Scavengers excess reagent

FIGURE 1.57 Scavenging of excess reagent with scavenger resin. (Reproduced with permission from Biotage AB.)... 

Similar to anhydride-bearing polymers, the synthesis of a poly(norborn-2-en-5-yl-methanol)-derived system, polymerized by a Gmbbs catalyst after scavenging excess of reagent, was reported by Hanson (Tab. 8.3) [42, 43]. 

Solution-phase synthesis [5] often needs purification or clean-up procedures after each reaction step to remove excess reagent. These methods include scavenging, extractions and associated plate transfers. All these procedures cause the loss of the desired compound. Although the purity can be improved after treatment, the chemical yield is seriously compromised. In contrast, SPOS has a unique advantage in purifying bound compound without losing compound mass. However, if the reaction is not complete at each step, the side products will form on resin and they cannot be removed while bound to the resin. The final yield and purity wiU both suffer as a result. A 90% yield for a four-step synthesis wiU produce the final product in a disappointing 65% yield. 

Combination of amines and dendrimers functionalized proton scavenger resins efficiently removes excess reagents from amide-formation reactions. Marsh et al.41 used a supported branched polyamine and... 

Fig. 1. General approaches to the removal of excess reagents using nucleophilic and electrophilic scavenger resins.

Polymer-bound scavengers are functionalized polymers that are designed to react with and bind excess reagents and by-products, facilitating their removal by simple filtration. The desired product remains in solution. 

Fluorous scavengers facilitate removal of excess reagents from complex reaction mixtures [28] (Scheme 3.25). Thus, the fluorous anhydrides 55 and isocyanates 56 enable the removal of excess amine reagents. Several custom-tailored fluorous scavengers with complementary reactivity are available for removal of other reactive species. 

Scheme 3.25 Examples of fluorous scavengers enabling simple removal of excess reagents from complex reaction mixtures [28].

Purification of products or intermediates by liquid-phase extraction has been variously reported. Examples are given in Table 3.14. The scope of liquid-phase extraction may be extended by adding highly polar scavengers before work-up. These react with excess reagents of low polarity to form products that are highly soluble in water and can easily be extracted by aqueous wash (see footnotes in Table 3.14). 

Sohd-phase scavengers can also be used both for removal of excess reagents and trace impurities, such as metal residues, for example palladium from cross-coupling reactions. A range of resins is available to remove a variety of metal residues, usually involving binding to a thiol or amino group. 

Figure 8.1 Use of scavengers to bind excess reagent/ byproduct, eliminating the need for chromatography.

---