LongLarf

An atom‐economic strategy to catalytically generate and intercept nitrile anion equivalents by hydrogen transfer catalysis has been developed. Addition of α,β‐unsaturated nitriles to a pincer‐based Ru−H complex affords keteniminates by selective 1,4‐hydride transfer, which were employed in a net hydrogenative acylation reaction.

Abstract

We present an atom‐economic strategy to catalytically generate and intercept nitrile anion equivalents using hydrogen transfer catalysis. Addition of α,β‐unsaturated nitriles to a pincer‐based Ru−H complex affords structurally characterized κ‐N‐coordinated keteniminates by selective 1,4‐hydride transfer. When generated in situ under catalytic hydrogenation conditions, electrophilic addition to the keteniminate was achieved using anhydrides to provide α‐cyanoacetates in high yields. This work represents a new application of hydrogen transfer catalysis using α,β‐unsaturated nitriles for reductive C−C coupling reactions.

Redox auxiliary for alkynes: An ortho‐alkylation of pyridine N‐oxide with alkynes, including ynamides and arylacetylenes, has been achieved by organic photocatalysis, which provides access to a series of α‐(2‐pyridinyl) benzyl carbonyl compounds. Pyridine N‐oxide functions as both a redox auxiliary and radical acceptor to achieve the challenging photocatalytic single‐electron oxidation of the carbon–carbon triple bond.

Abstract

A photocatalyzed ortho‐alkylation of pyridine N‐oxide with ynamides and arylacetylenes has been developed, which yields a series of α‐(2‐pyridinyl) benzyl amides/ketones. Mechanistic studies, including electrochemical studies, radical‐trapping experiments, and Stern–Volmer fluorescence quenching studies demonstrate that pyridine N‐oxide serves as both a redox auxiliary and radical acceptor to achieve the mild photocatalytic single‐electron oxidation of carbon–carbon triple bonds with the generation of a cationic vinyl radical intermediate.

Publication date: 2 August 2019

Source: Tetrahedron, Volume 75, Issue 31

Author(s): Jiaming Liu, Shang Gao, Ming Chen

Graphical abstract

Publication date: 9 August 2019

Source: Tetrahedron, Volume 75, Issue 32

Author(s): Tobias Kaper, Sven Doye

Graphical abstract

Practical and regioselective amination of arenes using alkyl amines

Practical and regioselective amination of arenes using alkyl amines, Published online: 22 April 2019; doi:10.1038/s41557-019-0254-5

The synthesis of aryl amines is traditionally achieved through aromatic nitration or transition metal-catalysed cross coupling. Now, photocatalysis provides a route for the direct and selective C–H amination of aromatics with alkyl amines, without the need for pre-functionalization. This reaction tolerates a variety of functional groups and can be used for late-stage modification.

Biomimetic heterogeneous catalysts: Multifunctional metalloporphyrin covalent ionic frameworks with a highly porous structure for cooperative catalysis by Lewis acidic metal sites and nucleophilic Br⁻ ions showed efficient catalytic activity in CO₂ cycloaddition to epoxides (see figure), and the catalytic performance could be further enhanced by altering the functional groups and the BET surface area.

Abstract

The development of multifunctional heterogeneous catalysts with high porosity and remarkable catalytic activity still remains a challenge. Herein, four highly porous metalloporphyrin covalent ionic frameworks (CIFs) were synthesized by coupling 5,10,15,20‐tetrakis(4‐nitrophenyl)porphyrin (TNPP) with 3,8‐diamino‐6‐phenylphenanithridine (NPPN) or 5,5′‐diamino‐2,2′‐bipyridine (NBPy) followed by ionization with bromoethane (C₂H₅Br) or dibromoethane (C₂H₄Br₂) and then metalization with Zn or Co. The resulting CIFs showed high efficiency in catalyzing the cycloaddition of propylene oxide (PO) with CO₂ to form propylene carbonate (PC). All of the Zn‐containing CIF catalysts were able to catalyze the cycloaddition reaction with a PC yield greater than 97 %. The TNPP/NBPy (CIF2) catalyst ionized with C₂H₄Br₂ and metalized with Zn (Zn‐CIF2‐C₂H₄) exhibited the highest catalytic activity among the synthesized catalysts. The high catalytic performance of Zn‐CIF2‐C₂H₄ is related to its high porosity (577 m² g⁻¹), high Br:metal ratio (1:3.89), and excellent synergistic action between the Lewis acidic Zn sites and the nucleophilic Br⁻ ions. Zn‐CIF2‐C₂H₄ is sufficiently stable that greater than 94 % PC yield could be obtained even after six cycles. In addition, Zn‐CIF2‐C₂H₄ could catalyze the cycloaddition of several other epoxides with CO₂. These highly porous materials are promising multifunctional and efficient catalysts for industrially relevant reactions.

