Feng chiral N,N′-dioxide ligands: uniqueness and impacts

Abstract

From the historical perspective, all widely accepted privileged chiral ligands tune the catalytic activity of the metal complexes that they form and impart stereoselectivity to asymmetric reactions exclusively relying on the coordination with phosphorus, nitrogen or hetero-atom hybrids. Nowadays, works with N,N’-dioxide amides, i.e., Feng ligands, break with this tradition and have shown that oxygen-coordinated ligands can also be privileged, allowing for the proliferation of chiral metal catalysts and highly enantioselective transformations.

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In nature, mirror-image molecules (enantiomers) show dramatically distinct activities, and thus a major focus of research worldwide involves having reactions yield products in an enantioselective manner.¹ Enzymes, either naturally occurring or those evolved in the laboratory,² provide a fascinating approach to asymmetric synthesis, but all too often display insufficient stability and a relatively narrow substrate scope. Other endeavors have been directed toward artificial chiral metal catalysis³ and organocatalysis,^4,5 for which the 2001 and 2021 Nobel prizes in Chemistry were awarded, specifically for the development of useful tools and reactions. Arguably, metal complexes with chiral organic ancillaries (namely ligands) are among the most versatile catalysts for constructing stereocenters from prochiral or racemic substrates. Although numerous breakthroughs have been made, identification of the right metal catalyst for complicated substrates or an unprecedented transformation is never an easy task. As such, rational ligand design stands as an essential step, perhaps the essential step, in chiral organometallic chemistry—and the ever-increasing needs for generalizing and predicting ligand–metal-substrate interactions set an ambitious goal: achieving truly privileged ligands.⁶

Thus far, all widely accepted privileged chiral ligands tune the activity of the metal complexes that they form and impart stereoselectivity to asymmetric reactions by exclusively relying on coordination with phosphorus or nitrogen atoms.^6,7,8,9 Over the past two decades, the Feng group has established a library of chiral N,N’-dioxide amides (Fig. 1), which have emerged as a class of unique and privileged ligands and have enabled the development of major mechanistically unrelated reactions with exceptional levels of regio-, chemo-, diastereo-, and enantioselectivity.^{10,11,12–15} Such chiral N,N’-dioxide amides are, naturally, known as Feng ligands.¹⁶ Since a few comprehensive reviews have nicely recalled the discovery and summarized the applications of Feng ligands, we provide in this highlight a brief discussion focused on the uniqueness and impacts of this class of ligands.

Fig. 1 Representative Feng ligand structures.

1. Uniqueness of Feng ligands

In chiral ligand design, deployment of symmetry^17,18,19 and scaffold rigidity^6,9,20 can generally minimize the potential number of isomers produced of any ligand–metal-substrate complex by minimizing the number of potential reaction pathways, thus imparting excellent levels of stereocontrol. Adhering to such approaches has proven to be successful and has aided in the discovery of several privileged ligands, including BINAP, bis(oxazolines), salen- and SPINOL-based ligands.⁶ Indeed, the high performance of a Feng ligand is intimately related to its C₂ symmetry. On the other hand, rather than being rigid, Feng ligands are generally flexible, though the first reported Feng ligand employed a somewhat rigid Ti⁴⁺ complex.²¹

Conformational flexibility

When starting from commercially available and inexpensive chiral amino acids, the synthesis of a Feng ligand can be completed in three high-yielding transformations: amidation, N-alkylation, and oxidation (Scheme 1a). Notably, inclusion of the amide N–H bond can significantly improve the stability of polar N-oxides and anchor the chirality at nitrogen stereocenters through intramolecular hydrogen bonding.²² As such, Feng ligands tolerate air and moisture such that no special handling is needed for their storage. More importantly, the ease of rotation about the C(sp³)–C(sp³) bonds and the amide C(sp³)–C(sp²) bonds contribute to the overall flexibility of Feng ligands (Scheme 1b).

Scheme 1 Synthesis and characterization of Feng chiral and corresponding metallic complexes.

