Hybrid bidentate phosphorus ligands in asymmetric catalysis: privileged ligand approach vs. combinatorial strategies

Biomacromolecule-Assisted Screening for Reaction Discovery and Catalyst Optimization

Reaction discovery and catalyst screening lie at the heart of synthetic organic chemistry. While there are efforts at de novo catalyst design using computation/artificial intelligence, at its core, synthetic chemistry is an experimental science. This review overviews biomacromolecule-assisted screening methods and the follow-on elaboration of chemistry so discovered. All three types of biomacromolecules discussed─enzymes, antibodies, and nucleic acids─have been used as "sensors" to provide a readout on product chirality exploiting their native chirality. Enzymatic sensing methods yield both UV-spectrophotometric and visible, colorimetric readouts. Antibody sensors provide direct fluorescent readout upon analyte binding in some cases or provide for cat-ELISA (Enzyme-Linked ImmunoSorbent Assay)-type readouts. DNA biomacromolecule-assisted screening allows for templation to facilitate reaction discovery, driving bimolecular reactions into a pseudo-unimolecular format. In addition, the ability to use DNA-encoded libraries permits the barcoding of reactants. All three types of biomacromolecule-based screens afford high sensitivity and selectivity. Among the chemical transformations discovered by enzymatic screening methods are the first Ni(0)-mediated asymmetric allylic amination and a new thiocyanopalladation/carbocyclization transformation in which both C-SCN and C-C bonds are fashioned sequentially. Cat-ELISA screening has identified new classes of sydnone-alkyne cycloadditions, and DNA-encoded screening has been exploited to uncover interesting oxidative Pd-mediated amido-alkyne/alkene coupling reactions.

Rh-Catalyzed Highly Enantioselective Hydrogenation of Functionalized Olefins with Chiral Ferrocenylphosphine-Spiro Phosphonamidite Ligands

A highly efficient Rh-catalyzed hydrogenation of functionalized olefins has been realized by a new family of highly rigid chiral ferrocenylphosphine-spiro phosphonamidite ligands. Excellent enantiocontrol (>99% ee in most cases) was achieved with a wide range of α-dehydroamino acid esters and α-enamides. This practicable catalytic system was further applied in the scalable synthesis of highly optically pure key intermediates of cinacalcet and d-phenylalanine.

Development of spirocyclic phosphoramidite-based hybrid diphosphorus ligands for enantioselective iridium-catalyzed hydrogenation of imines

Novel spirocyclic phosphine–phosphoramidite ligands for highly efficient and enantioselective Ir-catalyzed hydrogenation of various imines.

Diverse “roof shaped” chiral diamidophosphites: palladium coordination and catalytic applications

“Roof shaped” chiral diamidophosphites of various structures and denticities were obtained and tested in Pd-catalyzed asymmetric allylic substitution.

C 1-Symmetric diphosphorus ligands in metal-catalyzed asymmetric hydrogenation to prepare chiral compounds

Asymmetric hydrogenation has remained an important and challenging research area in industry as well as academia due to its high atom economy and ability to induce chirality. Among several types of ligands, chiral bidentate phosphine ligands have played a pivotal role in developing asymmetric hydrogenation. Although C₂-symmetric chiral bidentate phosphine ligands have dominated the field, it has been found that several C₁-symmetric ligands are equally effective and, in many cases, have outperformed their C₂-symmetric counterparts. This review evaluates the possibility of the use of C₁-symmetric diphosphorus ligands in asymmetric hydrogenation to produce chiral compounds. The recent strategies and advances in the application of C₁-symmetric diphosphorus ligands in the metal-catalyzed asymmetric hydrogenation of a variety of CC bonds have been summarized. The potential of diphosphorus ligands in asymmetric hydrogenation to produce pharmaceutical intermediates, bioactive molecules, drug molecules, agrochemicals, and fragrances is discussed. Although asymmetric hydrogenation appears to be a problem that has been resolved, a deep dive into the recent literature reveals that there are several challenges that are yet to be addressed. The current asymmetric hydrogenation methods mostly employ precious metals, which are depleting at a fast pace. Therefore, scientific interventions to perform asymmetric hydrogenation using base metals or earth-abundant metals that can compete with established precious metals hold significant potential.

