The synthesis of a new class of chiral pincer ligands and their applications in enantioselective catalytic fluorinations and the Nozaki-Hiyama-Kishi reaction

Copper-Catalyzed Enantioconvergent Radical N-Alkylation of Diverse (Hetero)aromatic Amines

The 3d transition metal-catalyzed enantioconvergent radical cross-coupling provides a powerful tool for chiral molecule synthesis. In the classic mechanism, the bond formation relies on the interaction between nucleophile-sequestered metal complexes and radicals, limiting the nucleophile scope to sterically uncongested ones. The coupling of sterically congested nucleophiles poses a significant challenge due to difficulties in transmetalation, restricting the reaction generality. Here, we describe a probable outer-sphere nucleophilic attack mechanism that circumvents the challenging transmetalation associated with sterically congested nucleophiles. This strategy enables a general copper-catalyzed enantioconvergent radical N-alkylation of aromatic amines with secondary/tertiary alkyl halides and exhibits catalyst-controlled stereoselectivity. It accommodates diverse aromatic amines, especially bulky secondary and primary ones to deliver value-added chiral amines (>110 examples). It is expected to inspire the coupling of more nucleophiles, particularly challenging sterically congested ones, and accelerate reaction generality.

3d Metal Complexes in T-shaped Geometry as a Gateway to Metalloradical Reactivity

ConspectusLow-valent, low-coordinate 3d metal complexes represent a class of extraordinarily reactive compounds that can act as reagents and catalysts for challenging bond-activation reactions. The pursuit of these electron-deficient metal complexes in low oxidation states demands ancillary ligands capable of providing not only energetic stabilization but also sufficiently high steric bulk at the metal center. From this perspective, pincer ligands are particularly advantageous, as their prearranged, meridional coordination mode scaffolds the active center while the substituents of the peripheral donor atoms provide effective steric shielding for the coordination sphere. In a T-shaped geometry, the transition metal complexes possess a precisely defined vacant coordination site, which, combined with the often observed high-spin electron configuration, exhibits unusually high selectivity of these compounds with respect to one-electron redox chemistry. In light of the intractable reaction pathways typically observed with related electronically unsaturated 3d transition metal complexes, the pincer coordination mode enables the isolation of low-valent compounds with more controlled and unique reactivity. We have thus investigated a series of T-shaped metal(I) complexes using three different types of pincer ligands, which may be regarded as "metalloradicals" due to their selectively exposed unpaired electrons.These compounds display remarkably high thermal stability and represent rarely observed "naked" monovalent metal species featuring both monomeric and dimeric structures. Extensive reactivity studies using various organic substrates highlight a strong tendency of these paramagnetic compounds to undergo one-electron oxidation, leading to the isolation of a plethora of metal(II) species with reduced organic ligands as unusual structural elements. The exploration of C₂ symmetric T-shaped Ni(I) complexes as asymmetric catalysts also shows success in enantioselective hydrodehalogenation of geminal dihalogenides. In addition, this specific class of low-valent, low-coordinate complexes can be further diversified by introducing redox-active pincer ligands or building homobimetallic systems with two T-shaped units.This Account focuses on the discussion of selected examples of iron, cobalt, and nickel pincer complexes bearing a [P,N,P] or [N,N,N] donor set; however, their electronic structure and radical-type reactivity can be broadly extended to other pincer systems. The availability of various types of pincer ligands should allow fine-tuning of the reactivity of the T-shaped complexes. Given the unprecedented reactivity observed with these compounds, we expect the studies of T-shaped 3d metal complexes to be a fertile field for advancing base metal catalysis.

Catalytic Enantioconvergent Allenylation of Aldehydes with Propargyl Halides

α-Allenol is a versatile synthon in organic synthesis. The catalytic asymmetric synthesis of α-allenols from readily available starting materials remains a prominent challenge, especially when simultaneous control over axial and central chirality is required. Herein, we describe the Cr-catalyzed enantioconvergent allenylation of aldehydes with racemic propargyl halides to rapidly access a wide range of chiral α-allenols with adjacent axial and central chiralities. This method features excellent regio-, diastereo- and enantioselectivity control with broad substrate scope, and provides facile access to all four stereoisomers when allied with a Mitsunobu reaction. Preliminary mechanistic studies support radical-based reaction pathways. The synthetic utility is demonstrated by the application in late-stage functionalization and the formal total synthesis of (+)-varitriol.

