Chiral Transition Metal Catalysts Research Articles

General Alkene 1,2-syn-Cyano-Hydroxylation Procedure Via Electrochemical Activation of Isoxazoline Cycloadducts.

Stereoselective alkene 1,2-difunctionalization is a privileged strategy to access three-dimensional C(sp3)-rich chiral molecules from readily available "flat" carbon feedstocks. State-of-the-art approaches exploit chiral transition metal-catalysts to enable high levels of regio- and stereocontrol. However, this is often achieved at the expense of a limited alkene scope and reduced generality. 1,3-Dipolar cycloadditions are routinely used to form heterocycles from alkenes with high levels of regioselectivity and stereospecificity. Nevertheless, methods for the ring-opening of cycloadducts to reveal synthetically useful functionalities require the use of hazardous reagents or forcing reaction conditions; thus limiting their synthetic applications. Herein, we describe the implementation of a practical, general and selective electrosynthetic strategy for olefin 1,2-syn-difunctionalization, which hinges on the design of novel reagents-consisting of a nitrile oxide 1,3-dipole precursor, equipped with a sulfonyl-handle. These can selectively difunctionalize alkenes via "click" 1,3-dipolar cycloadditions, and then facilitate the telescoped electrochemical single electron transfer activation of the ensuing isoxazoline intermediate. Cathodic reduction of the cycloadduct triggers a radical fragmentation pathway delivering sought-after stereodefined 1,2-syn-hydroxy nitrile derivatives. Our telescoped electrochemical procedure tolerates a wide range of functionalities, and─crucially─enables the difunctionalization of both electron-rich, electron-poor and unactivated olefins, with diverse degree of substitution; thus providing a robust, general and selective metal-free alternative to current alkene difunctionalization strategies. Capitalizing on these features, we employed our electrosynthetic method to enable the late-stage syn-hydroxy-cyanation of natural products and bioactive compounds, and streamline the de novo synthesis of pharmaceutical agents.

Chiral-at-Ruthenium Catalysts for Nitrene-Mediated Asymmetric C–H Functionalizations

ConspectusAsymmetric transition metal catalysis is an indispensable tool used both in academia and industry for forging chiral molecules in an enantioselective fashion. Its advancement relies in large part on the design and discovery of new chiral catalysts. In contrast to conventional endeavors of generating chiral transition metal catalysts from carefully tailored chiral ligands, the development of chiral transition metal catalysts containing solely achiral ligands (chiral-at-metal catalysts) has been neglected. This Account presents our recent work on the synthesis and catalytic applications of a new class of C2-symmetric chiral-at-ruthenium catalysts. These octahedral ruthenium(II) complexes are constructed from two achiral bidentate N-(2-pyridyl)-substituted N-heterocyclic carbene (PyNHC) ligands and two monodentate acetonitriles, and the dicationic complexes are typically complemented with two hexafluorophosphate anions. The chirality of these complexes originates from the helical cis-arrangement of the bidentate ligands, thereby generating a stereogenic metal center as the exclusive stereocenter in these complexes. The strong σ donor and π acceptor properties of the PyNHC ligands provide a strong ligand field that ensures a high constitutional and configurational inertness of the helical Ru(PyNHC)2 core, while at the same time, the trans-effect exerted by the σ-donating NHC ligands results in high lability of the MeCN ligands and, therefore, provides high catalytic activity. As a result, this chiral-at-ruthenium catalyst scaffold combines formidable structural robustness with high catalytic activity in a unique fashion. Asymmetric nitrene C-H insertion constitutes an efficient strategy for accessing chiral amines. The direct conversion of C(sp3)-H bonds into amine functionality circumvents the need for using functionalized starting materials. Our C2-symmetric chiral-at-ruthenium complexes display exceptionally high catalytic activity and excellent stereocontrol for various asymmetric nitrene C(sp3)-H insertion reactions. The ruthenium nitrene species can be generated from nitrene precursors, such as organic azides and hydroxylamine derivatives, which undergo ring-closing C-H aminations to afford chiral cyclic pyrrolidines, ureas, and carbamates in high yields and with excellent enantioselectivities at low catalyst loadings. Mechanistically, the turnover-determining C-H insertion is proposed to proceed in a concerted or stepwise fashion, depending on the nature of intermediate ruthenium nitrenes (singlet or triplet). Computational studies revealed that the stereocontrol originates from a better steric fit in combination with favorable catalyst/substrate π-π stacking effects for aminations at benzylic C-H bonds. In addition, we also present our research for exploring novel reaction patterns and reactivities of intermediate transition metal nitrenes. First, we discovered a novel chiral-at-ruthenium-catalyzed 1,3-migratory nitrene C(sp3)-H insertion to convert azanyl esters into nonracemic α-amino acids. Second, we found a chiral-at-ruthenium-catalyzed intramolecular C(sp3)-H oxygenation, thereby allowing for the construction of chiral cyclic carbonates and lactones via nitrene chemistry. We expect that our research program on catalyst development and reaction discovery will inspire the creation of novel types of chiral-at-metal catalysts and drive the development of new applications for nitrene-mediated asymmetric C-H functionalization reactions.

