Microwave reactions with polymer-supported reagents

Microwave reactions with polymer-supported reagents 

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Microwave reactions with polymer-supported reagents... 

Scheme 1.9 Typical examples of microwave-assisted reactions with polymer-supported reagents [14].

Oscarson et al. [5] described the hydrolysis of compound 20 to 9 (89%) by treatment with KIO clay in methanol-water for 2 min under the action of microwave irradiation at 150 °C. Because polymer-supported reagents worked out well in acylation reactions, they also submitted 20 to microwave irradiation with polymer supported poly(4-vinylpyridinium-p-toluenesulfonate) (PPTS) in ethanol-water at 150 °C for 2 min and obtained 9 in 94% yield (Scheme 12.19). 

Microwave chemistry is beginning to play a greater role also in solid-phase chemistry and in cases where polymer-supported reagents are utilized. The main drawback with these methodologies concerns the slow kinetics, which makes reaction scouting and optimization tedious. An impressive example of a microwave accelerated and polymer assisted process was recently described by Linclau. In this report, a set of carboxylic acids were fully 0-alkylated after only 3-5 min of microwave heating (Reaction Scheme 4). 

As an alternative approach, instead of performing the process with educts coupled to the support, polymer-supported reagents (PSRs) can be brought into contact with the educt dissolved in an appropriate solvent. In this case, the polymer support can be easily isolated from the product solution by filtration. Moreover, it is possible to apply a surplus of the PSR, and this frequently leads to higher product yields. Microwave heating has also been used to improve the PSR method, and some typical microwave-assisted reactions are shown in Scheme 1.9. More detailed information on this topic, and some appropriate references, are provided in Ref [14]. 

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