Applications of iron pincer complexes in hydrosilylation reactions

Due to its abundance, low cost and low toxicity, the first-row transition metal, iron is widely preferred as a catalyst in organic synthesis. The only drawback of lower selectivity due to high reactivity and low stability of the metal centre is tuned by using pincer ligands of different types. The different iron pincer complexes thus prepared are extensively used in catalyzing different types of organic reactions with great selectivity and functional group tolerance under moderate reaction conditions. In this review, we focus on the applications of iron pincer complexes in hydrosilylation reactions, especially the hydrosilylation of carbonyl derivatives and alkene/alkynes.

Iron pincer complexes are efficient in catalyzing various organic reactions with excellent selectivity and functional group tolerance at moderate reaction conditions. This review focuses on the applications of iron pincer complexes in hydrosilylation reactions.

Cobalt-Catalysed Asymmetric Addition and Alkylation of Secondary Phosphine Oxides for the Synthesis of P-Stereogenic Compounds

The catalytic asymmetric synthesis of P-chiral phosphorus compounds is an important way to construct P-chiral ligands. Herein, we report a new strategy that adopts the pyridinyl moiety as the coordinating group in the cobalt-catalysed asymmetric nucleophilic addition/alkylation of secondary phosphine oxides. A series of tertiary phosphine oxides were generated with up to 99 % yield and 99.5 % ee, and with broad functional-group tolerance. Mechanistic studies reveal that (R)-secondary phosphine oxides preferentially interact with the cobalt catalysts to produce P-stereogenic compounds.

Stereoselective synthesis of (Z)-1,3-bis(α,β-unsaturated carbonyl)-isoindolines from aldehydes and phenacyl azides under metal free conditions

Here in the present manuscript, we report our observation of an unprecedented stereoselective synthesis of 2H-isoindolin-1,3-ylidenes from 2-(formylphenyl)acrylates and phenacylazide in the presence of piperidine. Unlike in our previous findings, in which we accessed 3-keto-isoquinolines from the same starting materials under slightly modified reaction conditions, this unexpected one-pot tandem reaction allows an efficient and simple method to access a variety of highly functionalized ethyl (Z)-2-((Z)-3-(2-oxo-2-arylethylidene)-2,3-dihydro-1H-benzo[e]isoindol-1-ylidene)-acetates in very good to excellent yields (up to 91%). The present methodology is compatible with a wide variety of functional groups.

Chiral tridentate bis(oxazol-2-ylimino) isoindoline-based pincer ligands: isolation and characterization via deligation from in situ prepared Cd-ligand complexes

The first isolation and structural characterization of a series of chiral trinitrogen 1,3-bis(4,5-dihydrooxazol-2-ylimino)isoindoline-based pincer ligands are reported. Cadmium complexes, isolated as Cd(L2X)2 where L2X is the deprotonated form of L2XH = 1,3-bis(4,5-dihydro-4-(R)-phenyloxazol-2-ylimino)-isoindoline ((R,R)-5H) or 1,3-bis(4,5-dihydro-4-(S)-iso-propyloxazol-2-ylimino)isoindoline ((S,S)-6H) were prepared in situ via traditional or microwave-based techniques with the latter being more efficient but less able to be scaled up at this time. Ligands (R,R)-5H and (S,S)-6H were isolated via deligation from their respective cadmium complexes using a thiol-based ligand exchange protocol. The characterization of ligands and their respective cadmium complexes, in both the solid (X-ray crystallography) and solution (NMR spectroscopy) states are reported. Pd((S,S)-6)(OAc) is reported as a proof-of-concept of the ability to prepare 1 : 1 ligand to metal ratio complexes that are believed to be necessary as potential enantioselective catalysts.

Recent Strategies for Carbon-Halogen Bond Formation Using Nickel

Nickel catalysis has demonstrated the capability of performing a broad range of synthetically challenging transformations over the last decade. Though recent literature has focused on the formation of C-C and C-N bonds, a variety of breakthroughs in the field of C-X bond generation have also been reported. A diverse range of strategies using nickel have been developed, in an effort to expand the scope and synthetic utility of these halogenation methods. This Minireview will cover six emerging strategies in this field including: oxidatively induced C-X reductive elimination, triflate-to-halogen exchange reactions, directed C-H halogenation, non-directed electrophilic C-H halogenation of arenes, enantioselective α-fluorination of carbonyl containing compounds, and 1,2-difunctionalization-halogenation reactions. The final section has been split into two parts: nickel-catalyzed hydrohalogenation and nickel-catalyzed carbohalogenation reactions.

