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The combination of enzymes and metal complexes can be used to achieve the de-epimerization of diastereoisomeric mixtures. Thus, by means of this chemoenzymatic dynamic kinetic asymmetric transformation (DYKAT), a wide range of racemic diols and, to a lesser extent, amino alcohols and diamines, have been transformed into a single diastereoisomer in excellent yields, and diastereo- and enantioselectivities.

This Review compiles the evolution, mechanistic understanding, and more recent advances in enantioselective Pd-catalyzed allylic substitution and decarboxylative and oxidative allylic substitutions. For each reaction, the catalytic data, as well as examples of their application to the synthesis of more complex molecules, are collected. Sections in which we discuss key mechanistic aspects for high selectivity and a comparison with other metals (with advantages and disadvantages) are also included. For Pd-catalyzed asymmetric allylic substitution, the catalytic data are grouped according to the type of nucleophile employed. Because of the prominent position of the use of stabilized carbon nucleophiles and heteronucleophiles, many chiral ligands have been developed. To better compare the results, they are presented grouped by ligand types. Pd-catalyzed asymmetric decarboxylative reactions are mainly promoted by PHOX or Trost ligands, which justifies organizing this section in chronological order. For asymmetric oxidative allylic substitution the results are grouped according to the type of nucleophile used.

The successful rhodium-catalyzed asymmetric hydroformylation and hydroaminomethylation of α-substituted acrylamides is described using 1,3-phosphite-phosphoramidite ligands based on a sugar backbone. A broad scope of chiral aldehydes and amines were afforded in high yields and excellent enantioselectivities (up to 99%). Furthermore, the synthetic potential of this method is demonstrated by the single-step synthesis of the brain imaging molecule RWAY.

We have identified a successful family of simple P-stereogenic N-phosphine–phosphite ligands for the Rh-catalyzed asymmetric hydrogenation of olefins. These catalysts show excellent enantiocontrol ...

Asymmetric hydrogenation (AH) is one of the most attractive methods for the preparation of enantiomerically pure compounds due to its operational simplicity and perfect atom economy. Despite the extensive research dedicated to the AH of alkenes and the important progress achieved, some issues still need to be solved. Most catalysts only work with a limited number of alkenes and each type of alkene needs a specific catalyst for optimal enantioselectivity. In this respect, the AH of functionalized alkenes is mostly carried out with Ru– and Rh–diphosphine catalysts, while the AH of olefins without a coordinative functional group is mainly carried out with Ir–P,N catalysts. The AH of functionalized olefins has been thoroughly studied for decades; there are, however, some substrate types that are still a challenge (e.g. cyclic β-enamides). Compared to the AH of functionalized olefins, the reduction of unfunctionalized alkenes is much less developed. Most catalysts are still specific for the type of olefin geometry and its substitution pattern. Most successful cases have been reported for trisubstituted E-unfunctionalized alkenes and, to a lesser extent, for Z-trisubstituted, 1,1′-disubstituted and tetrasubstituted olefins. Broad substrate scopes are desirable to reduce the time dedicated to ligand design and preparation. Our group contributed in this field with a series of improved ligand libraries that considerably expanded the substrate scope. In this minireview, we reveal our progress in the iridium-catalyzed AH of a broad range of unfunctionalized olefins and cyclic β-enamides.

The palladium-catalyzed enantioselective allylic substitution by carbon or nitrogen nucleophiles is a key transformation that is particularly useful for the synthesis of bioactive compounds. Unfortunately, the selection of a suitable ligand/substrate combination often requires significant screening effort. Here, we show that a transition state force field (TSFF) derived by the quantum-guided molecular mechanics (Q2MM) method can be used to rapidly screen ligand/substrate combinations. Testing of this method on 77 literature reactions revealed several cases where the computationally predicted major enantiomer differed from the one reported. Interestingly, experimental follow-up led to a reassignment of the experimentally observed configuration. This result demonstrates the power of mechanistically based methods to predict and, where necessary, correct the stereochemical outcome., A predictive model has been created for a stereoselective palladium-catalysed allylic amination reaction. Derived only from quantum chemical data, the method is accurate enough to reveal multiple erroneous assignments in literature experiments.

We studied for the first time the potential of novel and simple Ir/thioether-NHC complexes in the asymmetric hydrogenation of unfunctionalized olefins and cyclic β-enamides. For comparison, we prep...

