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.

Cooperative asymmetric catalysts often offer advantages in terms of activity, stereoselectivity, and generality as compared to more traditional single point activation catalysts. In cooperative bimetallic catalysis, the intermetallic distance is a crucial parameter for the outcome of a reaction and an optimal synergy of both metal centers. We have recently developed a number of catalytic asymmetric reactions, which are efficiently catalyzed by a planar chiral ferrocene based bispalladacycle and for which the cooperativity of two Pd centers has already been demonstrated. To get more insight into the role of the Pd/Pd distance in such metallocene bismetallacycles, in the present study a corresponding ruthenocene based Pd2-complex has been prepared by the first direct diastereoselective biscyclopalladation of a chiral ruthenocene ligand. In addition, the first highly diastereoselective direct monocyclopalladation of a homochiral ruthenocene is reported. The effect of the increased Cp/Cp distance within the ruthenocene bispalladacycle has been examined in four catalytic asymmetric applications: the aza-Claisen rearrangement of Z-configured allylic N-aryltrifluoroacetimidates, the direct 1,4-addition of α-cyanoacetates to enones, a tandem azlactone formation/1,4-addition to enones and a tandem reaction to form quaternary α-aminosuccinimides by in situ azlactone formation, 1,4-addition to a nitroolefin, and a Nef-type nitro-to-carbonyl transformation as key steps. For each reaction studied, it was found that with some substrates the ferrocene based catalyst is superior, whereas for other substrates the ruthenocene backbone is more favorable. The ruthenocene based bispalladacycle can thus be considered to be a useful and complementary alternative for cooperative bimetallic catalysis.

Transition-metal-catalyzed cross-coupling reactions between sp 2 -hybridized C atoms are of prime importance in both target and diversity oriented synthesis. Ideal cross-coupling reactions would neither require any leaving groups nor stoichiometric reagents. In this article, we report the first direct dehydrogenative crosscouplings between aromatic C−H bonds (in most cases using indole substrates) and allylic alcohols, which do not require an additional classical stoichiometric oxidizing agent and provide β-arylketones as value-added products. Ruthenocene-or ferrocene-based bismetallacycles, in which either Pd(II) or Pt(II) are the catalytically active centers, were found to be particularly efficient catalysts. Control experiments suggest that the bismetallacycles initially transform the allylic alcohols into vinylketones, which then alkylate the aromatic substrate in the presence of the catalyst. The fact that the dehydrogenative coupling does not require a classical stoichiometric oxidizing agent is explained either by protonolysis of a metallacyclic M(II)-H intermediate or by a mechanism in which an excess of the allylic alcohol substrate serves as a sacrificial hydrogen acceptor. The title reaction is supported by cocatalytic amounts of Ni(OAc) 2 . In preliminary studies, it was observed that the title reaction can as well be applied to prochiral CH-acidic pronucleophiles such as α-cyanoacetates, representing the first examples for direct enantioselective β-ketoalkylations via allylic alcohols in the absence of an additional oxidant.

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Allylic substitution reactions provide a valuable tool for the functionalization of CH acidic pronucleophiles. Often, control over the stereocenter generated at the nucleophilic reactant is still a challenge. The majority of studies that address this issue employ metal complexes with a low metal oxidation state (e.g. Pd0) to form allyl complexes through oxidative addition. In this article we describe the use of heterobimetallic PtII/PdII complexes, which probably activate the olefinic substrates through an SN2′ pathway. The reaction of α‐cyanoacetates delivers linear allylation products with exclusive regioselectivity and high E/Z‐selectivity for the new C=C double bond. Although the enantioselectivities attained are moderate, they are significantly higher than with related mono‐PdII or ‐PtII catalysts or the corresponding bis‐PdII complex, which indicates cooperation of the different metals. Control experiments suggest simultaneous activation of both reaction partners.