Unusual Deactivation in the Asymmetric Hydrogenation of Itaconic Acid

During the asymmetric hydrogenation of itaconic acid with rhodium solvate complexes of the type [Rh(PP)(MeOH)(2)] BF4 (PP = DIPAMP, MeDuPHOS) a deactivation with increasing substrate concentration is observed. It is shown that this inhibition phenomenon is due to the in situ formation of an inactive rhodium(III)-alkyl complex. Two crystal structures of single crystals of the responsible complexes (1) and (2) support the deactivation pathway.