Microwave assisted continuous-flow conditions

Shore and coworkers [64] used a capillary reactor with a Pd thin film and microwave-assisted continuous-flow conditions for Suzttki-Miyara and Heck coupling reactions. The Pd film was prepared by passing Pd(OAc)2 solution into the 1150 pm eapillary at 150°C resulting in a highly porous catalyst composed of nanometer-size grains. 

Pericas et al. prepared polystyrene-supported chiral phosphinooxazoline (PHOX) ligands having a 1,2,3-triazole tether, which was constructed by a 1,3 dipolar cycloaddition. The catalyst (50) gave allylamine from racemic allyl acetate in high yield with excellent enantioselectivity under microwave-assisted continuous-flow conditions (Scheme 7.37) [141]. Although the enantioselectivity was not changed, the catalytic activity of the polymer catalyst was decreased after 3 h. 

Glasnov, T. N. and Kappe, C.O. (2007). Microwave-Assisted Synthesis under Continuous-Flow Conditions. Macrorwo/. Rqp/JCoOTOTMW., 28, 395-410. 

The microwave-assisted Biginelli reaction was also optimized by the same authors under continuous flow conditions [90]. An equimolar reaction mixture of benzaldehyde, ethyl acetoacetate and urea was injected at a flow rate of 2 mL/min at a ceiling temperature of 120°C providing 52% of the desired DHPM 41 in 13 min of total processing time (5 min residence time in the cell). This flow rate allows the preparation of compound 41 on a 25 g/h scale (Scheme 30). 

Should the economics of the above-described new process favor a switch from the present AIC process (see Case Study 2) to one based on dihydrohypoxanthine new investment might be justified in new production equipment.81 A switch to the microwave option versus the conventional heating option would depend on the outcome of process optimization work on both. If a rapid clean reaction in near-quantitative yield were realized by optimizing the microwave conditions, then it would prove worthwhile to engineer a simple continuous flow reactor for further development work. The case in favor of adopting the microwave-assisted process would be further enhanced if the solution from the microwave reactor could be used directly in the next process step (thus eliminating the need for isolation equipment). 

Microwave-assisted esterification by a heterogeneous acid catalyst has been studied in a low dielectric constant medium (see Scheme 35) [64]. A continuous-flow setup has been devised in the system and the heterogeneous acid catalyst (Amberlyst A15 sulphonic acid cation-exchange resin) 61 localized in a polyethylene active flow cell. Use of a low dielectric constant medium (hexane) ensured absorption of microwave radiation only to the reacting species. In this case, the findings suggest a comparable esterification reaction under both microwave and thermal conditions. Furthermore, the presence of water in the catalytic resin resulted in a reduction of the reaction rate irrespective of the type... 

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