Asymmetric catalysis. Part 128: Diastereomeric rhodium(I) complexes in the enantioselective hydrogenation of ketopantolactone

Abstract

In the hydrogenation of ketopantolactone, new rhodium complexes bearing (R,R)-diop and various bidentate chiral N,N′ co-ligands with (R)- or (S)-configuration were used. On the one hand, the N,N′ co-ligands consist of pyrroleimines, which derive from (R)- and (S)-1-phenylethylamine, (R)- and (S)-1-cyclohexylethylamine (and benzylamine), and on the other hand of pyrroleoxazolines and pyridineimines. Stereoselectivities of 31–33% ee for (R)-pantolactone were achieved using related compounds (RR-R) and (RR-S), respectively, with no double stereoselectivity being observed. It is assumed that during catalysis the pyrroleimines bind in a monodentate way at the sixth coordination site of the rhodium atom by the pyrrole nitrogen with the imine nitrogen carrying the different chiral substituents far away from the rhodium atom. Monodentate deltacyclane phosphanes, chloro ligands or solvent molecules, bound at the sixth coordination site of the catalyst, led to widely differing enantioselectivities in the ketopantolactone hydrogenation.