Further Developments and Applications of Oxazoline-Containing Ligands in Asymmetric Catalysis

The chiral oxazoline motif is present in many ligands that have been extensively applied in a series of important metal-catalyzed enantioselective reactions. This Review aims to provide a comprehensive overview of the most significant applications of oxazoline-containing ligands reported in the literature starting from 2009 until the end of 2018. The ligands are classified not by the reaction to which their metal complexes have been applied but by the nature of the denticity, chirality, and donor atoms involved. As a result, the continued development of ligand architectural design from mono(oxazolines), to bis(oxazolines), to tris(oxazolines) and tetra(oxazolines) and variations thereof can be more easily monitored by the reader. In addition, the key transition states of selected asymmetric transformations will be given to illustrate the features that give rise to high levels of asymmetric induction. As a further aid to the reader, we summarize the majority of schemes with representative examples that highlight the variation in % yields and % ees for carefully selected substrates. This Review should be of particular interest to the experts in the field but also serve as a useful starting point to new researchers in this area. It is hoped that this Review will stimulate both the development/design of new ligands and their applications in novel metal-catalyzed asymmetric transformations.

Iron- and Cobalt-Catalyzed Asymmetric Hydrofunctionalization of Alkenes and Alkynes

Transition metal catalyzed asymmetric hydrofunctionalization of readily available unsaturated hydrocarbons presents one of the most straightforward and atom-economic protocols to access valuable optically active products. For decades, noble transition metal catalysts have laid the cornerstone in this field, on account of their superior reactivity and selectivity. In recent years, from an economical and sustainable standpoint, first-row, earth-abundant transition metals have received considerable attention, due to their high natural reserves, affordable costs, and low toxicity. Meanwhile, the earth-abundant metal catalyzed hydrofunctionalization reactions have also gained much interest and been investigated gradually. However, since chiral ligand libraries for earth-abundant transition-metal catalysis are limited to date, the development of highly enantioselective versions remains a significant challenge.This Account summarizes our recent efforts in developing suitable chiral ligands for iron and cobalt catalysts and their applications in the highly enantioselective hydrofunctionalization reactions (hydroboration and hydrosilylation) of alkenes and alkynes. In ligand design, we envisioned that chiral unsymmetric NNN-tridentate (UNT) ligand scaffolds could promote these enantioselective transformations with earth-abundant metals. Therefore, several types of chiral UNT ligands were designed and prepared in our laboratory, utilizing readily available natural amino acids as chiral sources. In the very beginning, chiral oxazoline iminopyridine (OIP) ligands were proposed and investigated through the rational combination of nitrogen-containing ligand scaffolds. After a systematic survey of the ligand effects, the imine moiety in the rigid OIP ligands was replaced by a conformationally more flexible amine unit, leading to the construction of reactive oxazoline aminoisopropylpyridine (OAP) ligands. Subsequently, imidazoline iminopyridine (IIP) and thiazoline iminopyridine (TIP) ligands were prepared by altering the oxygen atom of oxazoline with nitrogen and sulfur linkers, respectively. To further expand the chiral ligand library, other tridentate ligands containing a twisted pincer, anionic, and nonrigid backbone were also designed and synthesized, including iminophenyl oxazolinyl phenylamine (IPOPA) and imidazoline phenyl picolinamide (ImPPA). The efficacy of these chiral UNT ligands for asymmetric induction in iron and cobalt catalysis has been demonstrated through asymmetric hydrofunctionalization of alkenes and asymmetric sequential hydrofunctionalization of alkynes, which exhibit excellent reactivity as well as high chemo-, regio-, and stereoselectivity with broad functional group tolerance. Notably, highly regio- and enantioselective hydrofunctionalization of challenging substrates, such as 1,1-disubstituted aryl alkenes and terminal aliphatic alkenes, was also achieved. Furthermore, the development of asymmetric sequential isomerization/hydroboration of internal alkenes and sequential hydrofunctionalization of alkynes further demonstrates the synthetic power of these catalytic systems. The chiral enantioenriched products obtained by these methodologies could be potentially utilized in organic synthesis, medicinal chemistry, and materials science. We believe that our continuous efforts in this field would be beneficial to the development of asymmetric earth-abundant metal catalysis.

