High-Speed Couplings and Cleavages in Microwave-Heated, Solid-Phase Reactions at High Temperatures

A rapid and efficient procedure of flash heating by microwave irradiation is described for the attachment of aromatic and aliphatic carboxylic acids to chloromethylated polystyrene resins via their cesium salts. Significant rate accelerations and higher loadings are observed when the microwave-assisted protocol is compared with the conventional thermal method. Reaction times are reduced from 12−48 h with conventional heating at 80 °C to 5−15 min with microwave flash heating at temperatures up to 200 °C. All transformations were carried out in a purpose-built microwave reactor with on-line temperature, pressure, and microwave power control. Kinetic comparison studies have shown that the observed rate enhancements can be attributed to the rapid direct heating of the solvent (NMP) by microwaves rather than to a specific nonthermal microwave effect. Furthermore, a microwave-assisted cleavage that allows acidolysis of the Merrifield linker with standard trifluoroacetic acid reagents has been developed.