Chiral Diene Ligands for Catalytic Asymmetric Reactions

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One of the most significant subjects for developing catalytic asymmetric reactions is the design and preparation of a chiral ligand which will fit in with a given reaction efficiently in catalytic activity and enantioselectivity. A number of chiral molecules containing phosphorus and/or nitrogen atoms have been prepared and used for transition metal-catalyzed asymmetric reactions, but they are not always useful as chiral ligands because of their low catalytic activity or enantioselectivity. In this research project, as conceptually new chiral ligands, we have introduced a series of chiral dienes which opened up a new dimension in the field of asymmetric catalysis. The chiral dienes are based on bicyclo[2.2.1]heptadiene (nbd^*), bicyclo[2.2.2]octadiene (bod^*), or bicyclo[3.3.1]nonadiene (bnd^*) skeleton. The chiral diene ligands were found to be very effective for rhodium-catalyzed asymmetric 1,4-addition of arylboron reagents to electron deficient olefins, 1,2-addition to N-sulfonylimines, and so on. The enantioselectivities observed with the chiral diene ligands are very high compared with those observed with other types of chiral ligands such as chiral bisphosphines. It is remarkable that the catalytic activity of the rhodium catalysts coordinated with the diene ligands is much higher than with conventional phosphorus ligands, which made it possible to realize the catalytic asymmetric reactions with a very small amount (<0.005 mol %) of the catalyst.