Bicyclic Bridgehead Phosphoramidite-Based Hybrid Diphosphorus Ligands: Design, Synthesis, and Application in Catalytic Asymmetric Hydrogenation

A strategy for chiral ligand design has been developed that allows for incorporation of an achiral bicyclic bridgehead phosphoramidite to generate a class of hybrid diphosphorus ligands for high activity and asymmetric control. Using this concept, a series of chiral phosphine-phosphoramidite ligands bearing the sole chirality at the ligand backbone have been prepared and successfully employed in the Rh-catalyzed asymmetric hydrogenation of 2-vinylanilides for the synthesis of optically active anilines bearing an ortho-tertiary benzylic stereocenter.

Aromaticity-Driven Access to Cycloalkyl-Fused Naphthalenes

We report the efficient synthesis of cycloalkyl-fused naphthalenes through the [4 + 2]-cycloaddtion/decarboxylative aromatization of alkyne-tethered aryne insertion adducts. These scaffolds were difficult to synthesize using conventional reactions. The reaction proceeds via the formation of a benzopyrylium intermediate followed by intramolecular [4 + 2] cycloaddition and a subsequent decarboxylation pathway. This method is also compatible with allene-tethered substrates to afford similar products. In addition, the one-pot synthesis of polysubstituted naphthalenes via aryne insertion/benzannulation has also been developed in good yield.

Rh(I) Complexes in Catalysis: A Five-Year Trend

Rhodium is one of the most used metals in catalysis both in laboratory reactions and industrial processes. Despite the extensive exploration on "classical" ligands carried out during the past decades in the field of rhodium-catalyzed reactions, such as phosphines, and other common types of ligands including N-heterocyclic carbenes, ferrocenes, cyclopentadienyl anion and pentamethylcyclopentadienyl derivatives, etc., there is still lively research activity on this topic, with considerable efforts being made toward the synthesis of new preformed rhodium catalysts that can be both efficient and selective. Although the "golden age" of homogeneous catalysis might seem over, there is still plenty of room for improvement, especially from the point of view of a more sustainable chemistry. In this review, temporally restricted to the analysis of literature during the past five years (2015-2020), the latest findings and trends in the synthesis and applications of Rh(I) complexes to catalysis will be presented. From the analysis of the most recent literature, it seems clear that rhodium-catalyzed processes still represent a stimulating challenge for the metalloorganic chemist that is far from being over.

Phosphorus Ligands in Hydroformylation and Hydrogenation: A Personal Account

Metal-catalyzed hydroformylation and hydrogenation heavily rely on ligands, among which phosphorous ligands play a pivotal role. This personal account presents a selection of three distinct classes of phosphorous ligands, namely, monodentate meta-substituted phosphinites, bis-phosphites, and P-chiral supramolecular phosphines, developed in our group. The synthesis of these ligands, isolation, characterization, and their performance in transition metal-catalyzed hydroformylation, isomerizing hydroformylation, and asymmetric hydrogenation of olefins is summarized. The state of the art development in iron-catalyzed hydroformylation of alkenes and our contributions to the field is discussed. Use of phosphines enabled iron-catalyzed hydroformylation of alkenes under mild conditions. Thus, this account demonstrates the central role of phosphorus ligands in industrially relevant transformations such as hydrogenation and hydroformylation. The seemingly matured field of ligand discovery still holds significant potential and will steer the field of homogeneous catalysis.

Design of chiral ferrocenylphosphine-spiro phosphonamidite ligands for ruthenium-catalyzed highly enantioselective coupling of 1,2-diols with amines

A series of chiral ferrocene-backbone phosphines-spiro phosphonamidite ligands was developed for ruthenium-catalyzed enantioselective access to a broad range of β-amino alcohols from 1,2-diols and amines via the borrowing-hydrogen prciniple.

Chiral Phosphoric Acid-Catalyzed Enantioselective Phospha-Michael-Type Addition Reaction of Diarylphosphine Oxides with Alkenyl Benzimidazoles

An enantioselective phospha-Michael-type addition reaction of diarylphosphine oxides with alkenyl benzimidazoles was demonstrated using a chiral phosphoric acid as the chiral Brønsted acid catalyst. Addition products having phosphorus and benzimidazole units were formed in high yields with excellent enantioselectivities in most cases. The reduction of the phosphine oxide unit in the addition product afforded the corresponding chiral phosphine, which is a potential benzimidazole-based chiral P,N-ligand, without loss of enantiomeric excess.

