Scavenging and purification

Isolation of a clean and homogeneous product is an integral part of any synthesis, and microwave heating has also been instrumental in improving many scavenging and purification techniques utilizing functionalized polymers. 

For all LP steam operations in which the total hardness of RW exceeds, say, 10 to 20 ppm CaC03, the use of a softener is preferable to control solely via internal precipitation treatment. Oxygen scavenging and other internal treatment programs should also be provided. Some MPHW/HPHW and lower pressure steam boiler designs may require additional purification beyond simple water softening. 

Unfortunately, many reactions do not occur with quantitative conversion and in near absolute purity. The work-up and purification of most chemical processes probably takes up most of a bench chemists time. Therefore techniques that simplify and accelerate these operations not only free up valuable time, but allow greater creativity and increased levels of output. Here again, supported systems can be used to aid the chemist in the guise of scavengers, quenching agents and catch and release systems. 

Following a similar strategy, an ingenious mixed resin bed quench and purification strategy was devised for the Dess-Martin periodinane mediated conversion of alcohols to carbonyls. This hypervalent iodine oxidant was viewed as containing an inherent masked carboxylic acid functionality that was revealed at the end of the reaction (Species (11) Scheme 2.30). Therefore purification was easily achieved by treatment of the reaction mixture with a mixed-resin bed containing both a thiosulfate resin and a polymeric base. The thiosulfate polymer was used to reduce excess hypervalent iodine lodine(V) and (III) oxidation states species to 2-iodoben-zoic acid (11), which was in turn scavenged by the polymeric base [51]. 

Scheme 2.31 Deprotection and purification strategy based on scavenger technology ...

A one pot formation and purification of a 5-arylidine 4-thiazolidinone library has also been reported using polymer scavenging as the principle method of purification. An automated synthesizer was employed to make a parallel array of 4080 4-thiazolidinones, prepared simultaneously from a 3-component condensation of mercaptoacetic acid with an amine and a carbonyl compound. Further structural decoration was then introduced using the libraries from libraries principle where the core template was derivatized via an aldol reaction with a second carbonyl unit at the 5-methylene position (Scheme 2.57) [84]. After both synthetic steps. 

The high-throughput purification in a discovery environment and the removal of transition metals using adsorption on or crystallization in the presence of activated carbon, glass-bead sponges, polymeric fibers, or silica-bound scavengers and the preparative isolation of radiolabeled compounds are out of the scope of this contribution. 

Use of a readily separable, polymer-supported reagent to facilitate a chemical transformation is not always a panacea. Yields should preferably be high, and the recovery and purification of the desired product from the reaction mixture should be practical. Polymer-supported scavenging agents to scavenge byproducts have been employed, as also have polymer-supported reagents that capture the desired product from a reaction mixture.64... 

PASP synthesis can be divided into two main approaches these being (a) the use of supported reagents and scavengers, and (b) the adoption of a catch-and-release strategy (Figure 1). Both of these techniques allow the production of clean products, without the need to resort to traditional purification techniques, such as column chromatography. In an ideal case, the incubation of a substrate with a supported reagent causes its complete transformation into a new chemical entity, with any... 

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