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Версия для печати | Главная > Центр > Научные советы > Научный совет по катализу > ... > 2019 год > № 89

№ 89

 

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  • Илья Иосифович Моисеев
    К 90-ЛЕТНЕМУ ЮБИЛЕЮ
  • НАУЧНЫЙ СОВЕТ ПО КАТАЛИЗУ ОХНМ РАН
    Отчет о научно-организационной деятельности в 2018 году
  • За рубежом
  • Приглашения на конференции



Илья Иосифович Моисеев
К 90-летнему юбилею

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Научный Совет по катализу ОХНМ РАН

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Научный Совет по катализу ОХНМ РАН (продолжение)

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MXene serves as support material for single-atom catalysts

Simple procedure yields catalyst that outperforms reference noble-metal catalysts

A simple preparation procedure converts ultrathin sheets of titanium carbide to a material that supports isolated platinum atoms, yielding a highly active catalyst that uses CO2, a greenhouse gas, to make valuable organic compounds (J. Am. Chem. Soc. 2019, DOI: 10.1021/jacs.8b13579). A common procedure for preparing MXenes, a family of 2-D metal carbides and nitrides, from mixed-metal carbide starting materials yields a related carbide dotted with individual titanium vacancies. Chen Chen of Tsinghua University and coworkers proposed that treating the defective material with a platinum salt would be an easy way to pin isolated platinum atoms across the MXene surface. So the researchers prepared the material, confirmed its structure, and tested its ability to catalyze reactions. They found that it was especially effective at mediating formylation of many types of amines with CO2 under mild conditions. For example, exposing aniline to CO2 at atmospheric pressure in the presence of the catalyst and a silane reductant produced N-phenyl formamide (shown) with a yield and selectivity of nearly 100%, far higher than that obtained using reference platinum catalysts. Analyses show that a small positive charge on the platinum atom (Ptδ+) contributes to catalytic performance by facilitating adsorption of reagent molecules.

 

 

Perovskites catalyze aldehyde alkylations

Organolead halides of solar-cell fame function as active photocatalysts in organic synthesis

Mention perovskites to C&EN readers, and many of them will think of solar cells—and for good reason: organolead halide compounds with the perovskite crystal structure and ABX3 stoichiometry have been the superstars of low-cost photovoltaics for a few years running. A new study shows the perovskites have other tricks up their sleeves. A team led by San Diego State University chemists Xiaolin Zhu and Yong Yan reports that methylammonium lead tribromide and the cesium analog, two of the most studied solar-cell perovskites, double as highly active photocatalysts for organic synthesis (J. Am. Chem. Soc. 2019, DOI:10.1021/jacs.8b08720). The researchers used standard methods to prepare the low-cost nanocrystal catalysts and explored their reactivity under blue-light illumination in tests with 2-bromoacetophenone and octanal. The reactions generated a mixture of products, including the aldehyde α-alkylation product (shown), other C-coupling products, and dehalogenated acetophenone. By tuning reaction conditions, the team was able to boost the selectivity of the industrially important α-alkylation reaction to 96%. The perovskites are 1,000 times as active as some iridium- and ruthenium-based catalysts and only a fraction of the cost, Yan points out.

 

 

Strained reagent opens up new cross-coupling reactivity

The twist on the Suzuki-Miyaura reaction offers a simple route to trisubstituted cyclobutanes

The Suzuki-Miyaura transformation is a classic way to form carbon-carbon bonds. In the reaction, a palladium catalyst typically couples two classes of molecules, aryl halides and aryl boronate complexes. Now researchers have taken a new approach to the traditional cross-coupling, with a method in which palladium reacts with the boronate compound not at the boron atom but at the neighboring carbon-carbon σ bond (Nat. Chem. 2018,  DOI: 10.1038/s41557-018-0181-x). The result is a route to trisubstituted cyclobutanes valued by drug designers.

Led by the University of Bristol’s Varinder K. Aggarwal, the team accesses this novel mode of reactivity by using a strange-looking, bicyclobutyl sulfoxide reagent that exists as a bench-stable solid. Because of the strain placed on it, the cyclobutane’s middle carbon-carbon bond is weakened so it doesn’t act like a normal sigma bond, Aggarwal explains. At first, the reagent “seemed so unusual and esoteric,” he says, “but it’s actually easy to make and it does all this beautiful chemistry.”

Starting from the bicyclobutane precursor, the researchers could form bicyclobutyl boronate intermediates that could react with a variety of coupling partners to form trisubstituted cyclobutane products (example shown). These products contain substitution patterns similar to those made via [2+2] cycloaddition reactions, without that transformations’ requirement that one partner contain an electron-withdrawing substituent.

Boston College’s James P. Morken, whose group has expanded the Suzuki-Miyaura reaction to work with vinyl boronate complexes, says the new reactivity is intriguing. “It will be exciting to see what other types of strained, or even unstrained, ring systems engage in this type of reaction,” he says.

Aggarwal says they’re eager to explore other classes of bicyclobutane compounds in a range of reactions.

 

 

Catalyst chirality switched with a flash of light

Molecular motor and chiral ligand combined to create photoresponsive catalyst


Ultraviolet light can switch the catalyst between two different forms.

Cells have a lot of control over enzymes. They can modify an enzyme to be more or less active, or to change the reaction that it catalyzes. But chemistry in the lab is different. “Normally when we design a ligand or catalyst we design it to do one specific transformation,” explains Ben Feringa of the University of Groningen, who won the 2016 Nobel Prize in Chemistry. “What we wanted to do is make an adaptive catalyst.”

To do that, Feringa’s group has combined a molecular motor with an existing asymmetric catalyst. The result is a catalyst that can switch its chirality, and as a result the stereochemistry of its products, with a flash of light (J. Am. Chem. Soc.2018,  DOI: 10.1021/jacs.8b10816).

The switchable catalyst features what is known as axial chirality. This type of chirality is often used in asymmetric catalysis and results from the nonplanar arrangement of groups around an axis. Feringa’s molecule contains a biphenol group with axial chirality and a rigid core that rotates when hit with ultraviolet light. Because of the tight conformational coupling of these two elements, switching the helicity around the axis also changes the chirality of the biphenol. The biphenol coordinates with a zinc ion to form the active catalyst.

As a proof of principle, the chemists tested the catalyst in an organometallic 1,2-addition reaction to produce different stereoisomers depending on which form of the catalyst was used.

The catalyst is “a real achievement in the dynamic coupling of several types of convoluted chiralities,” says Nicolas Giuseppone, an organic chemist at the University of Strasbourg.

“It nicely demonstrates that switchable chiral catalysts can be obtained by the designed integration of functional units with well-defined stereochemical properties,” adds  Alberto Credi of the University of Bologna. “Such a modular approach could in principle be applied to a wealth of biaryl structures,” he adds, suggesting this could lead to various chiral switches that could control different processes.

Feringa agrees. The dream, he says, is adaptive systems that could be switched from one reaction to another using an external signal. But, he admits, “I don’t know where the most useful applications will be.”

Chemical & Engineering News


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