1,3-Bis(pyridineylidene)isoindoline: an isoindoline chelate with a stretched electronic structure

A bridging carbon analog of the well-studied bis(pyridyl)iminoisoindoline (BPI) can be produced via a one step reaction between diiminoisoindoline and pyridine-2-acetonitrile. The resultant bis(pyridineylidene)isoindoline (BPYI) is structurally analogous to BPI and can readily form metal complexes. However, it exhibits a markedly different electronic structure with intense absorption bands in the visible region of the spectrum.

Reactivity of a T-shaped cobalt(I) pincer-complex

The reactivity of a paramagnetic T-shaped cobalt(i) complex, [(iPrboxmi)Co], stabilised by a monoanionic bis(oxazolinylmethylidene)-isoindolate (boxmi) NNN pincer ligand is described. The exposure to carbon monoxide as an additional neutral ligand resulted in the square-planar species [(iPrboxmi)Co(CO)], accompanied by a change in the electronic spin state from S = 1 to S = 0. In contrast, upon treatment with trimethylphosphine the formation of the distorted tetrahedral complex [(iPrboxmi)Co(PMe3)] was observed (S = 1). Reacting [(iPrboxmi)Co] with iodine (I2), organic peroxides (tBu2O2, (SiMe3)2O2) and diphenyldisulphide (Ph2S2) yielded the tetracoordinated complexes [(iPrboxmi)CoI], [(iPrboxmi)Co(OtBu)], [(iPrboxmi)Co(OSiMe3)] and [(iPrboxmi)Co(SPh)], respectively, demonstrating the capability of the boxmi-supported cobalt(i) complex to homolytically cleave bonds and thus its distinct one-electron reactivity. Furthermore, a square-planar cobalt(ii) alkynyl complex [(iPrboxmi)Co(CCArF)] was identified as the main product in the reaction between [(iPrboxmi)Co] and a terminal alkyne, 4-fluoro-1-ethynylbenzene. Putting such species in the context of the previously investigated hydroboration catalysis, its stoichiometric reaction with pinacolborane revealed its potential conversion into a cobalt(ii) hydride complex, thus confirming its original attribution as off-cycle species.

Reaction Pathways and Redox States in α-Selective Cobalt-Catalyzed Hydroborations of Alkynes

Cobalt(II) alkyl complexes supported by a monoanionic NNN pincer ligand are pre‐catalysts for the regioselective hydroboration of terminal alkynes, yielding the Markovnikov products with α:β‐(E) ratios of up to 97:3. A cobalt(II) hydride and a cobalt(II) vinyl complex appear to determine the main reaction pathway. In a background reaction the highly reactive hydrido species specifically converts to a coordinatively unsaturated cobalt(I) complex which was found to re‐enter the main catalytic cycle.

Tackling N-Alkyl Imines with 3d Metal Catalysis: Highly Enantioselective Iron-Catalyzed Synthesis of α-Chiral Amines

A readily activated iron alkyl precatalyst effectively catalyzes the highly enantioselective hydroboration of N-alkyl imines. Employing a chiral bis(oxazolinylmethylidene)isoindoline pincer ligand, the asymmetric reduction of various acyclic N-alkyl imines provided the corresponding α-chiral amines in excellent yields and with up to >99 % ee. The applicability of this base metal catalytic system was further demonstrated with the synthesis of the pharmaceuticals Fendiline and Tecalcet.

Catalytic Allylation of Aldehydes Using Unactivated Alkenes

Simple feedstock organic molecules, especially alkenes, are attractive starting materials in organic synthesis because of their wide availability. Direct utilization of such bulk, inert organic molecules for practical and selective chemical reactions, however, remains limited. Herein, we developed a ternary hybrid catalyst system comprising a photoredox catalyst, a hydrogen-atom-transfer catalyst, and a chromium complex catalyst, enabling catalytic allylation of aldehydes with simple alkenes, including feedstock lower alkenes. The reaction proceeded under visible-light irradiation at room temperature and with high functional group tolerance. The reaction was extended to an asymmetric variant by employing a chiral chromium complex catalyst.

