Chromates, polymer-supported

The influence of the nature of the polymer support on the oxidation power of complex chromates was investigated by T. Brunelet, C. Jouitteau, G. Gelbard, /. Org. Chem. 1986, 53, 4016-4022. 

Pyridinium and quaternary ammonium resins react with chromium trioxide, producing polymer-supported complex chromates that oxidize alcohols, and provide a very facile work-up.427... 

Higher pH chromate programs (pH 7.0 to 9.0), using chromate (3 to 15 ppm) with P04 (3 to 6 ppm) and/or Zn (0.5 to 2.5 ppm). Phosphonate/polymer support is required, the selection of which permits progressively lower chrome levels and higher cooling water pH. 

Gelbard et al. have prepared a numbor of neutral and acidic supported ammonium chromates. Neutral resins woe found to be generally more effective dian acidic resins. The large differences in leactiviQr between different unbound, soluble alkylammonium chromates (Section 2.7.6.1) are not so pronounced with the polymer-supported oxidants. Some commoits about the reactivity of these supported oxidants woe made (i) quaternary ammonium salts are more reactive than the tertiary pyridinium chromates (ii) the reactivity of a conqilex chromate XCtOr was found to increase as the basicity of X decreases. Best... 

Fortunately the same authors report a far more pleasant and less hazardous procedure utilizing a polymer-supported hydrogen chromate ion (equation 40). The yields are all in excess of 95%, and the experimental simplicity is commendable. 

Figure 3.13 Effect of diethylaluminum ethoxide to triethylaluminum ratio on polymer Melt Index (12.16) for the Bis(triphenylsUyl) chromate catalyst supported on silica. The aluminum alkyl mixture is added after the deposition of the chromate compound onto the silica. Reprinted from [17] with permission from John WUey and Sons.

Two papers this year have described the use of chromate ions for the conversion of alkyl halides or alcohols into aldehydes or ketones (Equation 4). The reaction can be conducted in HMPA in the presence of a crown ether, and although yields are good for allylic and benzylic halides (ca. 80%) they are not so high for saturated halides. " The chromate ion, however, can be supported on an insoluble polymer matrix as the tetra-alkylammonium salt and this both enhances the nucleophilicity of the ion and simplifies the work up procedure. Using the polymer-supported reagent a variety of primary and secondary alcohols were oxidized to the corresponding aldehyde or ketone in excellent yield (ca. 90%). ... 

Garrick WL, Turbett RJ, Karol FJ, et al Ethylene polymerization with supported bis(triphenylsilyl) chromate catalysts, /Polym Sci A Polym Chem 10(9) 2609—2620, 1972. 

The absence of water thus avoids the two problems of hydrolysis of the surface chromates and water-induced sintering of the support, both of which lead to a catalyst with lower activity. Reoxidation of the chromium after carbon monoxide reduction gives a different active site precursor, which produces a catalyst which gives a broader molecular weight distribution in the resulting polymer. Catalysts pre-reduced with carbon monoxide are more active than those reduced with ethylene in the reactor. 

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