Metal Stereogenicity in Asymmetric Transition Metal Catalysis.

Chiral transition metal catalysts represent a powerful and economic tool for implementing stereocenters in organic synthesis, with the metal center providing a strong chemical activation upon its interaction with substrates or reagents, while the overall chirality of the metal complex achieves the desired stereoselectivity. Often, the overall chiral topology of the metal complex implements a stereogenic metal center, which is then involved in the origin of the asymmetric induction. This review provides a comprehensive survey of reported chiral transition metal catalysts in which the metal formally constitutes a stereocenter. A stereogenic metal center goes along with an overall chiral topology of the metal complex, regardless of whether the ligands are chiral or achiral. Implications for the catalyst design and mechanism of asymmetric induction are discussed for half-sandwich, tetracoordinated, pentacoordinated, and hexacoordinated chiral transition metal complexes containing a stereogenic metal center. The review distinguishes between chiral metal catalysts originating from the coordination to chiral ligands and those which are solely composed of optically inactive ligands (achiral or rapidly interconverting enantiomers) prior to complexation (dubbed "chiral-at-metal" catalysts).

Enantio- and Diastereodivergent Sequential Catalysis Featuring Two Transition-Metal-Catalyzed Asymmetric Reactions.

This study demonstrates the feasibility and inherent benefits of combining two distinct asymmetric transition-metal-catalyzed reactions in one pot. The reported transformation features a Pd-catalyzed asymmetric allylic alkylation and a Rh-catalyzed enantioselective 1,4-conjugate addition, effectively converting simple allyl enol carbonate precursors into enantioenriched cyclic ketones with two remote stereocenters. Despite the anticipated challenges associated with controlling stereoselectivity in such a complex system, the products are obtained in enantiomeric excesses ranging up to >99 % ee, exceeding those obtained from either of the individual asymmetric reactions. In addition, since the stereoselectivity of both steps is under catalyst control, this one-pot reaction is enantio- and diastereodivergent, enabling facile access to all stereoisomers from the same set of starting materials.

Coaxing carbenes into aliphatic amines

Aliphatic amines are popular motifs in biologically relevant molecules, so chemists have myriad tools for assembling C–N bonds. Methods that make those bonds enantioselectively are particularly valuable because they can produce chiral drugs and unnatural amino acids, which are useful for studying biology. While chiral transition-metal catalysts are effective for making aromatic amines, they fall short when it comes to making aliphatic amines. These compounds tend to coordinate to the metal, poisoning the catalyst and scrambling its ability to work enantioselectively. Researchers led by Nankai University’s Shou-Fei Zhu and Qi-Lin Zhou have managed to get around these problems by creating a reaction that uses two catalysts in tandem (Science 2019, DOI: 10.1126/science.aaw9939). The reaction (example shown) features an achiral metal catalyst—a homoscorpionate copper complex—which forms a carbene from a diazo ester. The homoscorpionate ligand protects the copper from another reactant, an aliphatic amine, which attacks the carbene to make a