A method for generation of Pd/phosphite-oxazoline catalysts containing an alkyl backbone chain has been successfully applied to Pd-catalyzed allylic substitution reactions. By carefully selecting t...

A large family of phosphite-thioether/selenoether ligands has been easily prepared from accessible L-(+)-tartaric acid and D-(+)-mannitol and applied in the M-catalyzed (M=Ir, Rh) asymmetric hydrogenation of a broad number of substrates (46 in total). Its highly modular architecture has been crucial to maximize the catalytic performance. Improving most of the reported approaches, this ligand family presents a broad substrate scope. By selecting the ligand parameters high enantioselectivities (ee's up to 99 %) have therefore been achieved in a broad range of both, functionalized and unfunctionalized substrates. Interestingly, both enantiomers of the hydrogenation products can be usually achieved by changing the ligand parameters.

The asymmetric hydrogenation of unfunctionalized and olefins bearing a poorly coordinative functional groups have been considered a very puzzling transformation over many decades. Over the last years, the asymmetric hydrogenation of these challenging olefins has experienced an important advance. So, we here discuss the breakthroughs and progress, mechanistic insights and more recent advances using iridium- and cobalt-catalyst. We also cover the more recent applications in the preparation of natural products and drugs.

We have identified a successful family of simple P-stereogenic

In this chapter, we describe the development in homogeneous Ir-catalyzed asymmetric hydrogenation with particular emphasis on the achievements made during the last 10 years. We also present their application to the synthesis of complex molecules. The first section deals with the hydrogenation of unfunctionalized olefins or with poorly coordinative groups. The second section includes the advances made in the hydrogenation of functionalized olefins. The last two sections cover the hydrogenation of imines and ketones, respectively.

We have developed a family of Ir–P,oxazoline catalysts for asymmetric hydrogenation. These catalysts, with a simple modular architecture, have shown a high tolerance to the olefin geometry and subs...

The construction of a novel class of amino-phosphite/phosphinite/phosphine ligands containing a protected pyrrolidine-3,4-diol moiety is presented. These ligands are obtained from readily available sugars. They thus contain the advantages of carbohydrates in terms of selection of the stereogenic carbons, polyfunctional groups able to modulate the electronic and steric properties, and the general good stability of carbohydrate derivatives. They constitute a novel class of P,N-ligands that have been used in the enantioselective allylic substitutions of acyclic and cyclic substrates with varied electronic and steric requirements, using different C- and N-nucleophiles, with high enantioselectivities. Among the three groups of P,N-ligands (amino-P; P = phosphite, phosphinite, and phosphine groups) the new amino-phosphite ligands give the widest substrate and nucleophile scope, including the more challenging hindered linear and cyclic substrates. In particular, for carbohydrate-derived amino-phosphite ligands a...

This study identifies new phosphite-oxazoline ligands that have been successfully applied in the palladium-catalyzed intermolecular asymmetric Heck reaction. The design of the new phosphite-oxazoline ligands derives from a previous successful generation of phosphine-oxazoline ligands, by replacing the phosphine group with several π-acceptor biaryl phosphite moieties. With these simple modifications, the new phosphite-based ligands, unlike previous phosphine-oxazoline, not only present a modular design with numerous potential phosphite groups available, but they are also air-stable solids, which can be made in the same number of synthetic steps as the phosphine analogues. The substitution of the phosphine by a biaryl phosphite group extended the range of substrates and triflates sources that can be coupled with regio-, enantioselectivities and activities comparable to the few best ones reported. In addition, the ligands that provided the best selectivities contained an isopropyl oxazoline moiety instead of the tert-butyl group found in the related phosphine-oxazoline ligands, which is made from a much more expensive precursor. In this paper we have also carried out kinetic studies and a Hammett plot analysis to determine the rate determining step of this system in the regime of interest. We suggest a likely explanation for the fast Heck reaction of the phosphite-oxazoline catalysts.

