Microwave Specific Effect on Catalyticatropo-Enantioselective Ring-Opening Reaction of Biaryl Lactones

Abstract

The microwave specific effect on the catalytic atropoenantioselective ring-opening reaction of biaryl lactones was investigated. Under strictly controlled temperature conditions, the reaction was accelerated by microwave irradiation without any loss of the enantioselectivity. Also, the racemization rate of the atropo-optically active biaryl lactone was enhanced by the microwave irradiation.Significant advances have been made in the field of microwave (MW) chemistry since 1986, when the first report of an organic synthesis using microwaves was published.1 A microwave irradiation instrument for organic synthesis was developed and the applying of microwaves for organic reactions has currently become a very popular method because it has many advantages.2 Among them, the major reason to use a microwave oven is shortening of the reaction time and improving the chemical yields.2 The mechanism is typically explained by a rapid temperature increase (thermal effect).3 However, some phenomena, which cannot be only explained by a simple thermal effect (microwave specific effect), have also been reported and the existence of the microwave specific effect still remains controversial. 4 Current theories on how to rationalize microwave specific effects were recently advanced and reviewed. Recently, we reported the atropo-enantioselective ring-opening reaction of biaryl lactones with dynamic-kinetic resolution 5 catalyzed by AgBF 4 phosphine complexes. 6 The ring-opening products were obtained in good-to-high yields with good-to-high enantioselectivities, although a relatively long reaction time was required. Thus, in order to shorten the reaction time, microwave irradiation was applied to the methanolysis reaction. The completed reaction time by microwave irradiation was compared to that during conventional heating (as conventional heating conditions, the heat block (HB) was used). As a result, the reaction was clearly accelerated by the microwave irradiation about 2.7 times (90 min under microwave conditions versus 240 min under heat block conditions), and surprisingly, the enantioselectivity for the microwave irradiation reaction was almost the same as that under the conventional heating conditions (Scheme 1). For organic syntheses, the enantioselectivity generally decreased at a higher reaction temperature, though the reaction was accelerated. This is explained by the van't Hoff equation of enantioselectivity, which is derived from the Arrhenius equation and the definition of ee.This equation indicates that the enantioselectivity is expressed as a function of the reaction temperature. The reaction rate normally becomes about double for every 10°C temperature increase. However, if the temperature was increased 14.3°C from 21.9°C under conventional heating conditions to accelerate the reaction by about 2.7 times as obtained under microwave irradiation (Scheme 1), the enantioselectivity should decrease to 65.4% ee based on the van't Hoff plot of this reaction. Therefore, this result suggests that the reaction enhanceme...

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