Facile synthesis of C2-symmetric tridentate bis(thiazoline) and bis(oxazoline) ligands and their application in the enantioselective Henry reaction

Synthesis and Magnetic Studies of Two Neutral, Bis-Ligand Fe(II) Complexes Containing Carbazole-Bis(tetrazole) Ligands

Previously reported carbazole-bis(tetrazole) (CzT^R) ligands (where R = iPr and CH₂-2,4,6-C₆H₂Me₃) were used to synthesize air-stable, six-coordinate, octahedral bis-ligand Fe(II) complexes (CzT^R)₂Fe. The synthesis and characterization of these complexes using ¹H nuclear magnetic resonance (NMR), X-ray crystallography, Mössbauer spectroscopy, and density functional theory (DFT) calculations are reported. Analysis of the magnetic properties revealed that the isopropyl derivative displays thermally induced spin crossover (SCO) over a temperature range of 150-350 K. This transition appears as an abrupt two-step transition in the solid state but simplifies to a smooth one-step transition in solution. The two-step transition in the solid state has been postulated to be due to lattice and solvation effects. In contrast, the slightly bulkier substituted CH₂-2,4,6-C₆H₂Me₃ (CH₂Mes) Fe complex displays dramatically different magnetic behavior with no SCO and magnetic data suggesting low-spin Fe(II) with a possible TIP contribution. DFT calculations support the postulate that the change in magnetic behavior is primarily due to the nature of the ligand substituents.

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.

Copper-catalyzed asymmetric 1,6-conjugate addition of in situ generated para-quinone methides with β-ketoesters

A Cu-catalyzed asymmetric 1,6-conjugate addition of in situ generated para-quinone methides (p-QMs) with β-ketoester has been developed to construct the ketoester skeleton bearing adjacent tertiary-quaternary carbon stereocenter in good yields...

Enantioselective Synthesis of Cα-Tetrasubstituted N-Hydroxyl-α-amino Nitriles via Cyanation of Ketonitrones Using Me2(CH2Cl)SiCN

Here, we report an unprecedented catalytic enantioselective cyanation of ketonitrones enabled by the bifunctional cyanating reagent Me₂(CH₂Cl)SiCN. This approach allows facile access to optically active N-hydroxyl-α-amino nitriles that are of high synthetic value but difficult to acquire by other methods. The use of bifunctional cyanating reagent Me₂(CH₂Cl)SiCN not only achieves an enantioselectivity higher than that with TMSCN but also enables various diversification reactions of the resulting silylated adducts. This represents the first enantioselective catalytic nucleophilic addition reaction of unactivated ketone-derived nitrones, exhibiting the potential of such tetrasubstituted C═N bonds for asymmetric synthesis of N-hydroxy α-amino acids and other N-hydroxy tertiary amines.

Hydrosoluble Complexes Bearing Tris(pyrazolyl)methane Sulfonate Ligand: Synthesis, Characterization and Catalytic Activity for Henry Reaction

The catalytic activity of the water-soluble scorpionate coordination compounds [Cu(-NN’O-Tpms)2] (1), [Mn(Tpms)2] (2) and Li[FeCl2(-NN’N’’-Tpms)] (3) [Tpms = tris(pyrazolyl)-methane sulfonate, O3SC(pz)3], were studied towards the (Henry) reaction between benzaldehyde and nitromethane or nitroethane in aqueous medium to afford, respectively, 2-nitro-1-phenylethanol or 2-nitro-1-phenylpropanol, the latter in the syn and the anti diastereoisomeric forms. Complex 1 exhibited the highest activity under the optimum experimental conditions and was used to broaden the scope of the reaction to include several aromatic aldehydes achieving yields up to 94%.

Synthesis of nickel and palladium complexes with diarylamido-based unsymmetrical pincer ligands and application for norbornene polymerization

A set of diarylamido-based unsymmetrical [PNNox] pincer ligands containing a chiral oxazoline ring have been synthesized and their nickel and palladium complexes [(2-PPh2(R1)ArN(R1)Ar-2-(R)oxazoline)MCl] (R1 = 4-H, R = (S)-4-iPr, M = Pd (Pd1); R1 = 4-H, R = (S)-4-Bn, M = Pd (Pd2); R1 = 4-H, R = (S)-4-Ph, M = Pd (Pd3); R1 = 4-Me, R = (S)-4-Bn, M = Pd (Pd4); R1 = 4-Me, R = (S)-4-Ph, M = Pd (Pd5); R1 = 4-H, R = 4-Me2, M = Pd (Pd6); R1 = 4-H, R = Benzo[d]-, M = Pd (Pd7); R1 = 4-H, R = (S)-4-Bn, M = Ni (Ni1); R1 = 4-H, R = (S)-4-Ph, M = Ni (Ni2); R1 = 4-Me, R = (S)-4-Bn, M = Ni (Ni3); R1 = 4-Me, R = (S)-4-Ph, M = Pd (Ni4)) were tested to show high catalytic activities for polymerization of norbornene. After activation of methylaluminoxane (MAO), all the nickel and palladium complexes could catalyze the polymerization of norbornene to yield vinyl-type polymers with activities up to 40.3 × 105 g of PNB (mol of Pd)-1 h-1. The copolymerization of norbornene with functional norbornene comonomers was also investigated by catalyst Pd2, accompanied by decreased catalytic activity and low incorporation of functional comonomers.