Coordination flexibility

The typical Feng ligand is a neutral, polar, and tetradentate O-donor ligand with two oxygen atoms from amine oxides and the other two from amides. When the Feng ligand interacts with a metal ion, an exchange process occurs, in which oxygen atoms of N-oxides and amides bind to the metal center, and the amide N–H bonds become free to form hydrogen bonds with appropriate substrates (Scheme 1c). Upon such coordination, the alkyl linkage and the amide motifs would shield oppositely related regions (the upper left and lower right quadrants in Scheme 1c) to create a chiral pocket, in which the chelated metal center interacts with substrates in an enantioselective manner.¹¹ Note that N-oxides can function as donors of σ-electron density, while amides act as σ-donors and π-acceptors,²³ thus accommodating distinct metal ions having either nearly filled or empty d orbitals of correct symmetry.

Such unique conformational and coordination flexibility makes the Feng ligand design truly versatile. In fact, Feng ligands readily chelate with more than 20 metal ions, ranging from main-group and transition metals to rare-earth metals. Specifically, octahedral complexes are formed with metal ions of small ionic radius, including main-group and transition metals, as well as Sc³⁺. Also note that the chemical structure of the Feng ligand design significantly influences the manner of its coordination. Specifically, a distorted trigonal bipyramidal Co²⁺ complex with five coordinating ligands was recently reported by Feng and coworkers.²⁴ For larger metal ions, e.g., the majority of rare-earth metal salts, Feng ligands tend to chelate and afford complexes with increased coordination number and distorted geometries (Scheme 1d).¹⁴ And as a practical matter, these metallic complexes have proven to be enantioselective over a wide range of mechanistically unrelated reactions, which firmly underpins the “privilege” of the ligands.

2. Impacts of the Feng ligand

By virtue of the unique flexibility of the Feng ligand design in terms of conformation and coordination chemistry, a variety of Feng ligands have been prepared, characterized, and applied as privileged chiral Lewis acid catalysts in more than 50 asymmetric transformations, including conjugate addition reactions,²⁵ cycloaddition reactions,¹³ arrangement reactions,²⁶ and so forth (Fig. 2). The success of the Feng ligand design not only represents an invaluable addition to the toolkit of chiral ligand design, but also enhances the stereocontrol of reactions with understood mechanisms and enables unprecedented transformations to occur with high enantioselectivity. Rather than a comprehensive review, only a few examples of prime significance by the Feng group and other groups are illustrated here to highlight the impact of the Feng ligand design.

Fig. 2 Asymmetric transformations catalyzed by Feng ligand–metal complexes.

The relatively stable α-diazocarbonyl compound is a useful and versatile building block, and has been used in a broad range of asymmetric transformations that provide direct access to enantioenriched molecules.^27,28 Nevertheless, Lewis acid-promoted or catalyzed enantioselective rearrangement reactions still remain challenging. Roskamp and colleagues reported a SnCl₂-catalyzed aldol-type reaction of α-diazoesters with aldehydes to give each a diazonium betaine intermediate, which then underwent a concerted an H-shift/N₂ extrusion reaction to give a β-keto ester, i.e., the Roskamp reaction.²⁹ Since the asymmetric version of the Roskamp reaction employed α-diazoamides with chiral auxiliaries,³⁰ Feng and coworkers introduced a Feng ligand to Sc(OTf)₃ catalysis and achieved the first catalytic enantioselective Roskamp reaction (Scheme 2a).³¹ This was a landmark reaction in both diazo and malonate chemistry due to (1) side reaction pathways such as epoxidation and aryl group migration being completely shut down using the L₃-RaPr₂–Sc(OTf)₃ complex at a concentration as low as 0.05 mol%, (2) the textbook conclusion about inaccessible α-carbon stereogenic β-keto esters being invalidated,³² (3) this reaction being featured as a “Roskamp–Feng reaction”,³³ and (4) a very large number of catalytic enantioselective transformations of α-diazocarbonyls, including homologation with ketones^{34,35,36–38} and [2,3]-^39,40,41 and [3,3]-sigmatropic rearrangements,⁴² having been developed by virtue of the use of prestigious Feng ligand–metal complexes. More interestingly, a novel metal–carbenoid-combined high-spin Feng ligand–Co(II) catalysis was thereby developed, by which a highly enantioselective vinylogous N–H insertion of secondary aliphatic amines²³ and an asymmetric [2 + 1] cycloaddition of thioketones⁴³ were achieved.

Scheme 2 Representative Feng- ligand–metal-catalyzed asymmetric transformations.

