Applications of polymer-bound reagents

Support-bound transition metal complexes have mainly been prepared as insoluble catalysts. Table 4.1 lists representative examples of such polymer-bound complexes. Polystyrene-bound molybdenum carbonyl complexes have been prepared for the study of ligand substitution reactions and oxidative eliminations [51], Moreover, well-defined molybdenum, rhodium, and iridium phosphine complexes have been prepared on copolymers of PEG and silica [52]. Several reviews have covered the preparation and application of support-bound reagents, including transition metal complexes [53-59]. Examples of the preparation and uses of organomercury and organo-zinc compounds are discussed in Section 4.1. 

In 2002, Ley reported the application of resin-bound reagents and polymers towards the synthesis of carpanone [57]. In the final steps towards carpanone, a resin-bound Co(salen) catalyst was used to give the desired intermediate along with the formation of a small amount of aldehyde by-product. To remove this byproduct, a resin-bound tris-amine scavenger was used, yielding the desired product in high purity (Scheme 8.42). 

In a detailed investigation, Turner and coworkers have described the preparation and application of solid-supported cyclohexane-1,3-dione as a so-called capture and release reagent for amide synthesis, as well as its use as a novel scavenger resin [125]. Their report included a three-step synthesis of polymer-bound cyclohexane-1,3-dione (CHD resin, Scheme 7.104) from inexpensive and readily available starting materials. The key step in this reaction was microwave-assisted complete hydrolysis of 3-methoxy-cyclohexen-l-one resin to the desired CHD resin. 

Cohen, B. J. Karoly-Hafeli, H. Patchomik, A. Active Ester of Polymer-Bound 4-Hydroxy-3-nilrobenzophenone as Useful Acylating Reagents. Application to Peptide Synthesis, J. Org. Chem. 1984,49,922. 

The applications reported for polymer-supported, soluble oxidation catalysts are the use of poly(vinylbenzyl)trimethylammonium chloride for the autooxidation of 2,6-di-tert-butylphenol [8], of copper polyaniline nanocomposites for the Wacker oxidation reaction [9], of cationic polymers containing cobalt(II) phthalocyanate for the autooxidation of 2-mercaptoethanol [10] and oxidation of olefins [11], of polymer-bound phthalocyanines for oxidative decomposition of polychlorophenols [12], and of a norbornene-based polymer with polymer-fixed manganese(IV) complexes for the catalytic oxidation of alkanes [13], Noncatalytic processes can also be found, such as the use of soluble polystyrene-based sulfoxide reagents for Swern oxidation [14], The reactions listed above will be described in more detail in the following paragraphs. 

It appears unlikely that all organic reactions can benefit from the use of polymeric reagents or catalysts. Nevertheless, such reagents and catalyst do appear to be promising in many applications. The fact that polymer-bound reagents and catalyst are more expensive, however, precludes wide industrial applications, except in special situations. 

Although the iodoarene can often be recovered and used in the synthesis of At2IX [53], this process could be facilitated by the use of solid supported diaryliodonium salts. Polymer-bound reagents have, however, met with limited success in applications, possibly because of chemoselectivity problems resulting in the nucleophiles being transferred to the polymer backbone [42,54], Ionic liquid-supported Ar2lX were recently synthesized and applied in chemoselective O-arylations without the need for chromatography [55]. 

No single physical method can find general application for the analysis of all types of resin-bound reagents, and the detailed description of each of the methods is beyond the scope of this book. These methods are quick and nondestructive and hence preferred over chemical methods. With suitable modifications, many physical methods can be adopted for the analysis of functional groups in solid, insoluble polymers. Crowley and Rapoport (1976) have discussed the possibliities of using physical methods of analysis in solid-phase synthesis. Some of the specific applications of such methods are now briefly dealt with. 

In these cases, the polymer was used as an asymmetric support to induce the formation of optically pure product (cf. Worster et al., 1979). Few reports of the use of polymer-bound asymmetric reagents seem to exist in the literature. In this application, the reagent is used either to promote the asymmetric coupling of two groups or to add a group to a compound in an asymmetric manner. By far the largest number of applications have been those in which the polymer-bound asymmetric centers act as catalysts. Asymmetric catalysts, based on either amino acids or cinchona alkaloids, have been used to catalyze the Michael reaction in an... 

Crosl977 Crosby, G.A. and Kato, M., The Utilization of a Polymeric Phenylthiomethyllithium Reagent for the Homologation of Alkyl Iodides and Its Application for the Study of Intraresin Reactions of Polymer-Bound Functional Groups, J. Am. Chem. Soc., 99 (1977) 278-280. 

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