Experimental Investigation of Microwave-Specific Effect on Nazarov Cyclization

Abstract

Abstract A copper-catalyzed Nazarov cyclization using an aryl vinyl ketone derivative containing a 1,3-dicarbonyl moiety was carried out under microwave irradiation conditions. The Nazarov cyclization was dramatically accelerated and the kinetic rate of the microwave irradiation was 5.8 times faster than that by conventional heating conditions. A theoretical interpretation of the microwave-specific effect as physical phenomena of the molecules based on chemical kinetics and dynamics was attempted. The microwave-specific effect in terms of the pre-exponential factor in the Arrhenius equation is discussed, thus a hypothesis of the conformational equilibrium activation by microwaves is theoretically supported. To further investigate the microwave-specific effect, a catalytic asymmetric Nazarov cyclization was finally performed. As the enantioselectivity is defined as the function of the reaction temperature, observation of the same enantioselectivity implies the same reaction temperature. Interestingly, when the asymmetric Nazarov cyclization was conducted using a chiral copper catalyst, the Nazarov reaction was subsequently enhanced by the microwave irradiation without any loss of the enantioselectivity. It is suggested that the drastic enhancement with the retention of the enantioselectivity was caused not by a thermal effect, but by a microwave-specific effect on the enantioselective reaction.