Pyridine-2,6-bis(oxazolines), helpful ligands for asymmetric catalysts

Pd/Cu Dual Metal-Catalyzed Regioselective [2 + 2 + 2] Cycloaddition of Malononitriles with Alkynes to Densely Substituted Pyridines

Transition metal-catalyzed [2 + 2 + 2] cycloaddition of nitriles and two alkynes is an efficient method for assembling pyridines. However, examples employing palladium catalysis have rarely been disclosed, and the processes of reactivity and selectivity remain unclear. We report here a palladium/copper dual metal-catalyzed [2 + 2 + 2] cycloaddition of diynyl-tethered malononitriles and terminal alkynes to synthesize densely substituted pyridines. This method features a good substrate scope, synthetically useful yields, and perfect regioselectivity. The derivatization of the pyridine products demonstrates the potential application of this method in synthesizing heterocycles and as ligands in photocatalysis. Preliminary mechanistic studies suggest that the reaction undergoes aza-oxidative cycloaddition of Pd(0) with nitrile and alkyne, followed by alkyne insertion and reductive elimination. The presence of copper is crucial to its reactivity and regioselectivity.

Iron-Catalyzed C-H Alkylamination of Tyrosine Derivatives

An efficient method for the direct alkylamination of tyrosine derivatives via iron-catalyzed C-H amination has been developed. The method, using O-benzoyl-N,N-dialkylhydroxylamines as aminating agents, provides various C-amino-functionalized tyrosine derivatives in up to 77% yield. The utility of this method is showcased by its application to the direct introduction of drug molecules into tyrosine, facilitating access to structurally diverse amino-functionalized tyrosine derivatives.

Strategies for Accessing cis-1-Amino-2-Indanol

cis-1-amino-2-indanol is an important building block in many areas of chemistry. Indeed, this molecule is currently used as skeleton in many ligands (BOX, PyBOX…), catalysts and chiral auxiliaries. Moreover, it has been incorporated in numerous bioactive structures. The major issues during its synthesis are the control of cis-selectivity, for which various strategies have been devised, and the enantioselectivity of the reaction. This review highlights the various methodologies implemented over the last few decades to access cis-1-amino-2-indanol in racemic and enantioselective manners. In addition, the various substitution patterns on the aromatic ring and their preparations are listed.

Evaluation of the enantioselectivity of new chiral ligands based on imidazolidin-4-one derivatives

The new chiral ligands I-III based on derivatives of imidazolidin-4-one were synthesised and characterised. The catalytic activity and enantioselectivity of their corresponding copper(II) complexes were studied in asymmetric Henry reactions. It was found that the enantioselectivity of these catalysts is overall very high and depends on the relative configuration of the ligand used; cis-configuration of ligand affords the nitroaldols with major enantiomer S- (up to 97% ee), whereas the application of ligands with trans-configuration led to nitroaldols with major R-enantiomer (up to 96% ee). The "proline-type" ligand IV was also tested in asymmetric aldol reactions. Under the optimised reaction conditions, aldol products with enantioselectivities of up to 91% ee were obtained.

Synthesis, Characterization, and Catalytic Behaviors in Isoprene Polymerization of Pyridine-Oxazoline-Ligated Cobalt Complexes

A family of pyridine-oxazoline-ligated cobalt complexes L2CoCl23a-h were synthesized and characterized. Determined via single-crystal X-ray diffraction, complexes 3a and 3d, ligated by two ligands, displayed a distorted tetrahedral coordination of a cobalt center. The X-ray structure indicated the pyridine-oxazoline ligands acted as unusual mono-dentate ligands by coordinating only to Noxazoline. Upon activation with AlEt2Cl (diethylaluminum chloride), these cobalt complexes all exhibited high catalytic activity (up to 2.5 × 106 g·molCo-1·h-1), affording cis-1,4-co-3,4-polyisoprene with molecular weights of 4.4-176 kg mol-1 and a narrow Ð of 1.79-3.42, suggesting a single-site nature of the active sites. The structure of cobalt catalysts and reaction parameters, especially co-catalysts and the reaction temperature, all have significant influence on the polymerization activity but not on the microstructure of polyisoprene.