Activation through self‐cleavage: The structural and biochemical characterization of polyphenol oxidase 1 (PPO1) from Malus domestica (MdPPO1) reveals self‐cleavage as an activation route for plant PPOs. A self‐cleavage‐inducing peptide was identified that controls the proteolytic activation process and also works as an activator for other PPOs.

Abstract

The conversion of inactive pro‐polyphenol oxidases (pro‐PPOs) into the active enzyme results from the proteolytic cleavage of its C‐terminal domain. Herein, a peptide‐mediated cleavage process that activates pro‐MdPPO1 (Malus domestica) is reported. Mass spectrometry, mutagenesis studies, and X‐ray crystal‐structure analysis of pro‐MdPPO1 (1.35 Å) and two separated C‐terminal domains, one obtained upon self‐cleavage of pro‐MdPPO1 and the other one produced independently, were applied to study the observed self‐cleavage. The sequence Lys 355–Val 370 located in the linker between the active and the C‐terminal domain is indispensable for the self‐cleavage. Partial introduction (Lys 352–Ala 360) of this peptide into the sequence of two other PPOs, MdPPO2 and aurone synthase (CgAUS1), triggered self‐cleavage in the resulting mutants. This is the first experimental proof of a self‐cleavage‐inducing peptide in PPOs, unveiling a new mode of activation for this enzyme class that is independent of any external protease.

Lead: The first hydrazido plumbylene and its lighter congeners have been prepared utilizing a bulky N‐aminocarbazole (see figure).

Abstract

The synthesis of the N‐aminocarbazole R‐NH₂ (2) is reported. Subsequent reaction with bis[bis(trimethylsilyl)amido]tetrylenes E[N(SiMe₃)₂]₂ (E=Ge, Sn, or Pb) allowed the isolation of formal hydrazidotetrylene derivatives, R−N(H)EN(SiMe₃)₂ (3) that includes the first example of a hydrazidoplumbylene to date. Thermal decomposition of these compounds resulted in the elimination of “NH” and afforded the tetrylenes R‐EN(SiMe₃)₂ (4).

A powerful cobalt drive: Cobalt PNP pincer complexes were found to be efficient catalysts for the isomerization of allylic alcohols to ketones. The catalyst with phenyl groups on the phosphorus atoms was highly active in the absence of a base. A broad variety of substrates were successfully converted. The mechanism proceeds through alkene insertion in the cobalt–hydride bond followed by beta‐hydride elimination.

Abstract

Catalytic isomerization of allylic alcohols in ethanol as a green solvent was achieved by using air and moisture stable cobalt (II) complexes in the absence of any additives. Under mild conditions, the cobalt PNP pincer complex substituted with phenyl groups on the phosphorus atoms appeared to be the most active. High rates were obtained at 120 °C, even though the addition of one equivalent of base increases the speed of the reaction drastically. Although some evidence was obtained supporting a dehydrogenation–hydrogenation mechanism, it was proven that this is not the major mechanism. Instead, the cobalt hydride complex formed by dehydrogenation of ethanol is capable of double‐bond isomerization through alkene insertion–elimination.

Impact factors are still widely used in academic evaluations

Impact factors are still widely used in academic evaluations, Published online: 11 April 2019; doi:10.1038/d41586-019-01151-4

Survey finds that 40% of research-intensive universities mention the controversial metric in review documents — despite efforts to dampen its influence.

