Asymmetric Ring-Closing Metathesis with a Twist

摘要

Chiral,metal-based catalysts are an integral part of modern asymmetric synthesis.Normally,enantioselective transition-metal catalysis hinges upon the deliberate combination of optically active ligands and metal cations.Conversely,stereo-defined transition-metal complexes that contain only achiral ligands and thus display chirality exclusively at the metal center are relatively scarce,and their infrequent use in asymmetric catalysis is still perceived as a curiosity.Any stereoinduction arising from a conventional chiral complex is intuitively ascribed to the chiral ligands,and latent stereochemical information at the metal atom is often ignored.This situation is not as uncommon as one might think,in particular with non-C2-symmetric bidentate ligands.In fact,this additional stereogenic element might be formed during or even exist throughout a catalytic cycle;the catalytically active intermediate might then be one of several diastereomers.