The success of phosphine-oxazoline ligands (PHOX) inspired the progress in P-oxazoline ligand families by modifying either the ligand backbone, the electronic and/or steric properties of the phosphine group or by exchanging the phosphine to a phosphinite or a phosphite group. In this respect, the structures of chiral P-oxazoline ligands have become more diverse and new families of very efficient ligands have emerged, which have improved catalytic performance in some asymmetric transformations, with an increased versatility, both in the range of reactions and in the range of substrates/reagents. In addition, most of ligands are synthesized from easily accessible chiral amino alcohols, maintaining the short and efficient synthetic route developed for PHOX ligands. New ligands have been developed by replacing the oxazoline functionality with several other N-donor groups, e.g. imidazole, thiazole, oxazole, pyridine, etc., and O- and S-groups. This review offers a critical overview of the utility of these most successful bidentate heterodonor P-N, P-O and P-S ligand families applied in metal-mediated processes. We illustrate how, through proper ligand design, these heterodonor bidentate ligand families can be an excellent source of ligands, with superior catalytic performance in many asymmetric reactions than the best C2-symmetric N,N and P,P-ligands reported so far.

Simple enantioselective synthesis of 6,6-disubstituted pentafulvenes bearing chiral pendant hydroxy groups are attained by cascade reactivity using commercially available proline-based organocatalysts. Condensation of cyclopentadiene with the acetyl function of a 1,2-formylacetophenone, followed by cyclization of a resulting fulvene-stabilized carbanion with the formyl group, generates bicyclic chiral alcohols with initial er values up to 94:6. Exceptional enantio-enrichment of the resultant alcohols results upon crystallization – even near racemic samples spontaneously de-racemize. This enables new families of substituted cyclopentadienes that are both enantiomerically and diastereomerically pure to be rapidly attained.

This study presents a new series of readily accessible iridium- and rhodium-phosphite/oxazoline catalytic systems that can efficiently hydrogenate, for the first time, both minimally functionalized olefins and functionalized olefins (62 examples in total) in high enantioselectivities (ees up to >99%) and conversions. The phosphite-oxazoline ligands, which are readily available in only two synthetic steps, are derived from previous privileged 4-alkyl-2-[2-(diphenylphosphino)phenyl]-2-oxazoline (PHOX) ligands by replacing the phosphine moiety by a biaryl phosphite group and/or the introduction of a methylene spacer between the oxazoline and the phenyl ring. The modular design of the ligands has given us the opportunity not only to overcome the limitations of the iridium-PHOX catalytic systems in the hydrogenation of minimally functionalized Z-olefins and 1,1-disubstituted olefins, but also to expand their use to unfunctionalized olefins containing other challenging scaffolds (e.g., exocyclic benzofused and triaryl-substituted olefins) and also to olefins with poorly coordinative groups (e.g., α,β-unsaturated lactams, lactones, alkenylboronic esters, etc.) with enantioselectivities typically >95% ee. Moreover, both enantiomers of the hydrogenation product could be obtained by simply changing the configuration of the biaryl phosphite moiety. Remarkably, the new catalytic systems also provided excellent enantioselectivities (up to 99% ee) in the asymmetric hydrogenation of another challenging class of olefins – the functionalized cyclic β-enamides. Again, both enantiomers of the reduced amides could be obtained by changing the metal from Ir to Rh. We also demonstrated that environmentally friendly propylene carbonate can be used with no loss of enantioselectivity. Another advantage of the new ligands over the PHOX ligands is that the best ligands are derived from the affordable (S)-phenylglycinol rather than from the expensive (S)-tert-leucinol.

A modular ligand library of -amino acid hydroxyamides and thioamides was prepared from 10 different N-tert-butyloxycarbonyl-protected -amino acids and three different amino alcohols derived from 2, ...

A large library of thioether/selenoether-phosphite ligands have been tested in the Pd-catalyzed asymmetric allylic substitution reaction. The presented ligands are derived from cheap and available carbohydrates and they are air-stable solids and easy to handle. Their highly modular nature has made it possible to achieve excellent enantioselectivities in the substitution of a range of hindered and unhindered substrates (ees up to 99% and 91%, respectively). In addition, twelve C-, N- and O-nucleophiles can be efficiently introduced, independently of their nature. Among the whole library, ligands that contain an additional chiral centre in the alkyl backbone chain next to the phosphite group and an enantiopure biaryl phosphite group provided the best enantioselectivities. In general, there is a cooperative effect between these two chiral elements, and therefore, a matched combination between them is necessary to achieve the highest enantioselectivities. However, in the case of cyclic substrates, this cooperative effect is less pronounced and advantageously, both enantiomers of the product can be obtained by setting up the desired configuration of the biaryl phosphite group. Studies of the key Pd–π-allyl intermediates allowed us to better understand the enantioselectivities obtained experimentally.