Expanding the Boxmi Ligand Family: Synthesis and Application of NON and NSN Ligands

Two synthetic strategies for a new family of neutral NON ligands featuring a "bis(oxazolinylmethylidene)isobenzofuran" framework (boxman) are reported. A Pd-mediated cyclization reaction forming the isobenzofuran core constitutes the key reaction in the eight-step synthetic route to the nonbackbone-methylated target compound ^H,Rboxman. In contrast, the introduction of two additional methyl groups provides stereochemical control during backbone construction and thereby access to the methylated derivative ^Me,Rboxman, which was synthesized in five steps and improved yields. In addition, the synthetic sequence was transferred to the thio analogue, providing access to the NSN ligand ^H,Rboxmene. Subsequent complexation experiments with iron and cobalt chloride precursors afforded the four-coordinated chlorido complexes ^Me,RboxmanMCl₂ (R = Ph, iPr; M = Fe, Co) and established the boxman family as trans-chelating, bidentate bis(oxazoline) ligands. Application of the latter in the nickel(II)- and zinc(II)-catalyzed α-fluorination of β-ketoesters and oxindoles (up to 98% yield and 94% ee) demonstrated their suitability for enantioselective catalysis.

General C(sp2)-C(sp3) Cross-Electrophile Coupling Reactions Enabled by Overcharge Protection of Homogeneous Electrocatalysts

Cross-electrophile coupling (XEC) of alkyl and aryl halides promoted by electrochemistry represents an attractive alternative to conventional methods that require stoichiometric quantities of high-energy reductants. Most importantly, electroreduction can readily exceed the reducing potentials of chemical reductants to activate catalysts with improved reactivities and selectivities over conventional systems. This work details the mechanistically-driven development of an electrochemical methodology for XEC that utilizes redox-active shuttles developed by the energy-storage community to protect reactive coupling catalysts from overreduction. The resulting electrocatalytic system is practical, scalable, and broadly applicable to the reductive coupling of a wide range of aryl, heteroaryl, or vinyl bromides with primary or secondary alkyl bromides. The impact of overcharge protection as a strategy for electrosynthetic methodologies is underscored by the dramatic differences in yields from coupling reactions with added redox shuttles (generally >80%) and those without (generally <20%). In addition to excellent yields for a wide range of substrates, reactions protected from overreduction can be performed at high currents and on multigram scales.

Recent advances in transition-metal-catalyzed incorporation of fluorine-containing groups

Fluorine chemistry plays an increasingly important role in pharmaceutical, agricultural, and materials industries. The incorporation of fluorine-containing groups into organic molecules can improve their chemical and physical properties, which attracts continuous interest in organic synthesis. Among various reported methods, transition-metal-catalyzed fluorination/fluoroalkylation has emerged as a powerful method for the construction of these compounds. This review attempts to describe the major advances in the transition-metal-catalyzed incorporation of fluorine, trifluoromethyl, difluoromethyl, trifluoromethylthio, and trifluoromethoxy groups reported between 2011 and 2019.

Catalytic asymmetric allylation of aldehydes with alkenes through allylic C(sp3)-H functionalization mediated by organophotoredox and chiral chromium hybrid catalysis

We describe a hybrid system that realizes cooperativity between an organophotoredox acridinium catalyst and a chiral chromium complex catalyst, thereby enabling unprecedented exploitation of unactivated hydrocarbon alkenes as precursors to chiral allylchromium nucleophiles for asymmetric allylation of aldehydes. The reaction proceeds under visible light irradiation at room temperature, affording the corresponding homoallylic alcohols with a diastereomeric ratio >20/1 and up to 99% ee. The addition of Mg(ClO4)2 markedly enhanced both the reactivity and enantioselectivity.

Organocatalytic asymmetric syntheses of 3-fluorooxindoles containing vicinal fluoroamine motifs

An organocatalytic Mannich reaction of 3-fluorooxindoles has been developed. Using a commercially available cinchona alkaloid catalyst, a wide range of 3-fluorooxindoles was successfully reacted with N-sulfonyl aldimines to give biologically important 3-fluorooxindoles containing vicinal fluoroamine motifs with high efficiency and good enantioselectivity. This protocol uses readily available reactants and cheap organocatalysts, and it is operationally simple.

