Enantioselective Cyclopropanation Reactions with Planar-Chiral Pyridinium Ylides: A Substituent Effect and a Remote Steric Effect

Novel planar-chiral pyridinium ylides were designed, and generated in situ from the corresponding pyridinium salts with triethylamine. Ylides with a common parapyridinophane skeleton reacted efficiently with electron-deficient dicyanoalkenes, or malononitriles, to produce optically active cyclopropane derivatives with high enantioselectivity (up to 99% ee). Remote steric effects were observed on the enantioselectivities, where the R-2 groups of the pyridinophane core resulted in higher ee values of the products. Density function theory (DFT) calculations are in good agreement with our experimental results: the energetically favored transition state leads to the major stereoisomer, namely the trans-cyclopropane products.