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Версия для печати | Главная > Центр > Научные советы > Научный совет по катализу > ... > 2017 год > № 82

№ 82



  • IV Научная конференция БОРЕСКОВСКИЕ ЧТЕНИЯ
  • Воспоминания участников конференции о Г.К. Борескове
  • Жизненный путь академика Г.К. Борескова
  • За рубежом
  • Приглашения на конференции

IV Научная конференция БОРЕСКОВСКИЕ ЧТЕНИЯ

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Воспоминания участников конференции о Г.К. Борескове

Переход к элементу


Жизненный путь академика Г.К. Борескова

Переход к элементу


За рубежом

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Improved route to PAHA polyesters

Approach greatly improves customizability and efficiency of poly(α-hydroxyacid) synthesis

A two-catalytic-step process can efficiently convert versatile OCA monomers into a range of customizable PAHAs.

Poly(α-hydroxy acid), or PAHA, polyesters can form products such as biodegradable sutures and implants, as well as cell-penetrating nanoparticles for drug and gene delivery. But synthesizing PAHA polyesters hasn’t been easy. Now researchers report a new way to prepare PAHA polyesters with desirable sets of properties that have been difficult or impossible to access before.

The stiffness, stretchability, tensile strength, and other physical properties of these polymers can be customized by varying the sidechains of the lactide and glycolide monomers used to make the materials. The repertoire of PAHA polyesters has been limited because synthesizing these monomers requires multistep and low-yield reactions, and adding sidechains to them is difficult.

In 2006, Didier Bourissou of Paul Sabatier University and coworkers showed that PAHAs could instead be made from O-carboxyanhydride (OCA) monomers, which are much easier to prepare and modify. Nevertheless, the organocatalytic reactions used to polymerize OCAs are slow and have undesired side reactions. Also some of the polymer products have uncontrolled stereochemistry and broad and unpredictable molecular weight distributions.

Rong Tong and Quanyou Feng at Virginia Tech have now addressed these issues with a new method for rapid and controlled OCA polymerization (J. Am. Chem. Soc. 2017, DOI: 10.1021/jacs.7b01462). In the two-step approach, a photoredox nickel-iridium catalyst first decarboxylates OCAs. Then zinc alkoxide catalyzes a ring-opening polymerization of the decarboxylated OCAs to yield the PAHAs. The process is fast, minimizes undesired side reactions, and produces PAHAs with controlled stereochemistry and narrow and predictable molecular weight distributions.

“Our chemistry will allow us to prepare polyester materials with customized macroscopic properties such as rigidity, elasticity, and biodegradability,” Tong says. Virginia Tech has applied for a patent on the technique.

Bourissou agrees that the technique should make “a variety of PAHA polymers with different functionalities and microstructures readily accessible.” He points out that “one practical limitation is that the ring-opening polymerization has to be performed at low temperature, –15 °C or below, to achieve good control and avoid side-reactions.” Fortunately, he says, OCAs are highly reactive and polymerize rapidly even at low temperature.


New catalyst details could help turn carbon dioxide into something valuable

A win-win approach to curbing climate change could be capturing carbon dioxide from the atmosphere and converting it to valuable products. A study presented at the American Chemical Society national meeting in San Francisco may help advance that effort by revealing mechanistic details of a catalytic process that converts CO2 to a commodity chemical—methanol.

On industrial scales, catalysts composed of copper and zinc oxide supported on alumina hydrogenate carbon monoxide and CO2 to methanol. But these catalysts have shortcomings, according to Ping Liu, a chemist at Brookhaven National Laboratory.

Speaking at a symposium sponsored by the Division of Energy and Fuels, Liu pointed out that the Cu-ZnO catalysts are not very efficient or selective in producing methanol. These reactions also require high temperatures and high pressures of the reactant gases. What’s more, she said, chemical details of the active catalytic site remain elusive. That information could be the key to designing catalysts with improved energy and chemical efficiency, Liu says.

In an ongoing debate regarding the catalyst’s active site, various researchers have argued that highly active Zn-Cu alloy species are the key catalytic players. In contrast, Liu’s new work suggests that the action occurs at the atomic interface between ZnO and Cu (Science 2017, DOI: 10.1126/science.aal3573).

To reach that conclusion, Liu, Brookhaven colleagues José A. Rodriguez  and Shyam Kattel, and Columbia University’s  Jingguang Chen, prepared several types of Cu and ZnO reference catalysts, including one made of zinc nanoparticles deposited on copper, and another with ZnO nanoparticles on copper.

They analyzed and directly compared the CO2-to-methanol chemistry of all the catalysts using synchrotron-based photoelectron spectroscopy and computational methods. The computations predicted that Cu-ZnO surface species should be the most reactive form of the catalyst. They also predicted that the Zn-Cu species shouldn’t remain stable under reaction conditions. Instead, it should react with oxygen and form copper zinc oxide. And that’s exactly what Liu and coworkers found in the lab.

Now the group aims to use that information to optimize the interface between ZnO and Cu to improve the catalysts.

“This is a highly important study with excellent quality data and supporting theoretical calculations,” said  Charles T. Campbell, a catalysis specialist at the University of Washington, Seattle. CO2 hydrogenation to methanol is one of the most likely pathways for converting the greenhouse gas to a valuable product, Campbell asserted. He added that this study should help improve that catalytic process.


