Direct formation of tethered Ru(II) catalysts using arene exchange

Enantioselective Transfer Hydrogenation of α-Methoxyimino-β-keto Esters

α-Methoxyimino-β-keto esters are reported to undergo highly enantioselective catalytic transfer hydrogenation using the Noyori-Ikariya complex RuCl(p-cymene)[(S,S)-Ts-DPEN] in a mixture of formic acid-triethylamine and dimethylformamide at 25 °C. The experimental study performed on over 25 substrates combined with computational analysis revealed that a Z-configured methoxyimino group positioned alpha to a ketone carbonyl leads to higher reactivity and mostly excellent enantioselectivity within this substrate class. Density functional theory calculations of competing transition states were used in rationalizing the origins of enantioselectivity and the possible role of the methoxyimino group in the reaction outcome.

Dynamic Kinetic Resolution of β-Cyano α-Ketoesters via Asymmetric Transfer Hydrogenation

An efficient rhodium-catalyzed asymmetric transfer hydrogenation of β-cyano α-ketoesters via dynamic kinetic resolution has been developed. Despite the challenge posed by multiple functional groups, the reaction proceeded smoothly under mild conditions, generating versatile synthons with two adjacent stereocenters in high yields with excellent enantio- and diastereoselectivities. Furthermore, the power of this strategy is highlighted by the scale-up reaction and the follow-up synthesis of cytoxazone and paclitaxel intermediates.

Highly efficient synthesis of enantioenriched vicinal halohydrins via Ir-catalyzed asymmetric hydrogenation using dynamic kinetic resolution

A novel synthetic route was developed for the construction of chiral cis-vicinal halohydrins derivatives through Ir/f-phamidol-catalysed asymmetric hydrogenation of corresponding α-halogenated ketones with high yields (up to 99% yield), excellent diastereoselectivities (>20 : 1 dr), enantioselectivities (up to 99% ee), and high substrate catalyst ratio (S/C = 1000).

In-cell Catalysis by Tethered Organo-Osmium Complexes Generates Selectivity for Breast Cancer Cells

Anticancer agents that exhibit catalytic mechanisms of action offer a unique multi-targeting strategy to overcome drug resistance. Nonetheless, many in-cell catalysts in development are hindered by deactivation by endogenous nucleophiles. We have synthesised a highly potent, stable Os-based 16-electron half-sandwich ('piano stool') catalyst by introducing a permanent covalent tether between the arene and chelated diamine ligand. This catalyst exhibits antiproliferative activity comparable to the clinical drug cisplatin towards triple-negative breast cancer cells and can overcome tamoxifen resistance. Speciation experiments revealed Os to be almost exclusively albumin-bound in the extracellular medium, while cellular accumulation studies identified an energy-dependent, protein-mediated Os accumulation pathway, consistent with albumin-mediated uptake. Importantly, the tethered Os complex was active for in-cell transfer hydrogenation catalysis, initiated by co-administration of a non-toxic dose of sodium formate as a source of hydride, indicating that the Os catalyst is delivered to the cytosol of cancer cells intact. The mechanism of action involves the generation of reactive oxygen species (ROS), thus exploiting the inherent redox vulnerability of cancer cells, accompanied by selectivity for cancerous cells over non-tumorigenic cells.

Hydrogen-Bonding Ability of Noyori-Ikariya Catalysts Enables Stereoselective Access to CF3-Substituted syn-1,2-Diols via Dynamic Kinetic Resolution

Stereopure CF3-substituted syn-1,2-diols were prepared via the reductive dynamic kinetic resolution of the corresponding racemic α-hydroxyketones in HCO2H/Et3N. (Het)aryl, benzyl, vinyl, and alkyl ketones are tolerated, delivering products with ≥95% ee and ≥87:13 syn/anti. This methodology offers rapid access to stereopure bioactive molecules. Furthermore, DFT calculations for three types of Noyori-Ikariya ruthenium catalysts were performed to show their general ability of directing stereoselectivity via the hydrogen bond acceptor SO2 region and CH/π interactions.