Direct Regioselective Reductive Allylation of Imines: Application to Synthesis of Oxazines with Halogenated Reagent

Oxazines are an important class of compounds in oxazine ligands and medical chemistry. Here, we describe a linear-selective allylation of imines with allyl electrophiles via cross-electrophile coupling reactions, followed by cyclization with halogenated reagents, providing a new strategy to afford oxazine compounds with a tetrasubstituted carbon center. Mechanistic studies indicate that α-amino carbanion, generated by successive single-electron transfer processes, is a key intermediate for nucleophile attack on π-allylpalladium in photoredox/palladium catalysis.

Probing Electronic Effects in Tridentate Copper(I) Complexes by CIVP Spectroscopy

Ligand electronic effects play an important role in catalysis, where small changes to ligand structure can bring about large changes in catalytic activity. Therefore, accurate experimental quantification of ligand electronic properties plays a crucial role in understanding and tuning chemical reactivity. In this work, we used cryogenic ion vibrational predissociation (CIVP) spectroscopy to experimentally quantify electronic effects in terpyridine ligands, as simple model systems, by measuring CIVP spectra of their copper complexes tagged by N2 molecules. We used the N2 stretching vibration as a reporter chromophore to probe electronic effects of the investigated ligands and employed quantum chemical calculations to better understand how different substituents influence the vibrational frequencies of the stretching vibration of the chromophore. Our data show that the electronic character, as well as position and number of substituents, can affect the N≡N vibrational frequency, and that the N≡N bond serves as a sensitive probe for electronic and steric effects.

Design of C1-symmetric tridentate ligands for enantioselective dearomative [3 + 2] annulation of indoles with aminocyclopropanes

Chiral polycyclic indolines are widely present in natural products and have become the focus of extensive synthetic efforts. Here, we show the catalytic asymmetric dearomative [3 + 2] annulation of indoles with donor-acceptor aminocyclopropanes to construct tricyclic indolines. Key to the success of the reaction is the rational design of C₁-symmetric bifunctional tridentate imidazoline-pyrroloimidazolone pyridine ligand. Under 5 mol% of Ni(OTf)₂-ligand complex, diverse tricyclic indolines containing cyclopentamine moieties are obtained in good chemoselectivities, high diastereoselectivities, and excellent enantioselectivities. An unusual cis-configuration ligand is superior to the trans-configuration ligand and the corresponding C₂-symmetric tridentate nitrogen ligands in the annulation reaction. Mechanistic studies by control experiments and density functional theory calculations reveal a dual activation manner, where Ni(II) complex activates the aminocyclopropane via coordination with the geminal diester, and imidazolidine NH forms a H-bond with the succinimide moiety.

Regio- and Chemoselective Copper-Catalyzed Formal [2+2+2] Cycloaddition of Primary Amines with Arylacetylenes to 2,4,5-Trisubstituted Pyridines

Disclosed herein is an efficient strategy for the synthesis of 2,4,5-trisubstituted pyridines via CuI/NBS-catalyzed formal intermolecular [2+2+2] cycloaddition of easily available primary amines and nonactivated terminal alkynes. Moreover, this given reaction features a new mode of cycloaddition with high regio- and chemoselectivity, good atom- and step-economy, broad substrate scope, and wide functional group compatibility. Further mechanism studies indicate that this transformation starts with oxidative alkynylation of the amine to form a propargylamine intermediate, followed by radical addition to the alkyne and intramolecular cycloaddition, delivering the pharmacologically interesting multisubstituted pyridines.

Metal-Free Reductive Coupling of para-Quinone Methides with 4-Cyanopyridines Enabled by Pyridine-Boryl Radicals: Access to Pyridylated Diarylmethanes with Anti-Cancer Activity

Reported herein is a streamlined protocol to produce pyridylated diarylmethanes through pyridine-boryl radical induced reductive coupling between para-quinone methides (p-QMs) and 4-cyanopyridines using bis(pinacolato)diboron (B₂ pin₂ ) as a templated reagent. The metal-free process is characterized by an operationally simple approach, excellent chemoselectivity (1,2- vs. 1,6-selectivity), and a broad substrate scope with good functional group compatibility. The mechanistic studies provided important insights into the reductive cross-coupling process between diarylmethyl radical and pyridine-boryl radical. Moreover, part of the obtained pyridylated diarylmethane products were screened against a panel of cancer cell lines, and 3 v was confirmed to significantly inhibit the proliferation of head and neck squamous cell carcinoma (HNSCC) cells. This method offers a platform for the preparation of new lead compounds with antitumor activity.

