Abstract
New practical catalysts have been developed for asymmetric cyclopropanation of 2, 5-dimethyl-2, 4-hexadiene with tert-butyl diazoacetate. First, the effects of the substituents on the salicylaldehyde moiety in the copper Schiff-base complex on the catalytic activity and the stereoselectivities were investigated. As a result, a substitution at the 5-position of hydrogen with the nitro-group on the salicylaldehyde moiety was found to enhance the catalytic efficiency. In addition, a combination catalyst of the copper Schiff-base complex with a Lewis acid was found to enhance the catalytic efficiency and achieved 90% chemical yield and 91% ee at 20°C with 0.1 mol% catalyst loading. Then, new cationic bisoxazoline-copper complex catalysts with PF6-as the counter ion were demonstrated to achieve the highest stereoselectivities (trans/cis = 88/12, 96% ee at 0°C) with tert-butyl diazoacetate. Furthermore, the asymmetric induction mechanism of the cyclopropanation reaction catalyzed by each complex was studied with density functional calculations.
