Microwave-specific effect on enantioselective reactions

Microwave irradiation is widely applied to organic synthesis. Many advantages of microwave irradiation, including drastic shortening of reaction time, suppression of side reactions, and/or higher chemical yield, have been proposed. Generally, these effects have been understood merely as a simple thermal effect, since microwave irradiation rapidly raises the temperature of the reaction mixture. However, some observed results could not be understood only by the thermal effect. Recently our group reported that several enantioselective reactions were accelerated under microwave irradiation without loss of enantioselectivity. As these extraordinary results could not be explained by the simple thermal effect, the concept of the "microwave-specific effect" should be considered. We describe our recent experimental results to elucidate the microwave-specific effect. Investigations are described on enantioselective ring-opening reactions of biaryl lactone derivatives, racemization of an optically pure biaryl lactone derivative, enantioselective Claisen rearrangement reaction, enantioselective Conia-ene reaction and ring-closing metathesis reaction. These reactions or racemization of optically pure compounds were accelerated under microwave conditions. Such reaction acceleration of enantioselective reactions was observed without loss of enantioselectivity.