Synthesis of Metal-Free Chiral Covalent Organic Framework for Visible-Light-Mediated Enantioselective Photooxidation in Water

Although chiral covalent organic frameworks (CCOFs) presence grows in thermal asymmetric catalysis, their application in equally important asymmetric photocatalysis has yet to begin. Herein, we first report a propargylamine-linked and quaternary ammonium bromide decorated porphyrin-CCOF which can highly promote visible-light-driven enantioselective photooxidation of sulfides to sulfoxides in water and in air. This methodology has also been applied to the synthesis of (R)-modafinil, a wakefulness-promoting medication used for the treatment of excessive sleepiness. This research might open a new way for the application of CCOFs in asymmetric photocatalysis.

Heteroleptic iron(ii) complexes of chiral 2,6-bis(oxazolin-2-yl)-pyridine (PyBox) and 2,6-bis(thiazolin-2-yl)pyridine ligands – the interplay of two different ligands on the metal ion spin sate

The spin-crossover properties of [Fe(LR)L][ClO4]2 (LR = a chiral PyBox {L1R} or ThioPyBox {L2R} derivative) show subtle differences depending on the tridentate ‘L’ co-ligand.

New tertiary amine-derived C2-symmetric chiral pyridine-N,N′-dioxide ligands and their applications in asymmetric catalysis

Developing efficient methods for preparing privileged chiral ligands is an important endeavor in synthetic chemistry.

A Metal-Free Visible-Light Photoredox Construction and Direct C-H Functionalization of Pyridines: Green Synthesis of Polysubstituted Picolinaldehydes

The development of a novel environmental benign and sustainable synthetic method for highly efficient construction and direct C-H functionalization of N-heterocycles remains a pivotal central research topic for organic and medicinal chemistry. Herein, a novel visible-light-enabled biomimetic aza-6π electrocyclization for efficient assembly of diverse pyridines and further tandem Minisci-type reaction were developed. A broad spectrum of polysubstituted picolinaldehydes were readily constructed with high efficacy and good functional group tolerance under metal- and oxidant-free conditions.

Spin-States of Diastereomeric Iron(II) Complexes of 2,6-Bis(thiazolin-2-yl)pyridine (ThioPyBox) Ligands and a Comparison with the Corresponding PyBox Derivatives

This report investigates homoleptic iron(II) complexes of thiazolinyl analogues of chiral PyBox tridentate ligands: 2,6-bis(4-phenyl-4,5-dihydrothiazol-2-yl)pyridine (L¹Ph), 2,6-bis(4-isopropyl-4,5-dihydrothiazol-2-yl)pyridine (L¹iPr), and 2,6-bis(4-tert-butyl-4,5-dihydrothiazol-2-yl)pyridine (L¹t-Bu). Crystallographic data imply the larger and more flexible thiazolinyl rings reduce steric clashes between the R substituents in homochiral [Fe((R)-L¹R)₂]²⁺ or [Fe((S)-L¹R)₂]²⁺ (R = Ph, iPr, or t-Bu), compared to their PyBox (L²R) analogues. Conversely, the larger heterocyclic S atoms are in close contact with the R substituents in heterochiral [Fe((R)-L¹Ph)((S)-L¹Ph)]²⁺, giving it a more sterically hindered ligand environment than that in [Fe((R)-L²Ph)((S)-L²Ph)]²⁺ (L²Ph = 2,6-bis(4-phenyl-4,5-dihydrooxazol-2-yl)pyridine). Preformed [Fe((R)-L¹Ph)((S)-L¹Ph)]²⁺ and [Fe((R)-L¹iPr)((S)-L¹iPr)]²⁺ do not racemize by ligand redistribution in CD₃CN solution, but homochiral [Fe(L¹iPr)₂]²⁺ and [Fe(L¹t-Bu)₂]²⁺ both undergo partial ligand displacement in that solvent. Homochiral [Fe(L¹Ph)₂]²⁺ and [Fe(L¹iPr)₂]²⁺ exhibit spin-crossover equilibria in CD₃CN, centered at 344 ± 6 K and 277 ± 1 K respectively, while their heterochiral congeners are essentially low-spin within the liquid range of the solvent. These data imply that the diastereomers of [Fe(L¹Ph)₂]²⁺ and [Fe(L¹iPr)₂]²⁺ show a greater difference in their spin-state behaviors than was previous found for [Fe(L²Ph)₂]²⁺. Gas-phase DFT calculations (B86PW91/def2-SVP) of the [Fe(L¹R)₂]²⁺ and [Fe(L²R)₂]²⁺ complexes reproduce most of the observed trends, but they overstabilize the high-spin state of SCO-active [Fe(L¹iPr)₂]²⁺ by ca. 1.5 kcal mol^-1. This might reflect the influence of intramolecular dispersion interactions on the spin states of these compounds. Attempts to model this with the dispersion-corrected functionals B97-D2 or PBE-D3 were less successful than our original protocol, confirming that the spin states of sterically hindered molecules are a challenging computational problem.

