salen ligands and N-heterocyclic carbenes (NHC) are among the most abundant structural ligand motifs for metal catalysis. In this article we present a modular approach to chiral ligands merging both ligand motifs for the preparation of bimetallic catalysts, in which one metal (M1) is coordinated by the salen moiety and the other metal (M2) binds to two NHCs. After selective complexation of M1 = PdII, NiII ion into the salen N2O2 coordination site, heterobimetallic M1/Ag(I) complexes were synthesized which could be further utilized for the preparation of homo- and heterobimetallic M1/PdII complexes by an oxidative transmetalation to Pd(0) as a key step of the catalyst synthesis. The structures of the bimetallic complexes and the intermetallic distances strongly depend on the counterions of M2 = PdII, as revealed by X-ray and UV–vis studies. In the absence of an anionic ligand with suitable Lewis basicity the salen oxygen atoms serve as bridging ligands for both metals. A preliminary investigation into catalysis showed that the complexes are capable of catalyzing the 1,4-addition of oxindoles to 2-nitrostyrene.

The trans-selective catalytic asymmetric formation of β-lactones constitutes an attractive surrogate for anti-aldol additions. Recently, we have reported the first catalyst which is capable of forming trans-β-lactones with high enantioselectivity from aliphatic (and aromatic) aldehyde substrates by cyclocondensation with acyl bromides. In that previous study the concepts of Lewis acid and organic aprotic ion pair catalysis were combined in a salen-type catalyst molecule. Since a pyridinium residue on the salen periphery is essential for high trans- and enantioselectivity, we were interested in the question of whether substituents on the pyridinium rings could be used to further improve the catalyst efficiency, as they might have a significant impact on the effective charges within the heterocycles. In the present study we have thus compared a small library of aluminum salen/bispyridinium catalysts mainly differing in the substituents on the pyridinium residues. As one result of these studies a new catalyst was identified which offers slightly superior stereoselectivity as compared to the previously reported best catalyst. NBO calculations have revealed that the higher stereoselectivity can arguably not be explained by the variation of the effective charge.

Lewis acid catalysis and nucleophilic carbene catalysis are complementary fundamental concepts to accelerate and control chemical reactions of aldehyde substrates. Their efficient merger has recently been achieved using two separate catalyst species. The present report describes our efforts to develop a cooperative catalyst system which incorporates both features – Lewis acid and nucleophilic NHC – within the same catalyst entity. To generate free carbene moieties under very mild conditions, Ag‐NHC complexes were formed releasing the nucleophilic carbene upon treatment with PPh3. The result is the formation of an enol‐δ‐lactone as new enal dimerization product. Silver is essential for the reactivity mode thus suggesting that it plays a double role in the catalytic event.