Salen and NHC ligands are among the
most important ligands for
homogeneous catalysis. We have recently reported bimetallic complexes,
in which both motifs have been merged for the first time. However,
the intermetallic distances, which play a crucial role for cooperative
bimetallic catalysis, were probably not appropriate in these first-generation
hybrid catalysts. To generate heterobimetallic salen/NHC hybrid complexes
with intermetallic distances suitable for cooperative catalysis, chiral
macrocyclic hybrid ligands featuring a salen and two linked NHC donor
moieties have been prepared in the present study. For the ligand formation,
chiral enantiopure diamines as well as chiral enantiopure bisimidazoles
were employed, and a matched/mismatched situation was found depending
on the configuration of both chirality sources. Regioselective complexation
of Zn(II), Ni(II), and Pd(II) by the salen N2O2 coordination sphere was efficiently accomplished. Subsequent coordination
of the NHC units was achieved for Ag(I), Cu(I), Au(I), and Pd(II),
in the latter case by oxidative transmetalation with Pd2(dba)3. X-ray crystal structure analyses for Ni/Ag2 and Pd/Ag2 complexes show strongly puckered macrocycles,
in which one of the NHC-bound Ag(I) centers is in close proximity
to the salen-bound Ni(II) or Pd(II) centers and in which this Ag(I)
apparently interacts with both salen O-donor atoms. Preliminary data
for the 1,4-addition of an oxindole to a nitroolefin and for the Conia-ene
reaction of an α-cyanoacetate are reported.
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.
Diastereodivergency is a challenge for catalytic asymmetric synthesis. For many reaction types, the generation of one diastereomer is inherently preferred, while the other diastereomers are not directly accessible with high efficiency and require circuitous synthetic approaches. Overwriting the inherent preference by means of a catalyst requires control over the spatial positions of both reaction partners. We report a novel polyfunctional catalyst type in which a Ni(II) -bis(phenoxyimine) unit, free hydroxy groups, and an axially chiral bisimidazolium entity participate in the stereocontrol of the direct 1,4-addition of oxindoles to nitroolefins. Both epimers of the 1,4-adduct are accessible in excess on demand by changes to the ligand constitution and configuration. As the products have been reported to be valuable precursors to indole alkaloids, this method should allow access to their epimeric derivatives.
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.
Diastereodivergenz ist eine Herausforderung für die katalytische asymmetrische Synthese.Für viele Reaktionstypen ist die Bildung eines Diastereomers inhärent bevorzugt, während das andere Diastereomer nichtd irekt und effizient zugänglichist, sondern umständlicheSyntheseumwege erfordert. Das Umgehen dieser natürlichen Präferenz mithilfe eines Katalysators erfordert die räumliche Kontrolle über beide Reaktionspartner.W ir stellen einen neuartigen, polyfunktionellen Katalysator vor,i nd em eine Ni II -Bis(phenoxyimin)-Einheit, freie Hydroxygruppen und ein axial-chiraler Bisimidazolium-Baustein an der Stereokontrolle der direkten 1,4-Addition von Oxindolen an Nitroolefine beteiligt sind. Beide Epimere des 1,4-Adduktes sind nachBelieben durchV ariation der Konstitution und Konfiguration des Liganden zugänglich. Wieb ereits berichtet wurde,s ind die Produkte wertvolle Vorstufen für Indolalkaloide,u nd die vorliegende Methode sollte somit den Zugang zu den epimeren Derivaten erçffnen. Hintergrundinformationen zu diesem Beitrag sind im WWW unter http://dx.
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