Dibenzofuran-4,6-bis(oxazoline) (DBFOX). A novel trans-chelating bis(oxazoline) ligand for asymmetric reactions

The trans-chelating bis(oxazoline) ligand (R,R)-4,6-dibenzofurandiyl-2,2'-bis(4-phenyloxazoline) [(R,R)-DBFOX/Ph] coordinates metal ions to give C2-symmetric complexes which effectively catalyze a variety of asymmetric reactions. (R,R)-DBFOX/Ph·Ni(ClO4)2·3H2O, whose crystal structure is octahedral with three aqua ligands, can be stored under air for several months without loss of catalytic activity and promotes highly enantioselective reactions even in the presence of excess amounts of water, alcohols, amines, and acids. The complex shows remarkable chiral amplification in asymmetric Diels-Alder (DA) reactions. This review focuses on enantioselective reactions catalyzed by (R,R)-DBFOX/Ph·metal complexes.

Synthesis of 3-substituted 3-fluoro-2-oxindoles by deacylative alkylation

The fluorination of 3-acetyl-2-oxindoles with N-fluorobenzenesulfonimide under Lewis acid catalysis using Mg(ClO4)2 gives the starting compounds 3-acetyl-3-fluoro-2-oxindoles. These compounds are subjected to base-promoted deacylative alkylation (DaA) for the in situ generation of 3-fluoro-2-oxindole enolates under very mild reaction conditions using Triton B (1 equiv.) and alkyl halides and Michael acceptors as electrophilic reagents. The corresponding 3-alkylated-3-fluoro-2-oxindoles are obtained in good to very high yields. In addition, the palladium-catalyzed deacylative allylation is carried out with allylic alcohols using LiOtBu as the base and 6 mol% of Pd(OAc)2 and dppp, giving the resulting 3-allylated 3-fluoro-2-oxindoles in good yields. This methodology allows a simple synthesis of 3-alkylated-3-fluoro-2-oxindoles, which are difficult to obtain by other routes.

First-Row Transition Metal (De)Hydrogenation Catalysis Based On Functional Pincer Ligands

The use of 3d metals in de/hydrogenation catalysis has emerged as a competitive field with respect to "traditional" precious metal catalyzed transformations. The introduction of functional pincer ligands that can store protons and/or electrons as expressed by metal-ligand cooperativity and ligand redox-activity strongly stimulated this development as a conceptual starting point for rational catalyst design. This review aims at providing a comprehensive picture of the utilization of functional pincer ligands in first-row transition metal hydrogenation and dehydrogenation catalysis and related synthetic concepts relying on these such as the hydrogen borrowing methodology. Particular emphasis is put on the implementation and relevance of cooperating and redox-active pincer ligands within the mechanistic scenarios.

Asymmetric fluorination of indanone-2-carboxylates using a polystyrene-supported diphenylamine-linked bis(oxazoline) complex

A highly enantioselective fluorination of indanone-2-carboxylates catalyzed by a polystyrene-supported diphenylamine-linked bis(oxazoline) (PS-box)-Cu(OTf)₂ complex has been developed in a continuous flow system. The supported complex exhibited extremely efficient catalytic performance with high activity, affording the corresponding products in excellent yields (up to 99% yield) with excellent enantioselectivities (up to 99% ee) and more than 4000 turnover number (TON).

Ultrafast Iron-Catalyzed Reduction of Functionalized Ketones: Highly Enantioselective Synthesis of Halohydrines, Oxaheterocycles, and Aminoalcohols

A molecularly defined chiral boxmi iron alkyl complex catalyzes the hydroboration of various functionalized ketones and provides the corresponding chiral halohydrines, oxaheterocycles (oxiranes, oxetanes, tetrahydrofurans, and dioxanes) and amino alcohols with excellent enantioselectivities (up to >99 %ee) and conversion efficiencies at low catalyst loadings (as low as 0.5 mol %). Turnover frequencies of greater than 40000 h^-1 at -30 °C highlight the activity of this earth-abundant metal catalyst which tolerates a large number of functional groups.