POLITAG-M-F as Heterogeneous Organocatalyst for the Waste-Minimized Synthesis of β-Azido Carbonyl Compounds in Batch and under Flow Conditions

We herein report a waste minimization protocol for the β-azidation of α,β-unsaturated carbonyl compounds using TMSN3. The selection of the appropriate catalyst (POLITAG-M-F), in combination with the reaction medium, resulted in enhanced catalytic efficiency and a low environmental footprint. The thermal and mechanical stability of the polymeric support allowed us to recover the POLITAG-M-F catalyst for up to 10 consecutive runs. The CH3CN:H2O azeotrope has a 2-fold positive effect on the process, increasing the efficiency of the protocol and minimizing waste generation. Indeed, the azeotropic mixture, used as a reaction medium and for the workup procedure, was recovered by distillation, leading to an easy and environmentally friendly procedure for product isolation in high yield and with a low E-factor. A comprehensive evaluation of the environmental profile was performed by the calculation of different green metrics (AE, RME, MRP, 1/SF) and a comparison with other literature available protocols. A flow protocol was defined to scale-up the process, and up to 65 mmol of substrates were efficiently converted with a productivity of 0.3 mmol/min.

The Ir-Catalyzed Asymmetric Hydrogenation of α-Halogenated Ketones Utilizing Cinchona-Alkaloid-Derived NNP Ligand to Produce (R)- and (S)-Halohydrins

The asymmetric hydrogenation of α-halogenated ketones with iridium catalyst was developed, utilizing easily accessed cinchona-alkaloid-based NNP ligands. Various α-chloroacetophenones, heterocyclic thienyl and furanyl substrates, and even bromoketones were completely converted to the desired chiral halohydrins by this protocol. Both (R)- and (S)-chiral halohydrins can be prepared by changing the configurations of the chiral ligand NNP with up to 99.6% ee (enantiomeric excess) and 98.8% ee, respectively. Also, a gram-scale experiment was carried out efficiently.

Heterocycle-containing Noyori-Ikariya catalysts for asymmetric transfer hydrogenation of ketones

The synthesis of a range of N-(heterocyclesulfonyl)-functionalised Noyori-Ikariya catalysts is described. The complexes were prepared through a short sequence from C2-symmetric 1,2-diphenylethylene-1,2-diamine (DPEN) and were characterised by a range of methods including X-ray crystallography. The complexes were active catalysts for the asymmetric transfer hydrogenation (ATH) of a range of acetophenone derivatives, giving products of high ee in most cases, with notably good results for ortho-substituted acetophenones.

Ru-TsDPEN catalysts and derivatives in asymmetric transfer hydrogenation reactions

This review summarizes current developments, novel synthetic routes for Ruthenium tethered chiral catalyst, and its derivatives along with its application in asymmetric synthesis. The review also covers derivatization in tethering unit, modification in N-monofunctionalized ligand as well as ligation of other ligand with Ru metal in chiral catalyst. Apparently, the effect of a modified tethered catalyst in the enantioselective synthesis of chiral products as well as in synthetic chemistry is also discussed in detail.

Azeotropes as Powerful Tool for Waste Minimization in Industry and Chemical Processes

Aiming for more sustainable chemical production requires an urgent shift towards synthetic approaches designed for waste minimization. In this context the use of azeotropes can be an effective tool for “recycling” and minimizing the large volumes of solvents, especially in aqueous mixtures, used. This review discusses the implementation of different kinds of azeotropic mixtures in relation to the environmental and economic benefits linked to their recovery and re-use. Examples of the use of azeotropes playing a role in the process performance and in the purification steps maximizing yields while minimizing waste. Where possible, the advantages reported have been highlighted by using E-factor calculations. Lastly azeotrope potentiality in waste valorization to afford value-added materials is given.

