Figure 33 Cartoon of “catch and release” purification; a cationic product is captured on an anionic ion exchange resin. The neutral and anionic impurities are washed away, and the pure product is eluted from the resin.

Most reactivity-based scavengers are functionalized polystyrene resins. Nucleophilic impurities such as amines or thiols can effectively be removed by the use of electrophilic resins. Some examples of these reagents include polymer-bound isocyanates, sulfonyl chlorides, anhydrides, aldehydes, and chloroformates. Excess electrophiles, such as acid chlorides, isocyanates, aldehydes, alkyl halides, α, β-unsaturated carbonyl compounds, and sulfonyl chlorides, can be sequestered using polymer-bound nucleophiles such as amines and thiols [122-124]. Examples of several different classes of reactivity-based scavengers are shown (being used for purification of products) in Figure 34 [122-126]. The large number of different reagents available for these purification techniques allows “tuning” of the reactivity of a scavenger. For example, a thiol can be removed from a solution containing an amine by use of an a-iodoamide resin as an electrophile. This reagent will selectively react with the more nucleophilic thiol over the desired aminecontaining product [125]. The loading levels of polymer-bound scavengers are important; the swelling of resins requires the use of large solvent volumes, such that higher resin loading levels of scavenger are desirable to minimize dilution of the reaction mixture and amount of resin required. The following section highlights some more advanced scavengers that have been developed to remove specific impurities and demonstrate the power of this technique.