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Версия для печати | Главная > Центр > Научные советы > Научный совет по катализу > ... > 2020 год > № 93

№ 93

 

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  • Премия имени А.А. Баландина 2019 года
  • XI Международная конференция «Механизмы каталитических реакций»
  • За рубежом
  • Приглашения на конференции
  • Памяти Валерия Васильевича ЛУНИНА



Премия имени А.А. Баландина 2019 года

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XI Международная конференция «Механизмы каталитических реакций»

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Mechanical chirality gives new shape to catalysis

Mechanically chiral rotaxane creates a catalytic pocket a little like an enzyme

Researchers in the UK have used a mechanically planar chiral rotaxane as the basis for an enantioselective gold catalyst (Chem 2020, DOI: 10.1016/j.chempr.2020.02.006). Stephen Goldup and Andrew Heard at the University of Southampton say their work is a proof of concept that mechanical bonds can be used to build “pockets” for selective synthesis akin to those found in enzymes. Heard started by building and separating two mechanically chiral rotaxanes. Rotaxanes are mechanically inter-locked molecules in which a macrocycle is threaded onto a dumbbell-shaped compound or axle. Heard’s chiral rotaxanes differ from one an-other by the direction that the macrocyles loop around the central axles. By adding gold to a phosphine ligand at one end of the axle, Heard creat-ed chiral catalysts for a cyclopropanation reaction (shown). Each catalyst gave a different enantiomeric product and worked about as well as current catalysts. However, Goldup doesn’t expect that synthetic chemists will be using his new catalyst just yet—not least because these chiral rotaxanes are hard to make and isolate. The team now hopes to show that chiral ro-taxanes can help build enantioselective catalysts that are currently impos-sible to create otherwise—opening up new vistas of synthetic chemistry would make it well worth dealing with the fussy rotaxanes.

 

Photocatalyst converts fatty acids to diesel and jet-fuel molecules selectively

Method provides petroleum-free way to turn industrial biowaste into valuable commodity

Industrial waste containing bioderived long-chain fatty acids could serve as sustainable feedstocks for diesel and jet fuels, which today are pro-duced by refining petroleum sources. But several of the methods for re-moving oxygen from fatty acids to convert them to long-chain alkanes, the principal components of these fuels, require temperatures above 250 °C and high-pressure hydrogen, making them expensive and energy intensive. Decarboxylation methods, which convert fatty acids to alkanes by stripping carbon dioxide groups, run under milder conditions. But these methods tend to suffer from low selectivity: they generate a distr-bution of desirable and undesirable products. Now, Zhipeng Huang, Zhitong Zhao, and Feng Wang of the Dalian Institute of Chemical Physics and coworkers report that under mild conditions (30 °C and 0.2 MPa hy-drogen) and in the presence of ultraviolet light, a Pt-TiO2 catalyst decar-boxylates fatty acids selectively (Nat. Catal. 2020, DOI: 10.1038/s41929-020-0423-3). For example, the method converted pure stearic and linoleic acids to n-heptadecane in greater than 90% yield. In tests of crude soybean and tall-oil fatty acids, which are inedible by-products of soybean processing and the pulp industry, respectively, the method produced mixtures of long-chain alkanes at up to roughly 90% yield.

 

Straightforward path to linear alkylbenzenes identified

Nickel catalyst provides a route to the cleaning product chemicals with high yields and selectivity

The world used about 3.5 million metric tons of linear alkylbenzenes, worth about $4.8 billion, in 2019, mostly to make detergents and cleaning products. However, the industrial synthesis of these compounds tends to make mixtures containing branched alkylbenzenes, which don’t biodegrade easily and can pollute rivers, lakes, and oceans. Anti-Markovnikov transition metalcatalyzed hydroarylation, in which the aryl compound bonds to the less substituted carbon of an alkene, could make linear alkylbenzenes, but these reactions have been plagued by poor yields and selectivity.

Now John Hartwig of the University of California, Berkeley; Yoshiaki Nakao of Kyoto University; and coworkers have coupled select arenes with an unconjugated terminal alkene to form linear alkylbenzenes in one step with 85–96% yields and high selectivity (Nat. Chem. 2020, DOI: 10.1038/s41557-019-0409-4).

The current acylation and reduction methods to produce these compounds is through a Friedel-Crafts acylation, which requires multiple steps. The new reaction is the first to be done in one step, with approximately 50:1 linear to branched selectivity, Hartwig says. In addition, the catalyst, a nickel cycloocta-1,5-diene compound, could turn over 280 times and still give an 85% yield. This turnover is more than 10-fold higher than reac-tions involving catalytic hydroarylation of benzene, Hartwig says.

