bound to colloidal anion exchange resin was... 

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... 

Thiamine-catalyzed transformations are reversible, thus TV,/V-dialkyl hydrazones were selected as alternative acyl anion equivalents that were reported to react with electrophiles without acidic activation.41 One especially reactive example, formaldehyde hydrazone resin 13, was constructed from polymer-supported hydrazines and was employed in the first polymer-supported, uncatalyzed acyl anion additions (Fig. 8).38 As test substrates, nitroalkenes (as Michael acceptors) and activated aldehydes were selected. Reactivity of these acyl anion equivalents depended critically not only on the nature of the starting hydrazine, but also on the protocol for hydrazine formation. 

An alternative solution to the problem of purification is to use 1-dimethylaminonaphthalene 7 (DMAN),17 which can be removed from the product mixture by an acid wash. Reactions in the presence of this reagent must take place below -45 °C above this temperature the DMAN radical anion decomposes to give 1-lithionaphthalene.18 Polymer-supported arene catalysts can also be used.19 20... 

Cainelli, G., Cardillo, G., Orena, M., Sandri, S. Polymer supported reagents. Chromic acid on anion exchange resins. A simple and practical oxidation of alcohols to aldehydes and ketones. J. Am. Chem. Soc. 1976, 98, 6737-6738. 

Pioneering attempts at using cinchona alkaloids as a platform for chiral stationary phase preparation have been reported as early as in the mid-1950s by Grubhofer and Schleith [52]. The chiral anion exchange polymeric materials were prepared by immobilization of quinine (and other cinchona alkaloids) via the 9-hydroxyl group or quinuclidine nitrogen to a polymer support. However, this resulted in very low selectivities of these phases toward racemic mandelic acid as a test analyte. Results of the early studies have been reviewed in detail by Davankov [53]. 

Many more chemical processes have been developed using solid acid catalysts than with solid base catalysis, in particular at an industrial scale. However, most of those examples correspond to processes involving inorganic materials [161]. The first reports on the use of polymer-supported basic catalysts can be traced back to the use of anion exchange resins containing basic anions. The simplest, most general structure for this kind of resins is given by 32 (X = OH-, Scheme 10.9)... 

CB1981] and the same reaction of the related silyl enol ether (16) using polymer supported mandelic acid leads to (5)-mandelic acid in up to 94% e.e. [94TL2891]. Low temperature protonation of the anion derived from (17) produces mainly the cis product by kinetic control [94CB1495] and hydroboration of (18) gives the cis hydroxymethyl compound with high... 

---