Chemists get better acquainted with palladium catalysts

A team of chemists from industry and academia has taken a step back from the normal hustle and bustle of synthesizing new molecules to investigate what happens to workhorse palladium catalysts when they go catalyzing. The traditional thinking has been that a precursor Pd(II) salt is reduced to a catalytically active Pd(0) species when a ligand is introduced. Yet chemists have never had a complete picture of this reduction mechanism, and some scientists have found that under certain conditions a Pd(I) species can get involved in the formation of the Pd(0) active catalyst.

Researchers led by Franziska Schoenebeck,  a chemistry professor at RWTH Aachen University, and Thomas J. Colacot, global R&D manager at catalyst firm Johnson Matthey, have now completed a systematic experimental and computational study to better understand how this process really works. Along the way, they found that starting with a Pd(I) bridging dimer precatalyst offers faster reaction rates and better yields than  starting with a Pd(II) salt to generate the Pd(0) catalyst, as is commonly done in Buchwald-Hartwig aminations, Suzuki-Miyaura cross-couplings, and other palladium-catalyzed reactions. Colacot presented these new findings during a Division of Organic Chemistry symposium at the ACS national meeting in San Francisco.

When chemists unraveled the complete mechanism of Pd(0) active catalyst formation, they found that the bromide-bridging Pd(I) phosphine precatalyst shown produces better results in cross-coupling reactions than starting with a Pd(II) precursor.

Although other researchers have previously studied the addition of phosphine ligands to a Pd(II) bromide salt and determined how a bromide-bridging Pd(I) dimer forms, a full accounting of all the molecular participants had been lacking, Colacot said. Schoenebeck, Colacot, and their colleagues determined how the ratio of Pd(II) bromide precursor to phosphine ligand, as well as the order in which the ligand and other reagents are added, dictates whether or not the Pd(I) precatalyst forms ahead of the Pd(0) catalyst. Along the way, the researchers found a key hidden intermediate species, which was visible only when the reactions were run on a gram scale. This new species, a Pd(II)Br3 dimer, turns out to be the linchpin that holds the pathway together for formation of the Pd(I) precatalyst.

Ultimately, the researchers found that starting with the preformed Pd(I) precatalyst provides better results than starting with the Pd(II) bromide salt and adding the phosphine ligand (J. Am. Chem. Soc. 2017, DOI: 10.1021/jacs.7b01110). “This implies that absolute process precision is required in catalysis to get the optimal results,” Colacot said.

The new results “are quite surprising to me, especially since our efforts studying a Pd(I) complex showed them to be very poor catalysts,” says Stanford University chemistry professor Barry M. Trost. “This paper demonstrates that the amazing subtleties of the exact structures of the active catalyst can open new windows in palladium-catalyzed processes.”

The new findings already have Bruce H. Lipshutz of the University of California, Santa Barbara, and his group thinking about ways to improve their palladium-catalyzed reaction chemistry. The symposium where Colacot made his presentation was in Lipshutz’s honor as the 2017 recipient of the  HerbertC. Brown Award for Creative Research in Synthetic Methods.

The Schoenebeck and Colacot report “is a must-read,” Lipshutz said. “It’s an outstanding example of collaboration between academic and industrial groups that creates tremendous value for the catalysis community.”

Colacot said the Pd(I) precatalyst is commercially available and is being sold in kilogram amounts. In addition, other Pd(I) precatalysts based on the new information have been tested by the Schoenebeck group and are becoming available.


Pinning aryl groups on alkanes

Organosilicon-catalyzed reaction functionalizes even notoriously inert methane, without help from precious transition metals

Unactivated alkanes are difficult to functionalize, and most catalysts that derivatize them by opening hydrocarbon C–H bonds are based on precious transition metals.

Researchers have now developed a class of intermolecular C–H arylation reactions that use catalysts made from more-abundant materials: silicon and boron. The reaction adds aryl groups to C–H bonds of simple hydrocarbons, including to the notoriously inert bonds in methane, at mild temperatures (Science 2017, DOI: 10.1126/science.aam7975).

“Alkanes are bulk components of gasoline and as such are supercheap commodities, which, if converted to functionalized compounds, would become much more valuable,” comments Jay Siegel of Tianjin University, who developed a related intramolecular reaction but was not involved in the new study. “This is an area rich in prospects, with a bright future for chemical synthetic methods development.”

A hydrocarbon arylation reaction begins with catalyst preparation (left).

Hosea M. Nelson and coworkers at the University of California, Los Angeles, prepare the new organosilicon catalyst from an organosilane and a weakly coordinating carborane anion. The catalyst defluorinates an aryl fluoride starting material, likely generating an aryl cation intermediate that inserts electrophilically into a C–H bond of an alkane substrate to yield an arylated alkane. A key trimethylsilyl group on the aryl fluoride aids fluoride abstraction, helps the cation react quickly, and eases catalyst regeneration.

“Electrophilic reactions with methane are exceptionally rare, and the C–H functionalization of methane reveals the extraordinary reactivity of this system,” says Douglas Klumpp of Northern Illinois University, an expert on highly reactive electrophilic intermediates. The clever use of a trimethylsilyl group, he says, enabled the researchers “to tame a lion,” the aryl cation intermediate, “and that lion is able to do some very nice tricks.” However, Klumpp notes that one limitation of the chemistry is that “the aryl fluoride starting materials are expensive or difficult to obtain.”

“The chemistry isn’t ready for prime-time applications,” Nelson says. “It’s a new strategy that will hopefully fuel further study. We need to find ways to improve the reaction’s efficiency, selectivity, and substrate scope. We have filed a provisional patent and look forward to working with the chemical industry to develop practical applications.”

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