Enantioselective Synthesis of Pyridines with All-Carbon Quaternary Carbon Centers via Cobalt-Catalyzed Desymmetric [2+2+2] Cycloaddition

A Co-catalyzed enantioselective desymmetric [2+2+2] cycloaddition for synthesis of pyridines with all-carbon quaternary carbon centers has been developed. The regio- and enantioselectivities are controlled by the inherent nature of terminal alkynes and the substituents on the bisoxazolinephosphine ligands. Pyridines with 5-substitutents could be obtained with >20:1 regioselectivity and up to 94 % ee when terminal alkyl, alkenyl or silyl alkynes and DTBM/Ph-based NPN* ligand L6 were used. Terminal aryl alkynes and Ph/Bn-based ligand L4 leads to formation of pyridines with 6-substitutents in up to 99 % ee.

Gold-Catalyzed One-Pot Synthesis of Polyfluoroalkylated Oxazoles from N-Propargylamides Under Visible-Light Irradiation

A gold-catalyzed synthesis of polyfluoroalkylated oxazoles from N-propargylamides under visible-light irradiation has been developed. These reactions display excellent compatibility of radicals and gold catalysts under visible-light irradiation. Mechanistic experiments indicate that polyfluoroalkyl iodides play a dual role in enhanced compatibility of radicals and gold catalysts through assisted protodeauration of vinyl gold and reactivated the gold catalyst. In addition, PPh₃ AuNTf₂ not only activates N-propargylamide to generate vinyl gold intermediate, but also greatly promotes homolysis of polyfluoroalkyl iodides under blue light irradiation.

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.

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.

Helix-mediated over 1 nm-range chirality recognition by ligand-to-ligand interactions of dinuclear helicates

Long-range chirality recognition between the two chiral guest ligands can be tuned based on the helix distances (d_Ln–Ln = 11.5 and 14.0 Å) of bis-diketonate bridged dinuclear lanthanide complexes (2Th and 3Th, respectively) used as mediators. Both 2Th and 3Th form one-dimensional (1D) helical structures upon terminal binding of two chiral guest co-ligands (L^R or L^S). Long-range chiral self-recognition is achieved in self-assembly of 2Th with L^R and L^S to preferentially form homochiral assemblies, 2Th-L^R·L^R and 2Th-L^S·L^S, whereas there is no direct molecular interaction between the two guest ligands at the terminal edges. X-ray crystal structure analysis and density functional theory studies reveal that long-range chiral recognition is achieved by terminal ligand-to-ligand interactions between the bis-diketonate ligands and chiral guest co-ligands. Conversely, in self-assembly of 3Th with a longer helix length, statistical binding of L^R and L^S occurs, forming heterochiral (3Th-L^R·L^S) and homochiral (3Th-L^R·L^R and 3Th-L^S·L^S) assemblies in an almost 1 : 1 ratio. When phenyl side arms of the chiral guest co-ligands are replaced by isopropyl groups (L′^R and L′^S), chiral self-recognition is also achieved in the self-assembly process of 3Th with the longer helix length to generate homochiral (3Th-L′^R·L′^R and 3Th-L′^S·L′^S) assemblies as the favored products. Thus, subtle modification of the chiral guests is capable of achieving over 1.4 nm-range chirality recognition.

Long-range chirality recognition between the two chiral guest ligands can be tuned based on the helix distances (d_Ln–Ln = 11.5 and 14.0 Å) of bis-diketonate bridged dinuclear lanthanide complexes (2Th and 3Th, respectively).

Metal-free multicomponent reactions: a benign access to monocyclic six-membered N-heterocycles

The advent of multicomponent reactions in the synthesis of heterocycles and their ever burgeoning applications in drug development, materials chemistry, and catalysis, have attracted a great deal of current scientific interest. In particular, the metal-free multicomponent synthesis of six membered N-heterocycles has undergone intensive research over the last two decades offering an environmentally benevolent means contrary to traditional metal catalysed reactions. To the best of our knowledge, there exists no exclusive review on the metal-free multicomponent synthesis of six membered N-heterocyles, and hence the present report highlights the progress on metal-free multicomponent reactions with their advantages and mechanistic insights to access monocyclic six-membered N-heterocycles including pyridine, pyrimidine, pyrazine, triazine and their hydrogenated derivatives. The literature is covered since 2000, and the contents offer not only striking methods for divergent synthesis of six-membered N-heterocycles but also put forward some new insights into the exploration of metal-free multicomponent chemistry.