P-Stereogenic N-Phosphine-Phosphite Ligands for the Rh-Catalyzed Hydrogenation of Olefins

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 for α-dehydroamino acid derivatives and α-enamides (ee's up to >99%) and promising results for the more challenging β-analogues (ee's up to 80%). The usefulness of these catalytic systems was further demonstrated with the synthesis of several valuable precursors for pharmacologically active compounds, with ee's at least as high as the best ones reported previously (up to >99%).

Diamidophosphites from β-hydroxyamides: readily assembled ligands for Pd-catalyzed asymmetric allylic substitution

Novel diamidophosphites based on β-hydroxyamides were prepared, and their individual and in situ formed complexes were tested in Pd-mediated allylations.

Highly Diastereo- and Enantioselective Ir-Catalyzed Hydrogenation of 2,3-Disubstituted Quinolines with Structurally Fine-Tuned Phosphine-Phosphoramidite Ligands

A highly diastereo- and enantioselective Ir-catalyzed hydrogenation of unfunctionalized 2,3-disubstituted quinolines, especially 3-alkyl-2-arylquinolines, has been realized. The success of this hydrogenation is ascribed to the use of a structurally fine-tuned chiral phosphine-phosphoramidite ligand with a (S_a)-3,3'-dimethyl H₈-naphthyl moiety and (R_c)-1-phenylethylamine backbone. The hydrogenation displayed broad functional group tolerance, thus furnishing a wide range of optically active 2,3-disubstituted tetrahydroquinolines in up to 96% ee and with perfect cis-diastereoselectivity.

Oxalamide-based bisdiamidophosphites: synthesis, coordination, and application in asymmetric metallocatalysis

A new group of P*-chiral bisdiamidophosphites has been developed for Pd- and Rh-catalyzed asymmetric reactions with good to excellent enantioselectivities.

Efficient modular phosphorus-containing ligands for stereoselective catalysis

Over several years, our research team has contributed to ligand design for metal-catalyzed stereoselective transformations. This has been achieved with the synthesis and application to organic transformations of interest of an array of structurally diverse P-containing ligands. These range from highly modular enantiopure phosphine–phosphite ligands to supramolecularly regulated enantioselective phosphorus-based catalysts. Our research in supramolecular interactions has also led to the discovery of an unprecedented halogen-bonded rhodium-catalyst.

Computationally Guided Catalyst Design in the Type I Dynamic Kinetic Asymmetric Pauson-Khand Reaction of Allenyl Acetates

The Rh(I)-catalyzed allenic Pauson-Khand reaction (APKR) is an efficient, redox-neutral method of synthesizing α-acyloxy cyclopentenones. An enantioselective APKR could provide access to chiral, nonracemic α-acyloxy and α-hydroxy cyclopentenones and their corresponding redox derivatives, such as thapsigargin, a cytotoxic natural product with potent antitumor activity. Rapid scrambling of axial chirality of allenyl acetates in the presence of Rh(I) catalysts enables the conversion of racemic allene to enantiopure cyclopentenone product in a dynamic kinetic asymmetric transformation (DyKAT). A combined experimental and computational approach was taken to develop an effective catalytic system to achieve the asymmetric transformation. The optimization of the denticity, and steric and electronic properties of the ancillary ligand (initially (S)-MonoPhos, 58:42 er), afforded a hemilabile bidentate (S)-MonoPhos-alkene-Rh(I) catalyst that provided α-acyloxy cyclopentenone product in up to 14:86 er. Enantioselectivity of the Rh(I)-(S)-MonoPhos-alkene catalyst was rationalized using ligand-substrate steric interactions and distortion energies in the computed transition states. This asymmetric APKR of allenyl acetates is a rare example of a Type I DyKAT reaction of an allene, the first example of DyKAT in a cyclocarbonylation reaction, and the first catalyst-controlled enantioselective APKR.

