Rhodium‐Catalyzed Enantioselective Arylation of Aliphatic Imines

Chiral rhodium(I)‐catalyzed highly enantioselective arylation of aliphatic N‐sulfonyl aldimines with arylboronic acids has been developed. This transformation is achieved by the use of a rhodium/bis(phosphoramidite) catalyst to give enantiomerically enriched α‐branched amines (up to 99 % ee). In addition, this system enables efficient synthesis of (+)‐NPS R‐568 and Cinacalcet which are calcimimetic agents.     

Photoinduced Cobalt Catalysis for the Reductive Coupling of Pyridines and Dienes Enabled by Paired Single-Electron Transfer**

Selective hydroarylation of dienes has potential to provide swift access to useful building blocks. However, most existing methods rely on dienes stabilised by an aromatic group and transmetallation or nucleophilic attack steps require electron-rich aryl coupling partners. As such, there are few examples which tolerate wide-spread heteroarenes such as pyridine. Whilst allylic C−H functionalisation could be considered an alternative approach, the positional selectivity of unsymmetrical substrates is hard to control. Here, we report a general approach for selective hydropyridylation of dienes under mild conditions using metal catalysed hydrogen-atom transfer. Photoinduced, reductive conditions enable simultaneous formation of a cobalt-hydride catalyst and the persistent radical of easily-synthesised pyridyl phosphonium salts. This facilitates selective coupling of dienes in a traceless manner at the C4-position of a wide-range of pyridine substrates. The mildness of the method is underscored by its functional-group tolerance and demonstrated by applications in late-stage functionalisation. Based on a combination of experimental and computational studies, we propose a mechanistic pathway which proceeds through non-reversible hydrogen-atom transfer (HAT) from a cobalt hydride species which is uniquely selective for dienes in the presence of other olefins due to a much higher relative barrier associated with olefin HAT.     

Visible Light Induced Copper-Catalyzed Enantioselective Deaminative Arylation of Amino Acid Derivatives Assisted by Phenol

The exploration of value-added conversions of naturally abundant amino acids has received considerable attention from the synthetic community. Compared with the well-established asymmetric decarboxylative transformation, the asymmetric deaminative transformation of amino acids still remains a formidable challenge, mainly due to the lack of effective strategies for the C−N bond activation and the potential incompatibility with chiral catalysts. Here, we disclose a photoinduced Cu-catalyzed asymmetric deaminative coupling reaction of amino acids with arylboronic acids. This new protocol provides a series of significant chiral phenylacetamides in generally good yields and excellent stereoselectivity under mild and green conditions (42–85 % yields, up to 97 % ee). Experimental investigations and theoretical calculations were performed to reveal the crucial role of additional phenols in improving catalytic efficiency and enantiocontrol.     

Palladium-Catalyzed Arylation of Endocyclic 1-Azaallyl Anions: Concise Synthesis of Unprotected Enantioenriched cis-2,3-Diarylpiperidines

Unprotected cis-2,3-diarylpiperidines are synthesized through an unprecedented palladium-catalyzed cross-coupling reaction between aryl halides and elusive endocyclic 1-azaallyl anions. These intermediates are generated in situ by the deprotonation of 2-aryl-1-piperideines, precursors that are readily prepared in two operations from simple piperidines. An asymmetric version of this reaction with (2R, 3R)-iPr-BI-DIME as the ligand provides products in moderate to good yields and enantioselectivities. This study significantly expands the synthetic utility of endocyclic 1-azaallyl anions.     

Iron(II)‐ and Cobalt(II) Complexes with Tridentate Bis(imino)pyridine Nitrogen Ligands Bearing Chiral Bulky Aliphatic and Aromatic Substituents: Crystal Structure of [CoCl2{2, 6‐bis[R‐(+)‐(bornylimino)methyl]pyridine}]

Iron(II) and cobalt(II) complexes ( 7 ‐ 15 ) based on new aldimine 2, 6‐bis[(imino)methyl]pyridine ( 1 , 2 , 4 , 6 ) and ketimine (2, 6‐bis[(imino)ethyl]pyridine ( 3 , 5 ) ligands with bulky chiral aliphatic or aromatic terminal groups have been prepared and characterized by ¹H NMR, ¹³C NMR, IR‐, mass spectroscopy (EI), and elemental analysis. The complex [CoCl₂(BBoMP)]·¹/₂ CHCl₃ ( 13 ) (BBoMP: 2, 6‐bis{(R‐(+)‐(bornylimino)‐methyl}pyridine) crystallizes in monoclinic space group P2₁ with cell dimensions: a = 7.6603(11) Å, b = 28.3153(14) Å, c = 13.537(2) Å, V = 2908.1(6) ų, Z = 4. The coordination sphere around Co is distorted trigonal bipyramidal.     