Enantioselective C(sp 3 )‒H bond activation by chiral transition metal catalysts

Organic molecules are rich in carbon-hydrogen bonds; consequently, the transformation of C-H bonds to new functionalities (such as C-C, C-N, and C-O bonds) has garnered much attention by the synthetic chemistry community. The utility of C-H activation in organic synthesis, however, cannot be fully realized until chemists achieve stereocontrol in the modification of C-H bonds. This Review highlights recent efforts to enantioselectively functionalize C(sp3)-H bonds via transition metal catalysis, with an emphasis on key principles for both the development of chiral ligand scaffolds that can accelerate metalation of C(sp3)-H bonds and stereomodels for asymmetric metalation of prochiral C-H bonds by these catalysts.

Control of stereochemistry in atom transfer radical addition and step-growth radical polymerization by chiral transition metal catalysts

A series of chiral transition metal catalysts of ruthenium(II), iron(II), copper(I), and rhodium(I) were used to control the stereochemistry in the step-growth radical polymerization of 3-butenyl 2-chloropropionate (BCP) with both unconjugated CC and reactive C–Cl bonds in a single molecule and atom transfer radical addition (ATRA) as a model reaction for the polymerization. Among the various metal catalysts with chiral ligands, Ru2Cl4[(−)-DIOP]3 [DIOP=2,3-(isopropylidenedioxy)-2,3-dihydroxy-1,4-bis(diphenylphosphanyl)butane] induced asymmetric ATRA with 10% ee between CCl4 and 3-butenyl acetate (BAc), which has an unconjugated CC bond as a model olefin of the monomer. However, this ruthenium catalyst was not sufficiently active to induce ATRA between the C–Cl bond in methyl 2-chloropropionate (MCP), which is a model chloride for the monomer, and the CC bond in BAc, as well as the step-growth radical polymerization of BCP. Whereas chiral iron and copper catalysts, such as FeCl2/(−)-DIOP and CuCl/(−)-sparteine, efficiently allowed the ATRAs and the step-growth radical polymerization to result in the desired 1:1 adducts and oligomers, respectively, almost no optical activity was observed for the products. However, the diastereoselectivity was changed in both the ATRA and the polymerization by the metal catalysts with chiral or achiral ligands.

ChemInform Abstract: Dynamic Kinetic Resolution Based Asymmetric Transfer Hydrogenation of α‐Alkoxy‐β‐ketophosphonates. Diastereo‐ and Enantioselective Synthesis of Monoprotected 1,2‐Dihydroxyphosphonates.

Dynamic kinetic resolution driven, asymmetric transfer hydrogenation reactions of a wide range of 2-substituted α-alkoxy-β-ketophosphonates 3 were observed to proceed efficiently to give the corresponding 2-substituted α-alkoxy-β-hydroxy phosphonates 4 with excellent levels of diastereo- and enantioselectivity. These processes are promoted by using well-defined, commercially available, chiral transition metal catalysts and a 0.2:1 mixture of formic acid and triethylamine as the hydrogen source and solvent.

Dynamic Kinetic Resolution Based Asymmetric Transfer Hydrogenation of α-Alkoxy-β-Ketophosphonates. Diastereo- and Enantioselective Synthesis of Monoprotected 1,2-Dihydroxyphosphonates

Dynamic kinetic resolution driven, asymmetric transfer hydrogenation reactions of a wide range of 2-substituted α-alkoxy-β-ketophosphonates 3 were observed to proceed efficiently to give the corresponding 2-substituted α-alkoxy-β-hydroxy phosphonates 4 with excellent levels of diastereo- and enantioselectivity. These processes are promoted by using well-defined, commercially available, chiral transition metal catalysts and a 0.2:1 mixture of formic acid and triethylamine as the hydrogen source and solvent.