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).

Stereodivergent Catalysis

This review covers diastereo- and enantiodivergent catalyzed reactions in acyclic and cyclic systems using metal complexes or organocatalysts. Among them, nucleophilic addition to carbon-carbon and carbon-nitrogen double bonds, α-functionalization of carbonyl compounds, allylic substitutions, and ring opening of oxiranes and aziridines are considered. The diastereodivergent synthesis of alkenes from alkynes is also included. Finally, stereodivergent intramolecular and intermolecular cycloadditions and other cyclizations are also reported.

Recent trends in organocatalyzed asymmetric reduction of prochiral ketones

This review depicts the recent practices followed in organocatalyzed asymmetric reduction of prochiral ketones, highlighting the main organocatalysts used for the past seven years.

Catalytic enantioselective Henry reaction of α-keto esters, 2-acylpyridines and 2-acylpyridine N-oxides

Asymmetric Henry reaction of ketones catalyzed by a pre-prepared Ni complex with low catalyst loading.

A new series of bipyridine based chiral organocatalysts for enantioselective Henry reaction

We report the design and synthesis of a new series of binaphthol based chiral organocatalysts which can act as metal-free organocatalysts in the enantioselective Henry reaction with very good yield and ee.

Cyclometallation of the NNN pincer ligand in complexes of platinum

Platinum complexes of the diimino/carbazolyl NNN pincer ligand have been prepared and characterized. Facile cyclometallation of the mesityl substituent was observed.

Anionic chiral tridentate N-donor pincer ligands in asymmetric catalysis

Tridentate monoanionic ligands known as "pincers" have gained a prominent place as ligands for transition metals and, more recently, for main-group metals and lanthanides. They have been widely employed as ancillary ligands for metal complexes studied inter alia in bond activation steps relevant to catalytic processes. The central formally anionic aryl or heteroaryl unit acts as an "anchor" in the coordination to the metal, which kinetically stabilizes the resulting complexes. Their stability, activity, and reactivity can be tuned by subtle modifications of substitution patterns on the pincer ligand or by modifying the donor atoms. The challenges in pincer ligand design for enantioselective catalysis have been met by their assembly from rigid heterocycles and chiral ligating units in the "wingtip" positions, which generally contain the stereochemical information. The resulting well-defined geometry and shape of the reactive sector of the molecular catalyst favor orientational control of the substrates. On the other hand, the kinetic stability allows reduced catalyst loadings. Recently, a new generation of tridentate anionic N(∧)N(∧)N pincer ligands has been developed which give rise to highly enantioselective transformations. Their applications in asymmetric catalysis have focused primarily on the asymmetric Nozaki-Hiyama-Kishi coupling of aldehydes with halogenated hydrocarbons as well as Lewis acid catalysis involving enantioselective electrophilic attack onto metal-activated β-keto esters, oxindoles, and related substrates. These include highly selective protocols for Friedel-Crafts alkylations with Michael acceptors, electrophilic fluorinations, trifluoromethylations, azidations, and alkylations and subsequent transformations. Increasingly, these stereodirecting ligands are being employed in other types of transformations, including hydrosilylations, cyclopropanations, and epoxidations. The stability and well-defined nature of the molecular catalysts have made them attractive targets for mechanistic studies into a wide range of these transformations, thus providing the type of insight required for a more rational approach to catalyst development. This Account reviews work performed by us and other groups in the field and places it into perspective in relation to general research efforts in enantioselective catalysis.

Chiral lanthanide complexes: coordination chemistry, spectroscopy, and catalysis

Luminescent lanthanide complexes bearing amido-bisoxazoline ligands are reported. They were studied using time-resolved luminescence spectroscopy, and were probed for their activity in hydroamination/cyclisation and ring-opening polymerisation catalysis.

Dynamic kinetic asymmetric transformations of β-stereogenic α-ketoesters by direct aldolization

Dynamic kinetic asymmetric transformations (DyKAT) of racemic β-bromo-α-keto esters by direct aldolization of nitromethane and acetone provide access to fully substituted α-glycolic acid derivatives bearing a β-stereocenter. The aldol adducts are obtained in excellent yield with high relative and absolute stereocontrol under mild reaction conditions. Mechanistic studies determined that the reactions proceed through a facile catalyst-mediated racemization of the β-bromo-α-keto esters under a DyKAT Type I manifold.