Palladacycle-Catalyzed Highly Enantioselective Arylation of Sulfonylimines

Key words

1,2-addition - imines - amines - palladium

Significance

The rhodium- or palladium-catalyzed 1,2-addition of arylboron reagents to activated imines is a useful reaction for the synthesis of chiral amines. The palladium-catalyzed version is often affected by diminished yields due to competitive hydrolysis of the starting imine and/or ­homocoupling of the boronic acid. Peters and Schrapel address these issues with an efficient protocol using a chiral palladacycle catalyst that was developed in their group.

Comment

Virtually perfect enantioselectivity and high yields are achieved with a range of imine/boroxine combinations, with electron-poor boroxines giving lower yields. Boronic acids were found to be more reactive than boroxines, but the latter were chosen for this study to enable a reproducible protocol. The authors propose that an anionic acetate ligand on palladium facilitates transmetalation with the boroxine. The use of less coordinative anions (OTf^–, TFA^–) led to low or no reactivity. Preliminary results with cyclic ketimines show that the formation of a quaternary center is also possible.