Advances in Catalyst Systems for the Asymmetric Hydrogenation and Transfer Hydrogenation of Ketones

Hydrogenation of prochiral ketones using chiral transition-metal catalysts represents the cleanest way to access enantiomerically enriched secondary alcohols, which are important building blocks in fine chemicals synthesis. Despite excellent activity, selectivity and compatibility of metal complexes with variety of functional groups, no universal catalysts exist. In this article we summarize the advances in catalyst systems for the asymmetric homogenous and heterogenous hydrogenation of ketones that have been made in past decade. The development of catalysts is oriented in reaching as high as activity with low catalyst loadings, using “greener’’ conditions, and ensuring good recyclability of catalysts. Even though ruthenium complexes represent the largest part of the catalysts, other metals rapidly penetrate this field.

超分子的アプローチによるキラル遷移金属錯体触媒の精密制御

超分子的アプローチによるキラル遷移金属錯体触媒の精密制御

Induced Axial Chirality in Biocatalytic Asymmetric Ketone Reduction

Catalytic asymmetric reduction of prochiral ketones of type 4-alkylidene cyclohexanone with formation of the corresponding axially chiral R-configurated alcohols (up to 99% ee) was achieved using alcohol dehydrogenases, whereas chiral transition-metal catalysts fail. Reversal of enantioselectivity proved to be possible by directed evolution based on saturation mutagenesis (up to 98% ee (S)). Utilization of ketone with a vinyl bromide moiety allows respective R- and S-alcohols to be exploited as key compounds in Pd-catalyzed cascade reactions.

Artificial Metalloenzymes as Catalysts in Stereoselective Diels–Alder Reactions

Numerous enzymes are useful catalysts in synthetic organic chemistry, but they cannot catalyze the myriad transition-metal-mediated transformations customary in daily chemical work. For this reason the concept of directed evolution of hybrid catalysts was proposed some time ago. A synthetic ligand/transition-metal moiety is anchored covalently or non-covalently to a host protein, thereby generating a single artificial metalloenzyme which can then be optimized by molecular biological methods. In the quest to construct an appropriate experimental platform for asymmetric Diels-Alder reactions amenable to this Darwinian approach to catalysis, specifically those not currently possible using traditional chiral transition-metal catalysts, two strategies have been developed which are reviewed here. One concerns the supramolecular anchoring of a Cu(II)-phthalocyanine complex to serum albumins; the other is based on the design of a Cu(II)-specific binding site in a thermostable protein host (tHisF), leading to 46-98% ee in a model Diels-Alder reaction. This sets the stage for genetic fine-tuning using the methods of directed evolution.

Diastereo‐ and Enantioselective Reductive Aldol Reaction with Trichlorosilane Using Chiral Lewis Bases as Organocatalysts

The catalytic enantioselective tandem reaction is an efficient synthetic methodology in which optically active compounds are assembled from simple prochiral substrates via two (or more) distinct catalytic processes taking place under the same conditions. The synthetic efficiency is enhanced by avoiding the time-intensive and yield-reducing isolation and purification of synthetic intermediates and by decreasing the amounts of chemicals and solvents used. The asymmetric catalytic reductive aldol reaction is an efficient tandem transformation involving conjugate reduction of a,b-unsaturated carbonyl compounds followed by aldol reaction of the enolate intermediate with aldehydes or ketones. Chiral transition-metal catalysts have been used to control the stereochemistry of these transformations. We recently reported that achiral phosphorus oxides function as Lewis base organocatalysts to promote both the conjugate reduction of enones with trichlorosilane and the reductive aldol reaction of enones with aldehydes. Herein we report that enantioselective catalysis of this tandem reaction by chiral Lewis bases provides good to high diastereoand enantioselectivities. Scheme 1 outlines the current catalytic method. Our previous study had shown that the Lewis base catalyzed conjugate reduction with trichlorosilane proceeds via a six-membered transition state with an enone in the s-cis conformation to give the (Z)-trichlorosilyl enolate exclusively. Therefore, high syn selectivity is expected for the subsequent aldol process, assuming that the reaction proceeds through a chair-like cyclic transition state. Moreover, high enantioselectivity could also be achieved by judicious selection of chiral Lewis base catalysts (LB*). We first examined various chiral Lewis base catalysts (Figure 1) for the reductive aldol reaction of chalcone (1a) and benzaldehyde (2a) with trichlorosilane at 78 8C (Table 1). With (S)-BINAPO, the reaction in dichloromethane gave aldol adduct 3a with respectable stereoselectivities (Table 1, entry 1). By simply changing the solvent from dichloromethane to propionitrile, both the stereoselectivities and chemical yield dramatically improved (Table 1, entry 2). Other Lewis base catalysts were then examined using this solvent (Table 1, entries 3–6). (R,R)-DIOPO showed a comparable activity to BINAPO to afford similar enantioselectivity with a slight loss of diastereoselectivity (Table 1, entry 3). Although structurally similar to BINAPO, (S)[a] Prof. Dr. M. Sugiura, N. Sato, Y. Sonoda, Prof. Dr. M. Nakajima Graduate School of Pharmaceutical Sciences Kumamoto University 5-1 Oe-honmachi, Kumamoto 862-0973 (Japan) Fax: (+81)96-362-7692 E-mail : msugiura@kumamoto-u.ac.jp (M. Sugiura) nakajima@gpo.kumamoto-u.ac.jp (M. Nakajima) [b] Prof. Dr. S. Kotani Priority Organization for Innovation and Excellence Kumamoto University 5-1 Oe-honmachi, Kumamoto 862-0973 (Japan) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/asia.200900450. Scheme 1. The enantioselective reductive aldol reaction with trichlorosilane catalyzed by a chiral Lewis base catalyst.

