Chiral Metallacrown Ethers for Asymmetric Hydrogenation: Alkali‐Metal Ion Mediated Enhancement of Enantioselectivity

Abstract

Dedicated to Professor Albert S. C. Chan on the occasion of his 60th birthdayThe development of new effective ligands is a continual challenge in transition-metal-catalyzed asymmetric reactions. During the last several decades, a number of excellent chiral bidentate ligands have been reported. [1] Among them, there are no universal ligands reported since catalytic asymmetric transformations are often substrate-dependent. Subtle changes in conformation, sterics, and electronic properties of chiral ligands can lead to dramatic variations of reactivity and enantioselectivity. Therefore, the development of structurally and/or conformationally tunable chiral ligands will offer a great advantage for optimizing the enantioselectivity of a reaction. [2] Herein, we report a new kind of easily available and conformationally tunable phosphorus metallacrown ethers for asymmetric hydrogenation.Metallacrown ethers are a class of metallomacrocycles that are often formed by the chelation of a,w-bis(phosphorus-donor)polyether ligands to transition metals. [3,4] Like crown ethers, they have proven to be capable of binding alkali-metal cations selectively. In addition to their receptor properties, they are expected to be of interest as catalysts for organic reactions owing to the potential bifunctionality of such hard-soft bimetallic complexes and the tunability of the catalyst conformation. Surprisingly, the application of metallacrown ethers as catalysts is less studied. [4c, e] Most re-cently, Gray and co-workers reported a Rh I complex containing an a,w-bis(phosphite) ligand with two amido groups for the hydroformylation of styrene. [4c] It was found that the addition of alkali-metal salts could result in obvious increases in the regioselectivity. However, the use of chiral metallacrown for asymmetric catalysis has never been reported to the best our knowledge. Inspired by these facts and as a continuation of our ongoing endeavor to develop effective chiral supramolecular catalysts based on crown ethers, [5] for this study we choose easily available and length-tunable oligo(ethylene glycols) (OEG) as the backbone of the ditopic ligand. First complexation of the ligand with alkali-metal ions is expected to bring the two terminal ligating groups closer together, [6] furnishing a new type of template-induced bidentate ligand. [7,8] The subsequent coordination of the preformed ditopic ligand to the catalytically active transitionmetal ions thus gives the metallacrown ether catalyst (Scheme 1). With this template-induced strategy, the catalyst would be prepared more efficiently, avoiding the formation of supramolecular coordination polymers and/or oligomers. [4a, e] To exemplify our approach, we report here for the first time a class of supramolecular chiral binol-derived diphosphite ligands and the effect of alkali-metal ions on the enantioselectivity by taking Rh-catalyzed asymmetric hydrogenation of dehydroamino acid derivatives as the reaction prototype. Up to 17 % enhancement of ee value was observed in a favorable case.Firs...

Synthesis of Elusive Chloropnictenium Ions

“…The introduction of a macrocyclic crown ether skeleton with selective binding properties into the catalyst structure is expected to result in distinctive catalytic performance involving a host–guest recognition process 3. However, to the best of our knowledge, the application of metallacrown ethers as catalysts has not been widely studied,2d,2f–2l especially the use of chiral metallacrown ethers in asymmetric catalysis 2g,2l. Recently, we have developed a new class of readily available and tunable bis(phosphite) ligands with an oligo(ethylene glycol) backbone, and a pronounced enhancement of enantioselectivity was achieved in the rhodium‐catalyzed asymmetric hydrogenation of α‐dehydroamino acid esters by the addition of NaBArF {BArF = [3,5‐(CF 3 ) 2 C 6 H 3 ] 4 B} 2g.…”

“…However, to the best of our knowledge, the application of metallacrown ethers as catalysts has not been widely studied,2d,2f–2l especially the use of chiral metallacrown ethers in asymmetric catalysis 2g,2l. Recently, we have developed a new class of readily available and tunable bis(phosphite) ligands with an oligo(ethylene glycol) backbone, and a pronounced enhancement of enantioselectivity was achieved in the rhodium‐catalyzed asymmetric hydrogenation of α‐dehydroamino acid esters by the addition of NaBArF {BArF = [3,5‐(CF 3 ) 2 C 6 H 3 ] 4 B} 2g. Later, Vidal‐Ferran and co‐workers reported a similar chiral bis(phosphite)‐containing metallacrown ether catalyst bearing a 1,1′‐biphenyl unit in the oligo(ethylene glycol) backbone.…”

Podand‐Based Dimeric Chromium(III)–Salen Complex for Asymmetric Henry Reaction: Cooperative Catalysis Promoted by Complexation of Alkali Metal Ions

“…By coordination of podand ligands with transition metals, various metallacrown ethers have been fabricated 7. 8 Recently, our group realized the first example of asymmetric hydrogenation catalyzed by chiral metallacrown ethers (Scheme ), which were prepared in situ with bis(phosphite)‐functionalized podands and [Rh(cod) 2 ]BF 4 8a. The complexation of an alkali metal ion has shown remarkable effects on both catalytic activity and enantioselectivity in the asymmetric hydrogenation of α‐dehydroamino acid esters.…”

Supramolecularly fine-regulated enantioselective catalysts

“…Fan et al 22 identified metallacrown-ethers 23 as ideal scaffolds for developing supramolecularly regulated enantioselective catalysts: metallacrown derivatives simultaneously contain a plausible catalytic group (the metal centre) and a supramolecular motif capable of acting as a (Fig. 4), which contains six ethyleneoxy units, are indicated in Table 5, the reader is referred to the original work 22 for the results for the other two ligands). The efficiency of L4 could be enhanced by adding an RA (i.e.…”

“…Lower ees were obtained for the ortho-and meta-substituted analogues, although considerable regulation effects (enhancements of up to +6% in the ee; see entry 17 in Table 6) were observed. In between the publication dates of the studies on supramolecularly regulated metallacrown-based catalysts by Fan and He et al, 22,25 Vidal-Ferran et al reported the first generation of supramolecularly regulated -bisphosphites for asymmetric hydroformylations (AHFs). 26 The design of the ligand was similar to that of L4 but incorporated a conformationally labile [1,1'-biphenyl]-2,2'-diol motif at the regulation site.…”