Asymmetric Synthesis of Heterobimetallic Planar Chiral Ferrocene Pallada-/Platinacycles and Their Application to Enantioselective Aza-Claisen Rearrangements

Ligand exchange reactions are usually slower for Pt(II) in comparison to Pd(II) centers. Mainly for that reason Pt(II) catalysts have often shown a reduced catalytic activity as compared to their Pd(II) counterparts. We are interested in the question if this inherently slower ligand exchange might also provide a chance for heterobimetallic catalysts to accomplish an improved catalytic performance with substrates in which the reactive center has a lower binding constant than an additional Lewis basic moiety. For that purpose we have prepared the first diastereo- and enantiomerically pure mixed pallada-/platinacycles based on ferrocene. These complexes have been prepared by sequential direct diastereoselective cycloplatination and cyclopalladation. The investigation of the asymmetric aza-Claisen rearrangement of Z-configured trifluoroacetimidates showed that a heterodinuclear Pt-Pd bis-metallacycle is an excellent catalyst for this reaction type, in general allowing for very high enantioselectivities. Moreover, at a slightly elevated temperature (55 degrees C), the heterodinuclear platina-/palladacycle could in certain cases outperform the corresponding bis-Pd complex, previously known to be the by far most active highly enantioselective catalyst for the rearrangement of Z-configured trifluoroacetimidates. This effect, which might be surprising at first sight due to the low efficiency of other Pt catalysts for aza-Claisen rearrangements, might be explained by an enhanced lifetime of a productive monodentate olefin coordination of the substrate at the Pt center due to slower ligand exchange processes.