Continuous Enantioselective Hydrogenation with a Molecular Catalyst in Supported Ionic Liquid Phase under Supercritical CO2 Flow

The synthesis of enantiopure compounds using chiral transition-metal catalysts is largely dominated by batchwise procedures. [1] This typically results in considerable amounts of solvent waste from both reaction and purification steps, low space–time yields, and often precludes reuse of the precious catalyst. [2] The development of efficient and flexible flow systems could provide alternative modes of operation, combining the benefits of an integrated reaction and purification strategy with the molecular approach to catalyst design. Efficient immobilization of chiral catalysts in a liquid-like environment is an important prerequisite for the successful implementation of this strategy. [3] The concept of supported ionic liquid phase (SILP) catalysis describes a molecular catalyst that is dissolved in a small amount of an ionic liquid (IL) that is immobilized on the surface of a solid support, typically a porous oxidic material such as silica or alumina. Physisorption and capillary forces lead to surface coating and pore filling, yielding free-flowing powders that can be used in fixed or fluidized bed reactors. [4, 5] Due to a mean film thickness in the range of 10–20 �, catalysts dissolved in the supported layer are close to a large interface, and diffusion pathways are reduced in comparison to bulk biphasic systems, often leading to high reaction rates. [6–10] Despite their proximity to the surface, the molecular catalysts in the nanoliquid confinement of SILPs have been shown by kinetic analyses to perform in a genuinely homogeneous manner. [5–7] The ease of preparation and the modularity of SILP catalysts contribute to making them ideal candidates for continuous catalysis, combining many advantages of both homogeneous and heterogeneous systems. For a stable and efficient process, the choice of the mobile phase is decisive. SILP catalysts containing an organometallic complex have been applied successfully in continuous-flow catalysis involving gaseous substrates with moderate [11] to good stability. [12, 13] This approach is, however, restricted to volatile substrates and catalysts of sufficient thermal stability. Non-IL-miscible liquid phases would allow for milder reaction conditions and broaden the substrate scope to more interesting molecules, but usually lead to progressive desorption of the SILP by abrasion or gradual dissolution. [8, 14, 15] Cole-Hamilton and co-workers have recently demonstrated that supercritical carbon dioxide (scCO2) can be used as a mobile phase to transport nonvolatile substrates and products continuously over organometallic SILP catalysts for the hydroformylation of long-chain olefins. [16] By the combination of SILP catalysis with scCO2 flow, high activity paired with excellent stability could be achieved. [17] The infinitesimal solubility of ILs in scCO2 [18] allowed for retention of the SILP while organic products were continuously extracted. [19] A similar approach was recently described by the team of Iborra and Lozano for continuous-flow kinetic resolution of alcohols by using the lipase CAL-B in a SILP-type system and CO2 as the mobile phase. [20]