Ligand Induced CuII Transport Restricts Cancer and Mycobacterial Growth: Towards a Plug-and-Select Ion Channel Scaffold

Synthetic channels with high ion selectivity are attractive drug targets for diseases involving ion dysregulation. Achieving selective transport of divalent ions is highly challenging due their high hydration energies. A small tripeptide amphiphilic scaffold installed with a pybox ligand selectively transports Cu^II ions across membranes. The peptide forms stable dimeric pores in the membrane and transports ions by a Cu²⁺ /H⁺ antiport mechanism. The ligand-induced excellent Cu^II selectivity as well as high membrane permeability of the peptide is exploited to promote cancer cell death. The peptide's ability to restrict mycobacterial growth serves as seeds to evolve antibacterial strategies centred on selectively modulating ion homeostasis in pathogens. This simple peptide can potentially function as a universal, yet versatile, scaffold wherein the ion selectivity can be precisely controlled by modifying the ligand at the C terminus.

l-Threonine Transaldolase Activity Is Enabled by a Persistent Catalytic Intermediate

l-Threonine transaldolases (lTTAs) are a poorly characterized class of pyridoxal-5'-phosphate (PLP) dependent enzymes responsible for the biosynthesis of diverse β-hydroxy amino acids. Here, we study the catalytic mechanism of ObiH, an lTTA essential for biosynthesis of the β-lactone natural product obafluorin. Heterologously expressed ObiH purifies as a mixture of chemical states including a catalytically inactive form of the PLP cofactor. Photoexcitation of ObiH promotes the conversion of the inactive state of the enzyme to the active form. UV-vis spectroscopic analysis reveals that ObiH catalyzes the retro-aldol cleavage of l-threonine to form a remarkably persistent glycyl quinonoid intermediate, with a half-life of ∼3 h. Protonation of this intermediate is kinetically disfavored, enabling on-cycle reactivity with aldehydes to form β-hydroxy amino acids. We demonstrate the synthetic potential of ObiH via the single step synthesis of (2S,3R)-β-hydroxyleucine. To further understand the structural features underpinning this desirable reactivity, we determined the crystal structure of ObiH bound to PLP as the Schiff's base at 1.66 Å resolution. This high-resolution model revealed a unique active site configuration wherein the evolutionarily conserved Asp that traditionally H-bonds to the cofactor is swapped for a neighboring Glu. Molecular dynamics simulations combined with mutagenesis studies indicate that a structural rearrangement is associated with l-threonine entry into the catalytic cycle. Together, these data explain the basis for the unique reactivity of lTTA enzymes and provide a foundation for future engineering and mechanistic analysis.

Site-selective and diastereoselective functionalization of α-amino acid and peptide derivatives via palladium-catalyzed sp3 C–H activation

This review introduces palladium-catalyzed C–H functionalization of amino acids and peptides.

Metal-free visible-light-induced photoredox-catalyzed intermolecular pyridylation/phosphinoylation of alkenes

A visible-light-induced and photoredox-catalyzed intermolecular pyridylation/phosphinoylation of alkenes using 4-cyanopyridine and diphenylphosphine oxide under mild metal-free conditions has been reported.

Efficient stereoselective synthesis of chiral 3,3′-dimethyl-(2,2′-bipyridine)-diol ligand and applications in FeII-catalysis

A seven step synthesis of a chiral 2,2′-bipyridinediol ligand with 3,3′-dimethyl substituents was achieved starting from commercially available materials.