This work identifies a family of Ir/phosphite-sulfoximine catalysts that has been successfully used in the asymmetric hydrogenation of olefins with poorly coordinative or noncoordinative groups. In comparison with analogue Ir/phosphine-sulfoximine catalysts previously reported, the presence of a phosphite group extended the range of olefins than can be efficiently hydrogenated. High enantioselectivities, comparable to the best ones reported, have been achieved for a wide range of olefins containing relevant poorly coordinative groups such as α,β-unsaturated enones, esters, lactones, and lactams as well as alkenylboronic esters.

Air stable and readily available Ir-catalyst precursors modified with MaxPHOX-type ligands have been successfully applied in the challenging asymmetric hydrogenation of tetrasubstituted olefins under mild reaction conditions. Gratifyingly, these catalyst precursors are not only able to efficiently hydrogenate a range of indene derivatives (ee's up to 96%) but also 1,2-dihydro-napthalene derivatives and acyclic olefins (ee's up to 99%), which both constitute the most challenging substrates for this transformation.

Simple Ir-PHOX-based phosphite-oxazoline catalysts have been successfully applied in the asymmetric hydrogenation of cyclic β-enamides providing better enantioselectivities than previous effective Ru and Rh catalysts. This protocol allows the synthesis of 2-aminotetralines and 3-aminochromanes, key structural units found in many therapeutic agents and biologically active natural products, in high chemical yields and enantioselectivities (ee’s up to 99%). High enantioselectivities have also been achieved in the hydrogenation of cyclic α-enamides.

In this personal review, we present our efforts in the design of ligand libraries for the discovery of suitable metal catalysts for asymmetric hydrogenation and C-X bond-forming reactions.

Asymmetric transfer hydrogenation (ATH) has emerged as one of the most effective and sustainable synthetic tool for synthesizing enantiopure alcohols. Since Noyori’s group successfully applied Ru-catalysts modified with chiral β-amino alcohols or diamines as ligands, a large number of catalytic systems has been successfully developed. However, further improvement in terms of substrate scope, selectivity, and turnover frequency are required to make the process competitive with conventional hydrogenations. Overcoming these limitations requires research toward the design of new ligands. Such a task becomes easier if readily modulable chiral ligands are at hand. Sugar-based ligands are particularly useful for addressing this need. They are readily available, highly functionalized, and their modular constructions are easy. Series of chiral ligands can be screened in the search for high activities and selectivities for each type of substrate. This digest paper will discuss the progress on the use of sugar-based ligands in ATH reactions.

Excellent enantioselectivities are observed in palladium-catalyzed allylic substitutions of a wide range of substrate types and nucleophiles using a bidentate ligand composed of oxazoline and chirally flexible biaryl phosphite elements. This unusually wide substrate scope is shown by experimental and theoretical studies of its η3-allyl and η2-olefin complexes not to be a result of configurational interconversion of the biaryl unit, since the ligand in all reactions adopts an Sa,S configuration on coordination to palladium, but rather the ability of the ligand to adapt the size of the substrate-binding pocket to the reacting substrate. This ability also serves as an explanation to its excellent performance in other types of catalytic processes.

We report a reduced but structurally valuable phosphite/phosphinite-thioether ligand library for the Ir-hydrogenation of 40 minimally functionalized alkenes, including relevant examples with poorly coordinative groups. We found that enantiomeric excesses are mainly dependent on the substrate structure and on some ligand parameters (i.e., the type of thioether/phosphorus moieties and the configuration of the phosphite group), whereas the substituents of the biaryl phosphite moiety had little impact. By tuning the ligand parameters we were able to find highly selective catalysts for a range of substrates (ee’s up to 99%). These phosphite/phosphinite-thioether ligands have a simple backbone and thus yield simple NMR spectra that reduce signal overlap and facilitate the identification of relevant intermediates. Therefore, by combining HP-NMR spectroscopy and theoretical studies, we were also able to identify the catalytically competent Ir-dihydride alkene species, which made it possible to explain the enantio...

Artificial metalloenzymes combine the excellent selective recognition/binding properties of enzymes with transition metal catalysts, and therefore many asymmetric transformations can benefit from these entities. The search for new successful strategies in the construction of metal-enzyme hybrid catalysts has therefore become a very active area of research. This review discusses all the developed strategies and the latest advances in the synthesis and application in asymmetric catalysis of artificial metalloenzymes with future directions for their design, synthesis and application (Sections 2-4). Finally, advice is presented (to the non-specialist) on how to prepare and use artificial metalloenzymes (Section 5).