Escaping from Flatland: Stereoconvergent Synthesis of Three-Dimensional Scaffolds via Ruthenium(II)-Catalyzed Noyori-Ikariya Transfer Hydrogenation

Noyori-Ikariya-type ruthenium(II)-catalysts for asymmetric transfer hydrogenation (ATH) have been known for 25 years and have proved as a well-behaved and user-friendly platform for the synthesis of chiral secondary alcohols. A progress has been made in the past five years in understanding the asymmetric reduction of complex ketones, where up to four stereocenters can be controlled in a single chemical transformation. Intriguing multi-chiral molecular architectures are therefore available in few well understood and robust synthetic steps from commercially available building blocks and possess handles for additional functionalization. The aim of this Review is to showcase the availability of three-dimensional scaffolds and homochiral lead-like compounds via ATH and inspire their direct use in drug discovery endeavors. Basic mechanistic insights are provided to demystify the stereo-chemical outcomes, as well as examples of diastereoselective transformations of enantiopure alcohols to give a feeling of how these rigid non-planar molecules can be further elaborated.

Readily accessible sp3-rich cyclic hydrazine frameworks exploiting nitrogen fluxionality

Increased molecular complexity correlates with improved chances of success in the drug development process. Here, a strategy for the creation of sp3-rich, non-planar heterocyclic scaffolds suitable for drug discovery is described that obviates the need to generate multiple stereogenic centers with independent control. Asymmetric transfer hydrogenation using a tethered Ru-catalyst is used to efficiently produce a range of enantiopure cyclic hydrazine building blocks (up to 99% ee). Iterative C-N functionalization at the two nitrogen atoms of these compounds produces novel hydrazine and hydrazide based chemical libraries. Wide chemical diversification is possible through variation in the hydrazine structure, use of different functionalization chemistries and coupling partners, and controlled engagement of each nitrogen of the hydrazine in turn. Principal Moment of Inertia (PMI) analysis of this small hydrazine library reveals excellent shape diversity and three-dimensionality. NMR and crystallographic studies confirm these frameworks prefer to orient their substituents in three-dimensional space under the control of a single stereogenic center through exploitation of the fluxional behavior of the two nitrogen atoms.

Enantioselective synthesis of chiral multicyclic γ-lactones via dynamic kinetic resolution of racemic γ-keto carboxylic acids

Ru-Catalyzed asymmetric transfer hydrogenation of γ-keto carboxylic acids has been achieved, affording chiral multicyclic γ-lactones in high yields with excellent diastereo- and enantioselectivities.

Accessible Bifunctional Oxy-Tethered Ruthenium(II) Catalysts for Asymmetric Transfer Hydrogenation

A concise synthesis of new oxy-tethered ruthenium complexes effective for the asymmetric transfer hydrogenation of aromatic ketones is described. The oxy-tether was constructed via a defluorinative etherification arising from an intramolecular nucleophilic substitution of a perfluorinated phenylsulfonyl substituent. The obtained tethered complexes exhibited desirable catalytic activity and selectivity, especially in the asymmetric transfer hydrogenation of functionalized aromatic ketones. The robustness and rigidity of the tether contribute to their superior catalytic performance relative to the nontethered prototype complex.

A convenient enantioselective CBS-reduction of arylketones in flow-microreactor systems

A convenient, versatile, and green CBS-asymmetric reduction of aryl and heteroaryl ketones has been developed by using the microreactor technology. The study demonstrates that it is possible to handle borane solution safely within microreactors and that the reaction performs well using 2-MeTHF as a greener solvent.

Highly enantioselective transfer hydrogenation of racemic α-substituted β-keto sulfonamides via dynamic kinetic resolution

An efficient enantioselective transfer hydrogenation of racemic α-substituted β-keto sulfonamides via dynamic kinetic resolution has been achieved, affording α-substituted β-hydroxyl sulfonamides in good yields and excellent diastereo- and enantioselectivities.

Asymmetric Transfer Hydrogenation of 1,3-Alkoxy/Aryloxy Propanones Using Tethered Arene/Ru(II)/TsDPEN Complexes

A series of propanones containing combinations of aryloxy and alkoxy substituents at the 1- and 3-positions were reduced to the alcohols via asymmetric transfer hydrogenation using a tethered Ru(II)/TsDPEN catalyst. The enantioselectivities of the reductions reveal a complex pattern of electronic and steric effects which, when used in a matched combination, can lead to the formation of products of up to 68% ee (84:16 er) from this highly challenging class of substrate.