One-Pot Sequential Kinetic Profiling of a Highly Reactive Manganese Catalyst for Ketone Hydroboration: Leveraging σ-Bond Metathesis via Alkoxide Exchange Steps

A comprehensive experimental and computational mechanistic study of the highly enantioselective hydroboration of ketones catalyzed by a manganese(II) alkyl boxmi pincer complex is reported. The catalyst operates at low catalyst loadings (down to 0.01 mol %) under very mild conditions (typically -40 °C) and facilitates the reduction of both aryl alkyl and dialkyl ketones with excellent selectivity (up to >95%ee). Catalyst activation pathways were investigated, demonstrating that a manganese(II) hydride and a manganese(II) alkoxide species are part of the catalytic cycle and can be generated via σ-bond metathesis of the alkyl precursor with the borane or by alcoholysis. Extensive kinetic experiments based on a "one-pot sequential kinetic profiling" approach under various conditions in combination with kinetic simulations reveal that two catalytic cycles are effective with this earth-abundant base metal catalyst: (i) a minor MnH/borane-mediated insertion cycle, in which the subsequent, product-releasing metathesis step is rate determining ( k _m = 0.076 s^-1), giving a background reaction, which is zeroth order in substrate concentrations, and (ii) a major MnOR/borane-based alkoxide exchange process, leveraging the high-barrier metathesis via the affiliation to an insertion step. The latter features non-integer reaction orders in both reagents due to a combination of an adduct formation step ( k _a = 2.12 M^-1 s^-1, k _-a = 0.49 s^-1) and a substrate insertion step of comparable rates ( k _ai = 3.74 M^-1 s^-1). The kinetic findings are underpinned by high-level density functional theory calculations of the mechanism, control experiments, and kinetic isotope effect/Hammett/Eyring analysis in different concentration regimes. The study highlights the role of a rigorous mechanistic understanding of homogeneous catalytic processes in 3d metals for rational catalyst discovery and optimization.

Modern Approaches for Asymmetric Construction of Carbon-Fluorine Quaternary Stereogenic Centers: Synthetic Challenges and Pharmaceutical Needs

New methods for preparation of tailor-made fluorine-containing compounds are in extremely high demand in nearly every sector of chemical industry. The asymmetric construction of quaternary C-F stereogenic centers is the most synthetically challenging and, consequently, the least developed area of research. As a reflection of this apparent methodological deficit, pharmaceutical drugs featuring C-F stereogenic centers constitute less than 1% of all fluorine-containing medicines currently on the market or in clinical development. Here we provide a comprehensive review of current research activity in this area, including such general directions as asymmetric electrophilic fluorination via organocatalytic and transition-metal catalyzed reactions, asymmetric elaboration of fluorine-containing substrates via alkylations, Mannich, Michael, and aldol additions, cross-coupling reactions, and biocatalytic approaches.

Renaissance of pyridine-oxazolines as chiral ligands for asymmetric catalysis

Oxazoline-containing ligands have been widely employed in numerous asymmetric catalytic reactions. Pyridine-oxazoline-type ligands, a class of hybrid ligands, were designed earlier than bisoxazoline and phosphine-oxazoline ligands; however, their unique properties have only been discovered recently. Pyridine-oxazoline-type chiral ligands are rapidly becoming popular for use in asymmetric catalysis, especially for several new and efficient asymmetric methodologies. Several types of challenging asymmetric reactions have been discovered recently using pyridine-oxazoline-type ligands showing their special properties and potential for future application in a wide range of new catalytic methodologies. This review provides an overview of this field, with the aim of highlighting both ligand design and synthetic methodology development.

Catalytic asymmetric aldol addition reactions of 3-fluoro-indolinone derived enolates

Reported herein is a Cu(i)/bisoxazoline ligand-catalyzed aldol reaction of unprotected tertiary enolates generated in situ from 3-(1,1-dihydroxy-2,2,2-trifluoroethyl)-substituted derivatives of 3-fluoro-2-oxindoles. A range of α-fluoro-β-aryl/hetaryl/alkyl-β-hydroxy-indolin-2-ones containing C-F quaternary stereogenic centers of high pharmaceutical importance were furnished in good yields and satisfactory diastereo- and enantioselectivities. The reactions were conducted under operationally convenient conditions and displayed wide substrate/functional group generality including unprotected N-H on the tertiary enolates, and aromatic, hetero-aromatic and aliphatic aldehydes.

Organocatalytic Stereoselective Synthesis of Fluorinated 3,3'-Linked Bisoxindoles

A highly diastereoselective organocatalytic method that produces 3-fluoro-3'-hydroxy-3,3'-bisoxindoles and the corresponding 3-fluoro-3'-amino derivatives having two adjacent chirality centers from fluorooxindoles and isatins in high yields is described. The reaction occurs in protic solvents at room temperature, it can be upscaled without compromising yield and stereoselectivity, and chromatographic product purification is not required.