| Summary: | 碩士 === 國立中興大學 === 化學系 === 87 === We report herein the remarkable accelerating effect by microwave on two organic reactions which require much reduced reaction time than the conventional ones while maintaining comparable reaction yields.
First, a simple, fast and high yield method for the formation of carbamates via hydrogen chloride catalyzed reaction of sterically demanding isocyanates and alcohols under microwave irradiation has been established. The reaction of sterically hindered alcohols and isocyanates proceeded to completion in the presence of microwave in a few minutes to provide the desired carbamates in good yields as compared to 5 hours at room temperature for conventional stirred reaction. For example, merely 3~6 minutes were required when the reaction was carried out using 70 W (Low power output) in a domestic microwave oven to give 62~99% yield of the carbamates. When the reaction was irradiated in a focused, monomode microwave reactor (Synthewave S402) at 10% power level (30 W), only 3~5 minutes were necessary to furnish the desired product in 31~96% yield.
Second, microwave assisted cleavage of 1,3-diketones under alkaline conditions dramatically decreased the reaction time of a wide range of cyclic and acyclic b-diketones utilizing several different hydroxides in water. The reactions carried out under microwave irradiation, in general, afforded better yields than those under classical refluxed conditions.
Thus, under conventional heating, 1~72 hours were required to obtain 67~98% yield of the cleaved products. Irradiation at 520 W (Medium High power output) in a domestic microwave oven for 0.5~20 minutes produced the products in 16~98% yield. When performed at 100 °C, 4~120 minutes were needed to furnish 20~100% yield of the cleavage products in a focused microwave reactor.
To develop more environmentally friendly reaction conditions, the above reactions also have been conducted under solvent-free conditions (neat or on a solid support) in a focused microwave reactor. Unfortunately, inferior yields than those in solution were obtained instead.
In conclusion, microwave irradiation has been proved to be an effective method in accelerating organic reactions which demonstrates immense potential in industrial applications and therefore warrants further investigations.
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