Normally in these reactions, the alkenes isomerize between internal and terminal alkenes, Hartwig says. If the reaction is not selective for terminal alkenes, it will either give a mixture of linear and branched alkylbenzenes, or more or less stop. This new reaction only occurs with the termial alkene, which gets around the isomerization problem. The researchers didn’t intentionally design this selectivity into their system, but it’s a pathway that in principle could be generalized to similar types of X–H bonds adding to alkenes, Hartwig says.

Hartwig and coworkers found that the reaction goes through an alkyl nickel–aryl intermediate and involves an unexpected hydrogen transfer between catalyst ligands followed by reductive elimination. Further analysis of the mechanism revealed that the bulk of the N-heterocyclic carbene ligands did not affect the activity. The reactivity has more to do with noncovalent attractive interactions between ligands, instead of repulsive ones, Hartwig says.

These high-selectivity results from Hartwig and coworkers achieve a long-standing goal, says T. Brent Gunnoe, an organometallic chemist at the University of Virginia. “The use of large N-heterocyclic carbene ligands and noncovalent interactions to enhance the rate of catalysis is an unexpected result and a clever idea that could have broader impact in the design of other catalysts,” he says.

If the catalyst activity were high enough, this reaction would be easy to scale up to the industrial level, Hartwig says, because it’s a simple addition reaction, and there are no side products. However, it’s not perfect. “The biggest drawback of this system, in addition to catalyst lifetime, is its low tolerance for a lot of functional groups,” such as esters and nitriles, he says.

 

Nickel catalyst fends off air attack

Air-stable complex offers a more convenient approach to coupling reactions

Nickel catalysts have shot to synthetic stardom over the past decade or so, proving particularly useful in coupling reactions that form carbon-carbon or carbon-nitrogen bonds. The most popular catalyst for these reactions is bis(1,5-cyclooctadiene)nickel (Ni(COD)2), and although it’s very effective, the complex decomposes rapidly in air, making it troublesome to use.

Researchers at the Max Planck Institute for Coal Research have now developed an air-stable alternative to Ni(COD)2 that could make nickel ca-talysis much more accessible to users (Nat. Catal. 2019, DOI: 10.1038/s41929-019-0392-6). “Air stability has been a huge challenge,” says Josep Cornellà, who led the research, “and a lot of people don’t have access to glove boxes.”

Nickel is increasingly used in reactions traditionally dominated by palladium catalysts. Not only is nickel cheaper, Cornellà says, it can also catalyze a wider range of reactions than palladium. But the nickel catalysts used in these reactions are often extremely reactive, so they must be freshly generated within the reaction mixture. Chemists do this by mixing a precatalyst like Ni(COD)2 with other ligands, such as phosphines or bi-pyridines, which take the place of cyclooctadiene to form the active catalyst.

The new precatalyst is based on a nickel atom surrounded by three stil-bene ligands that bear trifluoromethyl groups (Ni(Fstb)3). These ligands enclose the nickel and protect it from oxygen. Cornellà’s team could make the red solid in 20 g batches by mixing a cheap precursor—nickel acetylacetonate—with the stilbene ligand and a reducing agent called triethylaluminum. The complex is stable for months in a freezer and only starts to decompose after several days at room temperature.

“People will use this immediately because it’s a drop-in replacement,” for Ni(COD)2, says Nilay Hazari of Yale University, who develops transition metal catalysts. “In my mind, it’s a very significant step forward.”

The Max Planck researchers used Ni(Fstb)3 to generate active catalysts in more than a dozen different reactions, including classics such as a Suzuki-Miyaura coupling, Buchwald-Hartwig carbon-nitrogen bond formations, and a Heck reaction. It generally performed as well as Ni(COD)2, giving the intended products in high yields.

Teamed with a carbene ligand, the new complex also catalyzed a coupling reaction between pentafluorobenzene and an alkyne, a feat that Ni(COD)2 cannot manage. Some reactions were less successful, though—a coupling reaction that involved butadiene or isoprene failed because these compounds could not displace the stilbene ligands from the complex.

Hazari says that Ni(Fstb)3 could be particularly helpful in catalyst development studies. For example, it could easily be loaded onto a 96-well plate with a different ligand in each well to assess which of the resulting active catalysts were most effective for a particular reaction. “For screening ligands, this is fantastic,” Hazari says.

However, he thinks there is still room for improvement. Triethylaluminum can ignite spontaneously in air, so it would be better to use a safer reducing agent to prepare the complex. It would also be helpful to improve the complex’s thermal stability, he adds. Cornellà and his colleagues are now testing a library of other stilbene ligands to fine-tune the reactivity of their complex.

Chemical & Engineering News


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Памяти Валерия Васильевича ЛУНИНА
(1940 – 2020 гг.)

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