We have identified a readily accessible phosphinooxazoline-based phosphite-oxazoline catalytic system, (S)-4-isopropyl-2-{2-[(3,3′,5,5′-tetra-tert-butyl-1,1′-biphenyl-2,2′-diyl)phosphite]phenyl}-2-oxazoline (L1 a), that can hydroborate a range of 1,1-disubstituted aryl olefins with high enantioselectivity (up to 94 %), excellent yields and perfect regioselectivity. The new phosphite-oxazoline ligands efficiently hydroborate a broader range of olefins than previous phosphinooxazoline ligands. In particular, a wide range of α-tert-butylstyrenes can be hydroborated that bear aryl substituents with different electronic and steric properties, which complements previous results with N-heterocyclic copper catalysts, the only other system reported to date that has achieved these reactions.

A set of iridium(I) and iridium(III) complexes is reported with triazolylidene ligands that contain pendant benzoxazole, thiazole, and methyl ether groups as potentially chelating donor sites. The bonding mode of these groups was identified by NMR spectroscopy and X-ray structure analysis. The complexes were evaluated as catalyst precursors in transfer hydrogenation and in acceptorless alcohol oxidation. High-valent iridium(III) complexes were identified as the most active precursors for the oxidative alcohol dehydrogenation, while a low-valent iridium(I) complex with a methyl ether functionality was most active in reductive transfer hydrogenation. This catalyst precursor is highly versatile and efficiently hydrogenates ketones, aldehydes, imines, allylic alcohols, and most notably also unpolarized olefins, a notoriously difficult substrate for transfer hydrogenation. Turnover frequencies up to 260 h–1 were recorded for olefin hydrogenation, whereas hydrogen transfer to ketones and aldehydes reached maxim...

A novel series of modular amino acid thioamide ligands functionalized with carbohydrates were introduced and employed in the rhodium-catalyzed asymmetric transfer hydrogenation (ATH) of aryl alkyl ...

We have replaced the oxazoline group with a thiazoline moiety in one of the most successful of the phosphite–oxazoline ligand families for the Ir-catalyzed hydrogenation of minimally functionalized olefins. A small but structurally important library of Ir phosphite–thiazoline precatalysts (Ir-L1–L2 a–e) has been developed by changing the substituents/configurations at the biaryl phosphite group. We found that the replacement of the oxazoline with a thiazoline moiety in the ligand design is beneficial in terms of substrate scope.

The first successful application of phosphite-pyridine ligands in the Ir-catalyzed asymmetric hydrogenation of di- and trisubstituted olefins is developed.

We have expanded the ligand design of one of the most successful phosphite-oxazoline ligands, derived from hydroxyl amino acid derivatives, in Pd-catalyzed allylic alkylation by replacing the oxazoline group with a thiazoline moiety. These phosphite-thiazoline ligands have been evaluated in the Pd-catalyzed allylic substitution of a wide range of mono-, di-, and trisubstituted substrates with several carbon nucleophiles, such as the less studied α-substituted malonates and β-diketones. For comparison we have also expanded our previous work on phosphite-oxazoline to carbon nucleophiles other than dimethyl malonate and to the alkylation of trisubstituted substrates. For hindered substrates such as disubstituted rac-1,3-diphenyl-3-acetoxyprop-1-ene and trisubstituted rac-1,1,-diphenyl-1-hepten-3-yl acetate and rac-1,3,3-triphenylprop-2-enyl acetate, enantioselectivities (ee values up to >99 %) are best obtained with phosphite-oxazoline ligands, whereas for unhindered cyclic substrates such as rac-3-acetoxycyclohexene, rac-3-acetoxycyclopentene, and rac-3-acetoxycycloheptene, enantioselectivities (ee values up to 94 %) improved considerably with phosphite-thiazoline ligands. In the case of monosubstituted substrates, the regio- and enantioselectivities obtained with both ligand types were comparable and excellent (regioselectivities up to 90 % and ee values up to 95 %). Therefore, by correctly combining substrate and ligand type (phosphite-oxazoline or phosphite-thiazoline), we have identified one of the few catalytic systems that provide high regio- and enantioselectivities in both enantiomers of the alkylation product for a wide range of hindered and unhindered mono-, di-, and trisubstituted substrates with several carbon nucleophiles. We have also discussed the synthesis and characterization of Pd-π-allyl intermediates to provide greater insight into the origin of enantioselectivity in these catalytic systems.