Azomethine Imines in Four- and Five-Membered Rings: Stable Cyclic Valence Isomers of an α-(Phenyldiazenyl)ketocarbene

The expected Wolff rearrangement of the ketocarbene intermediate 2 , formed upon rhodium(II )-catalyzed elimination of nitrogen from diazoketone 1 , does not occur. Instead, by incorporation of the neighboring diazene group the surprisingly stable, crystalline azomethine imines 3 and 4 are formed as cyclic valence isomers of 2 . Compound 4 , which contains a five-membered ring, is the first representative of a carba-sydnone and can also be obtained by acid-catalyzed isomerization of 3 at 50–70°C.     

Acceptorless Dehydrogenation of N‐Heterocycles by Merging Visible‐Light Photoredox Catalysis and Cobalt Catalysis

Herein, the first acceptorless dehydrogenation of tetrahydroquinolines (THQs), indolines, and other related N‐heterocycles, by merging visible‐light photoredox catalysis and cobalt catalysis at ambient temperature, is described. The potential applications to organic transformations and hydrogen‐storage materials are demonstrated. Primary mechanistic investigations indicate that the catalytic cycle occurs predominantly by an oxidative quenching pathway.     

Enantioselective epoxidations of alkenes catalyzed by (salen)Mn(III) in aqueous surfactant systems

An aqueous reaction medium, based on a surfactant solution of diethyltetradecylamine N-oxide (AOE-14), was developed for the enantioselective epoxidation of 1,2-dihydronaphthalene and of various cis-β-alkyl styrenes with increasing hydrophobicity, using bleach as oxidant and the Jacobsen chiral (salen)Mn(III) as catalyst. AOE-14 is able to both solubilize all reactants in water and bind the metal of the salen complex acting as coligand. Its use leads to good yields (>75%) and to ee values ranging from 75% up to 91% even in the case of cis-β-alkyl styrenes where lower cis/trans epoxide ratios are observed. The ratio of surfactant/substrate used is 1:1 or 4:1, much lower than those generally used in the literature.     

Controllable Isomerization of Alkenes by Dual Visible‐Light‐Cobalt Catalysis

We report herein that thermodynamic and kinetic isomerization of alkenes can be accomplished by the combination of visible light with Co catalysis. Utilizing Xantphos as the ligand, the most stable isomers are obtained, while isomerizing terminal alkenes over one position can be selectively controlled by using DPEphos as the ligand. The presence of the donor–acceptor dye 4CzIPN accelerates the reaction further. Transformation of exocyclic alkenes into the corresponding endocyclic products could be efficiently realized by using 4CzIPN and Co(acac)₂ in the absence of any additional ligands. Spectroscopic and spectroelectrochemical investigations indicate Co^I being involved in the generation of a Co hydride, which subsequently adds to alkenes initiating the isomerization.     

Palladium-Catalyzed Arylation of 9,10-Dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO) with Halogen-Substituted Phenols

Phenol functionalized 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO) derivatives are synthesized via palladium-catalyzed arylation of DOPO with halogen-substituted phenols. 2-,3-, and 4-Substituted halogen phenols are successfully applied to the cross-coupling reactions. This method is mild, convenient, and leads to high yields (65–95%).     

An NHC–Silyl–NHC Pincer Ligand for the Oxidative Addition of C−H,N−H,and O−H Bonds to Cobalt(I) Complexes

N‐Heterocyclic carbene based pincer ligands bearing a central silyl donor, [CSiC]⁻, have been envisioned as a class of strongly σ‐donating ligands that can be used for synthesizing electron‐rich transition‐metal complexes for the activation of inert bonds. However, this type of pincer ligand and complexes thereof have remained elusive owing to their challenging synthesis. We herein describe the first synthesis of a CSiC pincer ligand scaffold through the coupling of a silyl–NHC chelate with a benzyl–NHC chelate induced by one‐electron oxidation in the coordination sphere of a cobalt complex. The monoanionic CSiC ligand stabilizes the Co^I dinitrogen complex [(CSiC)Co(N₂)] with an unusual coordination geometry and enables the challenging oxidative addition of E−H bonds (E=C, N, O) to Co^I to form Co^III complexes. The structure and reactivity of the cobalt(I) complex are ascribed to the unique electronic properties of the CSiC pincer ligand, which provides a strong trans effect and pronounced σ‐donation.     

