2012 Volume 70 Issue 12 Pages 1281-1294
In asymmetric catalysis, many chiral ligands, atropisomeric (atropos) ones in particular, have so far been reported to attain excellent results. The word atropos consists of “a” meaning “not” and “tropos” meaning “turn” in Greek, and thus the dynamic behavior of a ligand with chirality (i.e. axis, planar, center, and helicity) can be referred to as tropos. In this paper, an advanced strategy for asymmetric activation using racemic catalysts is highlighted by the introduction of tropos ligands. The combination of a tropos ligand, a chiral activator (a chiral controller, in part), and a metal atom can, in principle, forms a single diastereomer. Through the asymmetric activation, the catalytic activity can be increased and the chiral environment of the catalyst obtained can be amplified for effective asymmetric catalysis. Additionally, the atropos enantiopure catalysts can also be isolated after the dissociation of chiral activators (referred to as controllers in this case) from the catalyst. Based on this concept, we have developed asymmetric catalysis based on tropos ligands, especially BIPHEPs.
