1416171-24-8Relevant articles and documents
Polyfunctional Imidazolium Aryloxide Betaine/Lewis Acid Catalysts as Tools for the Asymmetric Synthesis of Disfavored Diastereomers
Willig, Felix,Lang, Johannes,Hans, Andreas C.,Ringenberg, Mark R.,Pfeffer, Daniel,Frey, Wolfgang,Peters, René
, p. 12029 - 12043 (2019)
Enzymes are Nature's polyfunctional catalysts tailor-made for specific biochemical synthetic transformations, which often proceed with almost perfect stereocontrol. From a synthetic point of view, artificial catalysts usually offer the advantage of much broader substrate scopes, but stereocontrol is often inferior to that possible with natural enzymes. A particularly difficult synthetic task in asymmetric catalysis is to overwrite a pronounced preference for the formation of an inherently favored diastereomer; this requires a high level of stereocontrol. In this Article, the development of a novel artificial polyfunctional catalyst type is described, in which an imidazolium-aryloxide betaine moiety cooperates with a Lewis acidic metal center (here Cu(II)) within a chiral catalyst framework. This strategy permits for the first time a general, highly enantioselective access to the otherwise rare diastereomer in the direct 1,4-addition of various 1,3-dicarbonyl substrates to β-substituted nitroolefins. The unique stereocontrol by the polyfunctional catalyst system is also demonstrated with the highly stereoselective formation of a third contiguous stereocenter making use of a diastereoselective nitronate protonation employing α,β-disubstituted nitroolefin substrates. Asymmetric 1,4-additions of β-ketoesters to α,β-disubstituted nitroolefins have never been reported before in literature. Combined mechanistic investigations including detailed spectroscopic and density functional theory (DFT) studies suggest that the aryloxide acts as a base to form a Cu(II)-bound enolate, whereas the nitroolefin is activated by H-bonds to the imidazolium unit and the phenolic OH generated during the proton transfer. Detailed kinetic analyses (RPKA, VTNA) suggest that (a) the catalyst is stable during the catalytic reaction, (b) not inhibited by product and (c) the rate-limiting step is most likely the C-C bond formation in agreement with the DFT calculations of the catalytic cycle. The robust catalyst is readily synthesized and recyclable and could also be applied to a cascade cyclization.
Homo-and heterobimetallic Pd-, Ag-, and Ni-hybrid salen-bis-NHC complexes
Mechler, Melanie,Latendorf, Katja,Frey, Wolfgang,Peters, Rene
supporting information, p. 112 - 130 (2013/02/25)
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, Ni II 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 M 1/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.