Silver‐Catalyzed Arylation of (Hetero)arenes by Oxidative Decarboxylation of Aromatic Carboxylic Acids

A long‐standing challenge in Minisci reactions is achieving the arylation of heteroarenes by oxidative decarboxylation of aromatic carboxylic acids. To address this challenge, the silver‐catalyzed intermolecular Minisci reaction of aromatic carboxylic acids was developed. With an inexpensive silver salt as a catalyst, this new reaction enables a variety of aromatic carboxylic acids to undergo decarboxylative coupling with electron‐deficient arenes or heteroarenes regardless of the position of the substituents on the aromatic carboxylic acid, thus eliminating the need for ortho‐substituted aromatic carboxylic acids, which were a limitation of previously reported methods.     

Arylation of Acrylamide and Acrylonitrile with Arenediazonium Salts Catalyzed by Palladium Acetate

In recent years, palladium catalyzed arylation of olefins, the Heck reaction, has evolved as a powerful synthetic tool for organic chemists1. Since its discovery in the early seventies2, the reaction has undergone several modifications especially with regard to solvents, catalysts and additives. As a result, today Heck reaction is the most widely used transition metal catalyzed carbon-carbon bond forming reaction in organic synthesis3. However, despite all these modifications, little atten…     

Simultaneous Determination of Nickel(II) and Cobalt(II) by Second-Derivative Spectrophotometry

4-(2-Pyridylazo) resorcinol, PAR, is shown to be useful for simultaneous determination of cobalt(II) and nickel(II) using second-derivative spectrophotometric method with controlled experimental parameters. This method allows the determination of 0.20-1.25 ppm of nickel(II) and 0.25-1.50 ppm of cobalt(II) in mixtures with good precision and accuracy. This method has advantages of simplicity, speed and requires no prior separations.     

Arylation of diversely substituted hydrazines by tri- and pentavalent organobismuth reagents

Various hydrazine derivatives were studied with respect to arylation by triarylbismuthane and triarylbismuth diacetate reagents with emphasis on scope and limitations. Among these reagents, a few contained bulky substituents in their aromatic rings. The applied substrates spanned a range from simple hydrazides to triply protected hydrazines and included a large number of intermediates of principal synthetic interest. In the case of mono- and disubstituted hydrazines the results demonstrate apparent advantages of pentavalent over trivalent reagents, exemplified by fast, highly chemoselective monoarylation of acylhydrazines at the terminal nitrogen. In contrast, trisubstituted hydrazines are more efficiently substituted by trivalent reagents. With these substrates, a strong influence of steric factors was occasionally observed, as reflected in lower yields and even complete reaction inhibition. The introduction of two different aromatic substituents into disubstituted hydrazines using step-by-step or one-pot procedures was accomplished.     

Fluorimetric Determination of Thiamine with Cobalt(II)

Abstract

A rapid fluorimetric method is reported for the determination of thiamine based upon its interaction with Co(II) ion in the presence of base and a nonionic Triton X-100 micellar medium. The effect of different experimental variables upon the fluorescence intensity were assessed. The detection limit for thiamine is 5.0 × 10^?8M (λ_ex =375nm, λ_em =433nm). The method was successfully applied for determination of thiamine in variety of samples with good accuracy being achieved.     

Ultrasound-promoted Lipase-catalyzed Enantioselective Transesterification of (R,S)-Glycidol

Enantioselective transesterification of glycidol with vinyl butyrate as an acyl donor was investigated in the presence of Bacillus subtilis lipase(BSL2) as catalyst. Comparison studies demonstrate the advantage of ultrasound over the conventional shaking for the enzymatic reaction in non-aqueous media. The effects of reaction conditions(ultrasound power, temperature, water activity and pH) on the activity and enantioselectivity were also investigated. Under the optimum conditions, the synthetic activity of BSL2 was 2.95 μmol·min?1·mg?1 and the enantioselectivity(E value) was 52.2. Compared with conventional shaking, ultrasound made the synthetic activity and the enantioselectivity increase 9.5-fold and 1.4-fold, respectively. Furthermore, the repeated use of BSL2 for five cycles resulted in no obvious loss of enzyme activity, suggesting that the enzyme is stable under low power ultrasound conditions.