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 °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.