Practical copper-catalyzed asymmetric synthesis of chiral chrysanthemic acid esters

Practical copper salicylaldimine complex catalysts have been developed for the asymmetric synthesis of chiral chrysanthemic acid esters by the cyclopropanation reaction of 2,5-dimethyl-2,4-hexadiene with tert-butyl diazoacetate. First, the effects of the substituents on the salicylaldehyde moiety in the copper salicylaldimine complex (copper Schiff base complex) on the catalytic activity and the stereoselectivities were investigated. As a result, a substitution of hydrogen at the 5-position with the nitro group on the salicylaldehyde moiety was found to enhance the catalytic efficiency. In addtition, a combination catalyst of the copper Schiff base complex with Lewis acid was found to also enhance the catalytic efficiency and achieved 90% chemical yield and 91% ee at 20 °C with 0.1 mol % catalyst loading. Furthermore, the asymmetric induction mechanism of the cyclopropanation reaction catalyzed by the copper Schiff base complex was studied using density functional calculations.