¹H NMR is not a proof of hydrogen bonds in transition metal complexes

¹H NMR is not a proof of hydrogen bonds in transition metal complexes, Published online: 09 April 2019; doi:10.1038/s41467-019-09625-9

¹H NMR is not a proof of hydrogen bonds in transition metal complexes

Synlett
DOI: 10.1055/s-0037-1611790

The first examples of early-transition-metal-catalyzed hydroaminoalkylation reactions of allenes are reported. Initial studies performed with secondary aminoallenes led to the identification of a suitable titanium catalyst and revealed that under the reaction conditions, the initially formed hydroaminoalkylation products undergo an unexpected titanium-catalyzed rearrangement to form the thermodynamically more stable allylamines. The assumption that this rearrangement involves a reactive allylic cation intermediate provides a simple explanation of the fact that no successful early-transition-metal-catalyzed hydroaminoalkylations of allenes have previously been reported. As a result of the generation of the corresponding cation, the titanium-catalyzed intermolecular hydroaminoalkylation of propa-1,2-diene unexpectedly gives an aminocyclopentane product formed by incorporation of two equivalents of propa-1,2-diene.
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© Georg Thieme Verlag Stuttgart · New York

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Synlett
DOI: 10.1055/s-0037-1612416

Cadogan reductive cyclization of substituted 2-aryl-3-nitropyridines to give δ-carbolines was performed under MoO2Cl2(DMF)2 catalysis with triphenylphosphine as a ligand. A new approach for the synthesis of the alkaloid quindoline based on a Mo(VI)-catalyzed Cadogan reductive cyclization of 2-phenyl-3-nitro-5,6,7,8-tetrahydroquinoline followed by aromatization of the resulting 2,3,4,10-tetrahydro-1H-indolo[3,2-b]quinoline is proposed. Various о-nitroarylpyridines, obtained by reacting acylpyruvates and cyclic hydroxymethylene ketones with nitroacetophenone enamines, were used as starting compounds for the preparation of δ-carbolines. The synthesized δ-carbolines were found to act as phosphors; their photophysical properties were studied and a structure–property relationship was revealed.
[...]

© Georg Thieme Verlag Stuttgart · New York

Article in Thieme eJournals:
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Unnatural photosynthesis: Carbon dioxide is an attractive reagent for organic synthesis from the standpoint of global sustainability. This Minireview describes recent advances in the area of photoredox catalysis as an enabling strategy for promoting carboxylations that generate carboxylic acid products in reactions akin to a synthetic variant of nature's route to carbohydrates.

Abstract

Carbon dioxide is an attractive reagent for organic synthesis from the standpoint of global sustainability. Its widespread use, however, is hampered by the fact that it is poorly reactive. New catalysts and technologies that enable C−C bond constructions are thus of high intrinsic value. This Minireview describes recent advances in the area of photoredox catalysis as an enabling strategy for promoting carboxylations.

Pinch of salt: A highly enantioselective aza‐Darzens cyclization between cyclic imines and α‐halogenated ketones catalyzed by amino‐acid‐derived bifunctional phosphonium salts has been developed. A wide range of enantioenriched fused tri‐ and tetrasubstituted aziridines were synthesized with high yields and excellent diastereo‐ and enantioselectivities. Scale‐up synthesis and valuable transformations are also demonstrated.

Abstract

The first enantioselective aza‐Darzens reaction of cyclic imines with α‐halogenated ketones was realized under mild reaction conditions by using amino‐acid‐derived bifunctional phosphonium salts as phase‐transfer promoters. A variety of structurally dense tri‐ and tetrasubstituted aziridine derivatives, containing benzofused heterocycles as well as spiro‐structures, were readily synthesized in high yields with excellent diastereo‐ and enantioselectivities (up to >20:1 d.r. and >99.9 % ee). The highly functionalized aziridine products could be easily transformed into different classes of biologically active compounds.