Origin of the chemical stability of phosphine-phosphoramidites: structural study of an UPPhos-type crystal and application of UPPhos in the asymmetric hydrogenation of imines

Phosphine-phosphoramidites (PPAs) are heterobidentate ligands that have been developed in the last two decades and have been used successfully in asymmetric catalytic reactions. A single crystal of the PPA (11bS)-N-[(2S,4S)-4-(diphenylphosphanyl)pentan-2-yl]-N-methyldinaphtho[2,1-d:1',2'-f][1,3,2]dioxaphosphepin-4-amine, C₃₈H₃₅NO₂P₂, was prepared and structurally characterized by single-crystal X-ray diffraction and density functional theory (DFT) calculations. Structure elucidation revealed unique features which might have a significant effect in the excellent chemical stability of this type of molecule. The conformation of the molecule provides an optimal chelating structure. Iridium complexes of UPPhos were found to be efficient catalysts in the asymmetric hydrogenation of imines {UPPhos is (11bS)-N-[(2S,4S)-4-(diphenylphosphanyl)pentan-2-yl]-N-(propan-2-yl)dinaphtho[2,1-d:1',2'-f][1,3,2]dioxaphosphepin-4-amine}.

Mini-ISES identifies promising carbafructopyranose-based salens for asymmetric catalysis: Tuning ligand shape via the anomeric effect

This study introduces new methods of screening for and tuning chiral space and in so doing identifies a promising set of chiral ligands for asymmetric synthesis. The carbafructopyranosyl-1,2-diamine(s) and salens constructed therefrom are particularly compelling. It is shown that by removing the native anomeric effect in this ligand family, one can tune chiral ligand shape and improve chiral bias. This concept is demonstrated by a combination of (i) x-ray crystallographic structure determination, (ii) assessment of catalytic performance, and (iii) consideration of the anomeric effect and its underlying dipolar basis. The title ligands were identified by a new mini version of the in situ enzymatic screening (ISES) procedure through which catalyst-ligand combinations are screened in parallel, and information on relative rate and enantioselectivity is obtained in real time, without the need to quench reactions or draw aliquots. Mini-ISES brings the technique into the nanomole regime (200 to 350 nmol catalyst/20 μml organic volume) commensurate with emerging trends in reaction development/process chemistry. The best-performing β-d-carbafructopyranosyl-1,2-diamine-derived salen ligand discovered here outperforms the best known organometallic and enzymatic catalysts for the hydrolytic kinetic resolution of 3-phenylpropylene oxide, one of several substrates examined for which the ligand is "matched." This ligand scaffold defines a new swath of chiral space, and anomeric effect tunability defines a new concept in shaping that chiral space. Both this ligand set and the anomeric shape-tuning concept are expected to find broad application, given the value of chiral 1,2-diamines and salens constructed from these in asymmetric catalysis.

Chiral phosphine-phosphoramidite ligands for highly enantioselective hydrogenation of N-arylimines

The asymmetric hydrogenation of N-arylimines with the chiral phosphine-phosphoramidite ligand, (Sc,Sa)-PEAPhos 2b, has been developed, in which high turnover numbers and excellent enantioselectivity were achieved.

Heterobimetallic catechol-phosphine complexes with palladium and a group-13 element: structural flexibility and dynamics

Group-13 metal acetylacetonates [M(acac)3] (M = Al, Ga, In) or Al(OiPr)3 react with a complex [Pd(catphosH)2] that may act as chelating ligand towards a second metal, or with a mixture of catechol phosphine (catphosH2) and [PdCl2(cod)], to give heterometallic complexes featuring either dinuclear M(catphos)2Pd or trinuclear M{(catphos)2Pd}2 motifs. Characterisation of the products by crystallographic and solution NMR studies gives insight into the structural diversity and flexibility of the coordination environments of the group-13 elements and their impact on the stability of the multinuclear complexes. The results indicate that gallium and indium are the most suitable elements for the stabilisation of di- and trinuclear assemblies, respectively. Dynamic NMR spectroscopy allowed to follow the dynamic averaging of the coordination environments of the four distinguishable catechol phosphines in the indium complex [M{(catphos)2Pd}2]H. The results revealed that the isomerisation follows a complicated pathway involving several distinguishable proton transfer steps, and allowed to propose a mechanistic explanation for the observed isomerisation.