Effect of the aromatic substituent on the para-position of pyridine-bis(oxazoline) sensitizers on the emission efficiency of their EuIII and TbIII complexes

Two efficient lanthanide ion sensitizers 2,6-bis(oxazoline)-4-phenyl-pyridine (PyboxPh, 1) and 2,6-bis(oxazoline)-4-thiophen-2-yl-pyridine (Pybox2Th, 2) were synthesized. 1 crystallizes in the monoclinic space group P21/c with cell parameters a = 16.3794(4) Å, b = 7.2856(2) Å, c = 11.7073(3) Å, β = 97.229(1)° and V = 1385.97(6) Å3. 2 crystallizes in the monoclinic space group P21/n with cell parameters a = 5.9472(2), b = 16.0747(6), c = 14.3716(5) Å, β = 93.503(1)° and V = 1371.35(8) Å3. Photophysical characterization of 1 shows that its triplet state energy is located at 22 250 cm-1 and efficient energy transfer is observed for EuIII and TbIII. Solutions of [Ln(PyboxPh)3]3+ in dichloromethane display an emission efficiency of 37.2% for Ln[double bond, length as m-dash]Eu and 24.0% for Ln[double bond, length as m-dash]Tb. The excited state lifetimes for EuIII and TbIII are 2.227 ms and 723 μs, respectively. The triplet state energy of 2 is located at 19 280 cm-1 and is therefore too low to efficiently sensitize TbIII emission. However, the sensitization of EuIII is effective, with an emission quantum yield of 14.5% and an excited state lifetime of 714 μs. This shows that the derivatization of the chelator is strongly influenced by the aromatic substituents on the para-position of the pyridine ring. New isostructural 1 : 1 complexes of PyboxPh with EuIII (3) and TbIII (4) were also isolated and crystallize in the triclinic space group P1[combining macron] with cell parameters a = 9.1845(2) Å, b = 10.3327(2) Å, c = 11.9654(2) Å, α = 98.419(1)°, β = 108.109(1)°, γ = 91.791(1)°, V = 1064.08(4) Å3 and a = 7.8052(1) Å, b = 11.8910(1) Å, c = 14.2668(2) Å, α = 72.557(1)°, β = 86.355(1)°, γ = 77.223(1)°, V = 1231.95(3) Å3, respectively.

Enantioselective Alkynylation of Unstabilized Cyclic Iminium Ions

An enantioselective copper-catalyzed alkynylation of unstabilized cyclic iminium ions has been developed. Whereas such alkynylations typically utilize pyridinium, quinolinium and isoquinolinium intermediates, this method enables use of cyclic iminium ions unstabilized by resonance. With the use of a Lewis acid and copper catalyst, these iminium ions are generated in situ from readily available hemiaminal methyl ethers and transformed into highly enantioenriched α-alkynylated cyclic amines. A variety of terminal alkynes can be incorporated in high yields and enantiomeric excesses.

C2-Symmetric Dinickel Catalysts for Enantioselective [4 + 1]-Cycloadditions

Dinickel naphthyridine-bis(oxazoline) catalysts promote enantioselective intermolecular [4 + 1]-cycloadditions of vinylidene equivalents and 1,3-dienes. The products of this reaction are methylenecyclopentenes, and the exocyclic alkene is generally obtained with high Z selectivity. E- and Z-dienes react in a stereoconvergent fashion, providing cycloadducts with the same sense of absolute stereochemistry and nearly identical ee values. This feature allows dienes that are commercially available as E/Z mixtures to be used as substrates for the cycloaddition. A DFT model for the origin of asymmetric induction is provided.

Direct Access to Highly Enantioenriched α-Branched Acrylonitriles through a One-Pot Sequential Asymmetric Michael Addition/Retro-Dieckmann/Retro-Michael Fragmentation Cascade

A highly enantioselective synthesis of α-branched acrylonitriles is reported featuring a one-pot sequential asymmetric Michael addition/retro-Dieckmann/retro-Michael fragmentation cascade. The method, which relies on a solid, bench-stable, and commercially available acrylonitrile surrogate, is practical, scalable, and highly versatile and provides a direct access to a wide range of enantioenriched nitrile-containing building blocks. Most importantly, the method offers a new tool to incorporate an acrylonitrile moiety in an asymmetric fashion.

Complementing Pyridine-2,6-bis(oxazoline) with Cyclometalated N-Heterocyclic Carbene for Asymmetric Ruthenium Catalysis

A strategy for expanding the utility of chiral pyridine-2,6-bis(oxazoline) (pybox) ligands for asymmetric transition metal catalysis is introduced by adding a bidentate ligand to modulate the electronic properties and asymmetric induction. Specifically, a ruthenium(II) pybox fragment is combined with a cyclometalated N-heterocyclic carbene (NHC) ligand to generate catalysts for enantioselective transition metal nitrenoid chemistry, including ring contraction to chiral 2H-azirines (up to 97 % ee with 2000 TON) and enantioselective C(sp3 )-H aminations (up to 97 % ee with 50 TON).