Abstract

Aerobic oxidative dehydrogenation reactions of benzylamines to imines were studied and the efficiencies of various ground‐ and excited state quinone organocatalysts were compared. Long wave‐absorbing anthraquinones readily catalyze the aerobic photodehydrogenation of primary and secondary benzylamines to benzylidenebenzylamines with high rate and selectivity, and the reaction mechanism was studied by laser flash photolysis. Further, branched α‐benzyl dibenzyl‐amines undergo a regioselective photocatalytic dehydrogenation to branched aldimines, which can efficiently be converted into cis‐1,3‐diaryl tetrahydro‐isoquinolines through diastereoselective super acid‐mediated Pictet‐Spengler cyclizations.

Radical developments: Photochemical/secondary amine dual catalysis has proven to be an efficient strategy for radical umpolung reactions. Two new processes provide an elegant solution to a long‐standing chemoselectivity problem for the synthesis of 1,4‐dicarbonyl compounds.

Stop and smell the roses: The Prins reaction was discovered in 1919 while Hendrik Jacobus Prins researched the synthesis of novel fragrance ingredients. Subsequently, the reaction found widespread use in the total synthesis of natural products. This overview of fragrance ingredients synthesized with the Prins reaction celebrates the centenary of the discovery of the reaction.

Abstract

The Prins reaction, an acid‐catalyzed condensation of alkenes with aldehydes, is used extensively in the fragrance industry. This year we celebrate the 100th anniversary of the discovery of this reaction. In honor of this occasion we present an overview of the diverse applications of the Prins reaction in the synthesis of flavor and fragrance ingredients. To pay tribute to the inventor of the Prins reaction, Hendrik Jacobus Prins, we also provide some insight into his life, scientific, and entrepreneurial accomplishments.

Publication date: 10 May 2019

Source: Tetrahedron, Volume 75, Issue 19

Author(s): Zhang Zhang, Yong-Hong Liu, Xi Zhang, Xi-Cun Wang

Graphical abstract

Direct and selective: This Minireview deals with direct and selective oxygenation of C−H bonds to C−O bonds with exclusive focus on oxygen as oxidant, as the byproduct is water or hydrogen peroxide. Moreover, achieving these reactions with metal‐free catalysts and contributes to green and sustainable chemistry. Recent reports on C−H oxygenation with metal‐free catalysts and molecular oxygen under visible‐light conditions are summarized.

Abstract

Direct and selective oxygenation of C−H bonds to C−O bonds is regarded as an effective tool to generate high‐value products. However, these reactions are still subject to challenges such as harsh reaction conditions, use of expensive transition metal catalysts, and involvement of stoichiometric oxidants. To avoid these, molecular oxygen would be ideal as oxidant, as the byproduct is water or hydrogen peroxide. Additionally, achieving these reactions by using metal‐free catalysts would contribute to green and sustainable chemical synthesis. This Minireview summarizes recent reports on C−H oxygenation reactions with metal‐free catalysts and molecular oxygen under visible‐light conditions.

Freedom from waste: A calcium‐based cycle provides an excellent way to sustainably convert various sources of carbon into acetylene; a valuable product for industry and research. The proposed cycle may utilize carbon wastes, biomass refinery wastes, and organic compounds. Recyclability, large‐scale production, and future possibilities for fossil‐resource‐independent manufacturing lead to calcium carbide being considered as a key chemical platform.

Abstract

Calcium carbide, a stable solid compound composed of two atoms of carbon and one of calcium, has proven its effectiveness in chemical synthesis, due to the safety and convenience of handling the C≡C acetylenic units. The areas of CaC₂ application are very diverse, and the development of calcium‐mediated approaches resolves several important challenges. This Review aims to discuss the laboratory chemistry of calcium carbide, and to go beyond its frontiers to organic synthesis, life sciences, materials and construction, carbon dioxide capturing, alloy manufacturing, and agriculture. The recyclability of calcium carbide and the availability of large‐scale industrial production facilities, as well as the future possibility of fossil‐resource‐independent manufacturing, position this compound as a key chemical platform for sustainable development. Easy regeneration and reuse of the carbide highlight calcium‐based sustainable chemical technologies as promising instruments for total carbon recycling.