Reactant Sequestration

Several resins have been used frequently in reactant sequestration. Ami-nomethylpolystyrene 1 and the more highly functionalized polyamine resins 2 and 3 have been reported to sequester excesses of solution-phase electrophiles, including isocyanates, isothiocyanates, sulfonyl chlorides, acid chlorides, anhydrides, aldehydes, and imines. Cross-site reactivity is not an issue with the more densely functionalized sequestering resins so their use in an automated laboratory environment offers a significant resin and volume economy compared to less densely functionalized resins. 

Appropriately functionalized resins can sequester excess nucleophiles from solution-phase reactions. Thus the calcium sulfonate resin 4 captures tetra-n-butylammonium fluoride (TBAF) from a variety of desilylation reactions.22,24 Polymer-bound tetra-n-butylammonium sulfonate and insoluble calcium fluoride are formed. The applicability of this strategy was illustrated for deprotection of (3-trimethylsilylethyl esters as well as silyl ethers. 

A quaternary ammonium hydroxide ion exchange resin 6 was shown to sequester phenols, hydroxypyrazoles, and other weakly acidic heterocycles.25 The sequestered nucleophiles could also be used as polymer-supported reactants. Similarly, the guanidine-functionalized resin 7 was also shown to be a useful capture agent for weakly acidic nucleophiles, including phenols and cyclic iV-acyl sulfonamides.26 

A methyl isocyanate-functionalized resin 8 has been used to sequester excesses of amines or hydrazines from solution phase when these reactants were used in urea-forming reactions or pyrazole-forming reactions.19,21 Finally, the a-bromoketone resin 9 was shown to efficiently sequester thioureas from solution in Hantzsch aminothiazole-forming reactions.27 

Resin capture can be faster and more efficient than classical methods of purification (e.g., chromatography). Chemoselective sequestration requires minimal amounts of solvent for separating reactants from solution-phase products. Gradient elution techniques, common in chromatographic separations, are avoided, saving time and solvent. Additionally, concurrent use 

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Many reactions catalyzed by the addition of simple metal ions involve chelation of the metal. The familiar autocatalysis of the oxidation of oxalate by permanganate results from the chelation of the oxalate and Mn (III) from the permanganate. Oxidation of ascorbic acid [50-81-7] C HgO, is catalyzed by copper (12). The stabilization of preparations containing ascorbic acid by the addition of a chelant appears to be negative catalysis of the oxidation but results from the sequestration of the copper. Many such inhibitions are the result of sequestration. Catalysis by chelation of metal ions with a reactant is usually accomphshed by polarization of the molecule, faciUtation of electron transfer by the metal, or orientation of reactants. 

So far, we have considered only the purification methods for the rapid clean up of reaction mixtures that are facilitated by sequestration of either by-products, excess reactants or spent reagents. The idea that one can used a suitably functionalized solid support to selectively capture the required product away from any contaminating impurities, filter and then re-release it in a pure form is also an important purification concept (Fig. 2.3) [31]. 

Direct sequestration of a reactant by an insoluble resin is impractical if the kinetics is sluggish and impossible if the solution-phase reactant does not contain a functionality to enable direct sequestration. These limitations led several research groups to use bifunctional solution-phase linking reagents, also referred to as sequestration-enabling-reagents. 33... 

Poorly reactive (poorly sequestrable) byproducts are more frequently encountered than poorly sequestrable reactants. A few reports have appeared describing the use of soluble bifunctional linking reagents to chemically tag... 

Carbon dioxide is considered to be an interesting alternative to most traditional solvents [17, 18] because of its practical physical and chemical properties it is a solvent for monomers and a non-solvent for polymers, which allows for easy separation. To a somewhat lesser extent, it can also be a sustainable source of carbon [19]. The use of CO2 as a reactant is considered to contribute to the solution of the depletion of fossil fuels and the sequestration of the greenhouse gas CO2. One example in this area is the copolymerization of carbon dioxide with oxiranes to aliphatic polycarbonates [19-22]. 

At the present time, this type of regulation is not clearly understood and must be examined in the future. In this way, the special role of the large vacuole, which contains high concentrations of reactants has to be studied this compartment may play a central role not only in the sequestration of metabolites and inhibitors, but, also in supplying substrates and cofactors. 

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