Recent advances in enantioselective [2 + 2 + 2] cycloaddition

Enantioselective cycloaddition using chiral transition metal catalysts is an atom-economical and efficient synthetic tool for the construction of chiral carbo- and heterocyclic skeletons. This short account discloses our recent results of inter- and intramolecular enantioselective [2 + 2 + 2] cycloadditions of alkyne and/or alkene moiety(ies). Chiral iridium complexes catalyzed the alkyne trimerization for the generation of axial chirality(ies), and chiral rhodium ones catalyzed alkyne-alkyne-alkene cyclization for the generation of a quaternary carbon including spirocyclic system.

Systematizing Structural Motifs and Nomenclature in 1,n′‐Disubstituted Ferrocene Peptides

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 200 leading journals. To access a ChemInform Abstract, please click on HTML or PDF.

イリジウム，ロジウム錯体を用いる触媒的付加環化反応の開発

Transition metal-catalyzed cycloaddition is an atom-economical and powerful synthetic tool for the construction of cyclic carbon skeletons. Various types of cycloadditions, including [2 + 2 + 1], [2 + 2 + 2], [4 + 2] cycloaddition, etc., have been reported. Their asymmetric versions using chiral transition metal catalysts have also been reported to give enantiomerically-enriched multi-cyclic compounds.First, an iridium-catalyzed enantioselective Pauson-Khand-type reaction is summarized. Pauson-Khand (-type) reaction is a [2 + 2 + 1] cycloaddition of an alkyne, alkene and carbon monoxide, and gives synthetically useful cyclopentenones. Rhodium- and iridium-catalyzed Pauson-Khand-type reactions using an aldehyde as a CO source were also mentioned. Second, two types of enantioselective [2 + 2 + 2] cycloadditions are described : One is an iridium-catalyzed [2 + 2 + 2] cycloaddition of diynes and monoalkynes for the synthesis of chiral teraryls with two axial chiralities. Another is a rhodium-catalyzed [2 + 2 + 2] cycloaddition of enynes and monoalkynes for the synthesis of bicyclic cyclohexa-1, 3-dienes with a chiral quaternary carbon center. Third, a rhodium-catalyzed enantioselective [2 + 2] cycloaddition of alkynes and alkenes for the synthesis of chiral cyclobutenes is mentioned.

Systematizing structural motifs and nomenclature in 1,n′-disubstituted ferrocene peptides

Ferrocene peptide conjugates display an array of structural features including helical ferrocene based chirality and a number of different intramolecular hydrogen bonding patterns. In this tutorial review we present a rigorous nomenclature for these systems, followed by a section that summarises and categorises the structures known to date. The issues discussed herein are of general relevance for all metallocene-based chiral transition metal catalysts and peptide turn mimetics.

Asymmetric 1,3‐Dipolar Cycloadditions of 2‐Diazocyclohexane‐1,3‐diones and Alkyl Diazopyruvates

AbstractThe 1,3‐dipolar cycloaddition reactions of 2‐diazocyclohexane‐1,3‐dione (7a; Table 1) and of alkyl diazopyruvates (11a–e; Table 3) to 2,3‐dihydrofuran and other enol ethers have been investigated in the presence of chiral transition metal catalysts. With RhII catalysts, the cycloadditions were not enantioselective, but those catalyzed by [RuIICl2(1a)] and [RuIICl2(1b)] proceeded with enantioselectivities of up to 58% and 74% ee, respectively, when diazopyruvates 11 were used as substrates. The phenyliodonium ylide 7c yielded the adduct 8a in lower yield and poorer selectivity than the corresponding diazo precursor 7a (Table 2) upon decomposition with [Ru(pybox)] catalysts. This suggests that ylide decomposition by RuII catalysts, contrary to that of the corresponding diazo precursors, does not lead to Ru‐carbene complexes as reactive intermediates. Our method represents the first reproducible, enantioselective 1,3‐cycloaddition of these types of substrates.