A Tethered Ru-S Complex with an Axial Chiral Thiolate Ligand for Cooperative Si-H Bond Activation: Application to Enantioselective Imine Reduction

An axial chiral version of the 2,6-dimesitylphenyl group attached to sulfur is reported. Its multistep preparation starts from (S)-binol, and the thiol group is established by a racemization-free thermal Newman-Kwart rearrangement. The new chiral thiolate ligand decorated with one mesityl group is used in the synthesis of a tethered ruthenium chloride complex. Its spectroscopic characterization revealed solvent-dependent epimerization at the ruthenium center. The major diastereomer is crystallographically characterized. Chloride abstraction with tetrakis[3,5-bis(trifluoromethyl)phenyl]borate (NaBAr^F₄ ) yields the corresponding coordinatively unsaturated ruthenium complex with the Ru-S bond exposed. Si-H bond activation at this Ru-S bond proceeds in syn fashion but with moderate facial selectivity (d.r.=70:30), generating diastereomeric chiral-at-ruthenium hydrosilane adducts. Their application to catalytic imine hydrosilylation led to promising enantioinduction (40 % ee), thereby providing proof of concept for asymmetric catalysis involving cooperative Si-H bond activation.

Asymmetric hydrogenation of α-hydroxy ketones with an iridium/f-amphox catalyst: efficient access to chiral 1,2-diols

We successfully applied f-amphox ligands to the Ir-catalyzed asymmetric hydrogenation of α-hydroxy ketones to afford chiral 1,2-diols with excellent results.

Synthesis of Tetrahydroisoquinoline Alkaloids and Related Compounds through the Alkylation of Anodically Prepared α-Amino Nitriles

α-Amino nitrile 2a was conveniently prepared in two individual steps from chiral hexafluorophosphate salt isoquinolinium (-)-8b including anodic cyanation as an efficient means to activate the sp(3) C1-H bond of the THIQ nucleus. The lithiation of 2a was carried out in THF at -80 °C in the presence of LDA to produce a stable α-amino carbanion which was condensed on a large variety of alkyl halides. The resulting quaternary α-amino nitriles were subjected to a stereoselective reductive decyanation in ethanol in the presence of NaBH4 as the hydride donor to yield N-Boc-1-alkyl-THIQs (+)-10a-g in up to 97:3 er's after removal of the chiral auxiliary group. Examination of the ORTEP view of THIQ (+)-1f revealed that the newly created stereogenic center had an absolute S configuration. Likewise, (-)-xylopinine was synthesized in four workup steps in an overall 63% yield from α-amino nitrile (+)-2b. In this process, crystallization of an enantioenriched mixture (90:10) of (-)-norlaudanosine with 1 equiv of (-)-N-acetyl-l-leucine afforded the leucinate salt (+)-13 (99:1 dr). Similarly, (+)-salsolidine was displaced from its (-)-DBTA salt (-)-12 in 99:1 er, which was determined by proton and carbon NMR spectroscopy in the presence of thiophosphinic acid (+)-14 as the chiral solvating agent.

γ-Sultam-cored N,N-ligands in the ruthenium(ii)-catalyzed asymmetric transfer hydrogenation of aryl ketones

Excellent enantioselectivities were obtained in the asymmetric transfer hydrogenation of benzo-fused ketones employing Ru(ii)-[syn-6]; dynamic kinetic resolution of rac-S7 was realized.

Easy To Synthesize, Robust Organo-osmium Asymmetric Transfer Hydrogenation Catalysts

Asymmetric transfer hydrogenation (ATH) is an important process in organic synthesis for which the Noyori-type Ru(II) catalysts [(arene)Ru(Tsdiamine)] are now well established and widely used. We now demonstrate for the first time the catalytic activity of the osmium analogues. X-ray crystal structures of the 16-electron Os(II) catalysts are almost identical to those of Ru(II). Intriguingly the precursor complex was isolated as a dichlorido complex with a monodentate amine ligand. The Os(II) catalysts are readily synthesised (within 1 h) and exhibit excellent enantioselectivity in ATH reactions of ketones.

Asymmetric transfer hydrogenation of α-amino β-keto ester hydrochlorides through dynamic kinetic resolution

The development of Ru-catalyzed asymmetric transfer hydrogenation of α-amino β-keto ester hydrochlorides is described.