Cross-linked polymer supports

Thus, isolation of oxoammonium salts on insoluble, cross-linked polymer supports was investigated along with their implementation in polymer-assisted solution-phase synthesis.23 These isolated oxoammonium salts could be employed in a water-free system to generate highly reactive oxidation agents without the overoxidation problems normally seen in the presence of water. 

We envisioned that a cross-linked, polymer-supported dioxaphosphorane i.e., diethoxydiphenylpolystyrylphosphorane DDPP) (25) might allow for expeditious product isolation while simultaneously incorporating e characteristically mild cyclodehydrating properties of DTPP. As a bonus, it seemed reasonable to expect the steric bulk and rigidity of the polymeric backbone to favorably influence the level of regioselective release of the phosphine oxide, and perhaps provide a more efficient method for enhancing the enantio-enrichment within chiral cyclic ethers. 

Chemistry on soluble polymer matrices has recently emerged as a viable alternative to solid-phase organic synthesis (SPOS) involving insoluble cross-linked polymer supports. Separation of the functionalized matrix is achieved by solvent or heat precipitation, membrane filtration, or size-exclusion chromatography. Suitable soluble polymers for liquid phase synthesis should be crystalline at room temperature, with functional groups on terminal ends or side chains, but must not be not cross-linked they are therefore soluble in several organic solvents. 

Polymers as solids are ubiquitous in our modern society. They are some of the most common synthetic materials. Biologically derived macromolecules are also abundant. Whether it is a piece of wood, a natural fiber, or a lobster shell, nature uses solid organic macromolecular materials as key architectural material. This abundance of examples of synthetic and natural solid polymeric materials is mirrored in the prevalence with which insoluble cross-linked polymer supports are used in synthesis and catalysis [23-25]. However, while solid-phase synthesis and related catalysis chemistry most commonly employ cross-linked supports that resemble those originally used by Merrifield [26], the polymers found in nature are neither always insoluble nor always cross-linked. Indeed, soluble polymers are as common materials as their insoluble cross-linked analogs. Moreover, nature quite commonly uses soluble polymers as reagents and catalysts. Thus, it is a bit surprising that synthetic soluble polymers are so little used in chemistry as supports for reagents, substrates, and catalysts. 

Possible alternatives to cross-linked polymer supports are soluble and colloidal polymers. They would require large scale ultrafiltration for industrial use. Although ultrafiltration is not yet economical for desalination of seawater, it might be for a separation of a more expensive product. One example is the catalytic partial hydrogenation of soybean oil (361 with soluble polymer-bound transition metal complexes. Solid inorganic supports such as silica gel and alumina are usually not subject to these physical attrition and filtration problems. 

Itsuno et al. [21] synthesized a cross-linked polymer support with a chiral 1,2-diamine for enantioselective ruthenium transfer hydrogenation catalysis of aromatic ketones. 

Yet, as described in the preceding chapters, the inherent categorical imperative of the method demanding completion and control of all chemical transformations on cross-linked polymer supports did not allow a direct breakthrough to automatic synthesis of real proteins. Though from the point of view of chemical problems there are still shortcomings in this respect, today several mechanical instruments are available on the market to perform manually or program-controlled gel-phase syntheses. 

Highly cross-linked polymer-supported BINAPHOS ligands were effective for the hydroformylation of styrene and other functionalized olefins (ee s up to 89%). Recovery and reuse of the catalyst was possible at low stirring conditions [28]. 

In various cases, Koga s bases 75 and 76 performed better with respect to enantioselectivity [77]. The groups of Williard [78] and Majewski etal. [79] introduced - soluble or cross-linked - polymer-supported lithium bases, as illustrated by the chiral deprotonating agent 77. Aside from lithium amides that were used by far most frequently, chiral magnesium bisamide 74 and polymer-supported regents derived thereof were also applied in enantioseiective deprotonations of prochiral ketones [80]. 

Scheme 13.15 The cross-linked-polymer-supported ionic liquid.

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