The privileged Feng ligand–metal complexes have also been used in elegant radical transformations. In 2021, Feng and coworkers developed an efficient enantioselective radical carboazidation and diazidation reaction of electron-deficient alkenes, in which an L₃-RaAd–Fe(OTf)₂ complex was used as a redox catalyst for the reductive generation of radical species, as well as azido group transfer (Scheme 2b).⁴⁴ In direct photochemical reactions, such as [2 + 2] cycloaddition⁴⁵ and Norrish–Yang type II cyclization,⁴⁶ use of Feng catalysts yielded high diastereo- and enantioselectivity. More interestingly, Feng and coworkers found that a few Feng ligand–metal-substrate complexes assembled in situ can function as unique photoredox reagents. Upon being excited, these complexes trigger single-electron transfer oxidation of thioethers and silanes, thereby giving nucleophilic radical species for precision radical coupling reactions⁴⁷ and radical addition to ketones⁴⁸ in a highly enantioselective manner (Scheme 2c).

Feng ligands have been commercially available at Sigma-Aldrich and J&K Scientific (Fig. 3) since 2014, which has inspired many other research groups to address issues associated with development of novel reactions and syntheses of molecular complexes. For example, Kobayashi and co-workers leveraged the excellent water tolerance of the L₃-PrMe₂–Sc(III) complex in aqueous Mukaiyama aldol reactions,⁴⁹ and they later designed the pH-responsive Feng ligand L₃-RaCnPy that functioned as an artificial metalloenzyme (Scheme 3a).⁵⁰ Yamamoto and colleagues leveraged the L₃-PrPr₂–Gd(III) and L₂-PrPr₂–Mg(II) complexes for the first enantioselective aminolysis of aromatic trans-2,3-epoxy sulfonamides (Scheme 3b)⁵¹ and amination of β-ketoesters with nitrosocarbonyl compounds generated in situ.^52,53 Jia and colleagues demonstrated the superiority of L₃-PrPr₂–Sc(OTf)₃ to “privileged” bifunctional hydrogen-bonding catalysts in the key asymmetric Michael addition reaction of a tricyclic benzofuranone with methyl vinyl ketone towards the total synthesis of (−)-galanthamine and (−)-lycoramine (Scheme 3c).⁵⁴ Given the unique catalytic activity of L₃-PiMe₃–Sc(OTf)₃, Xie and coworkers developed a large-scale enantioselective tandem Michael addition reaction of oxindoles with alkynones, which afforded the key intermediate for asymmetric total synthesis of monoterpenoidindolealkaloids, including (−)-tubifoline, (−)-tubifolidine, (−)-dehydrotubifoline, and (−)-strychnine (Scheme 3d).⁵⁵ Moreover, privileged Feng ligand–metal complexes have also provided facile access to a variety of structurally diverse nitrogen-heterocyclic compounds via asymmetric cycloaddition reactions.^56,57,58,59 Huang and colleagues⁶⁰ achieved convergent synthesis of chiral vicinal amino alcohols via an enantioselective reductive radical-type coupling reaction of nitrones with aromatic ketyl radicals generated from photocatalyzed single-electron transfer reduction of carbonyls (Scheme 3e). Paton, Smith, and colleagues⁶¹ recently demonstrated that the use of L₃-PrPr₂–Sc(OTf)₃ could lower the triplet energy of acrylanilides by forming complexes with the acrylanilides, to which selective energy transfer from a photosensitizer proceeded, thereby enabling an asymmetric 6π photocyclization reaction (Scheme 3f).

Fig. 3 Commercially available Feng ligand.

Scheme 3 Application of Feng ligands in complex molecule syntheses by other research groups.

3. Conclusions

In summary, the highly flexible, C₂-symmetric chiral N,N′-dioxide amides developed by Feng and coworkers, i.e., Feng ligands, feature tetradentate O-coordination with a broad scope of main-group, transition, and rare-earth metals. The formed chiral metal catalysts have enabled numerous asymmetric transformations that provide invaluable access to many enantio-enriched structurally complex molecules. The Feng ligand has clearly become a highly competent member of the family of privileged ligands. With its ever-increasing popularity and impacts in synthetic organic chemistry, recent examples have opened avenues for controlling the stereoselectivity of radical-related bond-forming processes by virtue of Feng ligand–metal Lewis acid catalysis.^{44–46,60,61} In the near future, we expect a greatly expanded chemical space of Feng ligands in manipulating such transient intermediates with greener processes, as well as applications in material science.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

We are grateful to the National Natural Science Foundation of China (no. 22188101) and the Fundamental Research Funds for the Central Universities (no. WK9990000111).

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