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Версия для печати | Главная > Центр > Научные советы > Научный совет по катализу > ... > 2020 год > № 95-96

№ 95-96

 

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  • VI Всероссийская научная молодежная школа-конференция
    «Химия под знаком СИГМА: исследования, инновации, технологии»
  • 3-я Всероссийская конференция «Методы исследования
    состава и структуры функциональных материалов»
  • За рубежом
  • Приглашения на конференции
  • Памяти Баира Сыдыповича БАЛЬЖИНИМАЕВА
  • Памяти Валерия Кузьмича ДУПЛЯКИНА
  • Памяти Романа Алексеевича БУЯНОВА



VI Всероссийская научная молодежная школа-конференция «Химия под знаком СИГМА: исследования, инновации, технологии»

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3-я Всероссийская конференция «Методы исследования состава и структуры функциональных материалов»

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3-я Всероссийская конференция «Методы исследования состава и структуры функциональных материалов.
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Chemists shine light on new way to think about reductive elimination

Computational study confirms alternative mechanism for C-C bond forming reaction


The Pd-catalyzed allylic alkylation goes through a transition state involving 7 atoms.

Reductive elimination reactions, in which a transition metal catalyzes new bond formation between two molecules, are one of organic chemists’ most powerful tools. For example, one flavor of these reactions, palladium-catalyzed allylic alkylations can make new C–C bonds while tolerating many functional groups. In addition, these mild reactions are one of the few ways chemists can generate compounds containing hindered quater-nary carbons.

Through a detailed computational study, Brian Stoltz and coworkers at the California Institute of Technology have figured out that some Pd-catalyzed reductive eliminations go through a different mechanism than previously thought, providing a new understanding of how reductive eliminations work (J. Am. Chem. Soc., 2020, DOI: 10.1021/jacs.0c09575).


Based on their computational work, chemists built a familiar arrow pushing diagram
to explain what happens in the transition state of the Pd-catalyzed allylic alkylation.

Traditionally, chemists thought the mechanism involved transition states featuring bond breaking and forming between three key atoms. Stoltz and coworkers studied a specific Pd-catalyzed reductive elimination reaction in which the desired C-C bond forms within the same molecule (shown). The chemists determined that the transition state for this reaction involves bond forming and breaking between seven atoms. Describing this seven-centered reductive elimination process allows chemists to expand the way they think about this fundamental reaction, Stoltz says. This mechanism is not the way chemists learn how reductive eliminations work in graduate school, he says, but it suggests that there are other ways that reductive elimination can happen. “It could change the way people think about what’s possible.”

For this computational study, the researchers focused solely on the Pd-centered transition state. Generally, this transition state is difficult for chemists to study experimentally because it’s not the rate-limiting step of the reaction, so it exists briefly. Chemists have suspected that this transi-tion state involves 7 key atoms, and is organized in a ring, like those found in pericyclic reactions. Not only did Stoltz and coworkers confirm this suspicion, but also they showed that the Pd’s d-orbitals allow the transition state to be aromatic, which means it’s stabilized through ex-tended conjugation of orbitals across the seven atoms. This stabilization allows the transition-state energy to be low enough for the reaction to proceed. In addition, they found that because of the geometry of Pd’s d orbitals this aromaticity doesn’t fit the traditional structure, instead it is shaped like a Möbius strip, which is a connected loop with one surface and a half turn.

The researchers also used a type of chemical bonding theory to explain the mechanism of the reaction in terms of arrow pushing. This drawing method in which arrows depict where electrons move in a chemical reac-tion is the way that many organic chemists are taught to understand these transformations.

Through detailed analysis, the researchers showed the inner workings of an important organometallic reaction, says Dean Tantillo, a computational organic chemist at the University of California, Davis. They used hard-core quantum chemistry to build a model using familiar, intuitive concepts, he says.

This study “allows us to think about some of the things we can improve on,” in the current reaction, and gives a high-level theoretical backing for the feasibility of new kinds of reaction, Stoltz says.

 

TiO nanocrystals exhibit unusual slow-motion blinking

Made via template-based synthesis, the defective crystals actively mediate photocatalysis

Like microscopic fireflies, semiconductor nanocrystals, also known as quantum dots, light up intermittently. For more than 20 years, researchers have worked to understand and control these random fluctuations in light emission because the “blinking” limits the stability of quantum dot–based devices like solar cells and light-emitting diodes. The observation of what may be a new type of blinking behavior suggests that researchers have more work to do (Angew. Chem., Int. Ed. 2020, DOI: 10.1002/anie.202005143). A team led by Tao Zhang, Tewodros Asefa, and Alexei M. Tyryshkin of Rutgers University and Eliška Mikmeková of the Czech Academy of Sciences reports that treating a polymer-derived porous carbon template with a titanium dioxide precursor yields TiO2 nanocrystals—limited to 10 nm in diameter and riddled with oxygen vacancies—that blink in an unprecedented way. In contrast to years of studies reporting rapid light–induced blinking with flashes often lasting just a fraction of a second—sometimes longer—the new work describes electron beam–induced flashes lasting roughly 15 s. In addition, when the crystals are irradiated with light, the resulting charge separation is unusu-ally stable, making the particles active photocatalysts, as shown by tests in which the team used them to reduce carbon dioxide with water.

 

Road to chiral alkylamines paved with iridium

Cationic catalyst adds amines to internal alkenes

Alkylamines are one of the most useful compounds in a chemist’s bag of tricks. Pharmaceuticals, agrochemicals, and polymers often contain these functional groups. Ideally, chemists would like to make alkylamines from widely available feedstocks, but the existing methods for doing so don’t excel at reacting with internal alkenes. John Hartwig and coworkers at the University of California, Berkeley, have now figured out how. The group used a positively charged iridium catalyst and an amine to add an NH bond to an internal alkene. This generates chiral alkylamines with high regioselectivity and moderate to high yields (Nature, 2020, DOI: 10.1038/s41586-020-2919-z).

Researchers have tried to make similar compounds in the past but have not been able to get good yields of the desired chiral products. The key to Hartwig’s synthesis was choosing bidentate aminopyridine derivatives both as ligands and reactants, which helps control the chirality of the product while preventing unwanted side reactions. During the reaction, two aminopyridine molecules bind to the Ir center. One is displaced by the incoming alkene while the other remains on the catalyst. The researchers think this configuration locks the geometry so the alkene can only add from one side of the catalyst molecule. The amine group on the aminopyridine helps to block unwanted side reactions, leading to chiral alkylamines with regioselectivity near 100% in some cases. The pyridine moiety can then be removed to give the final amine.

The cationic Ir catalyst also speeds up the reductive elimination step to release the alkylamine, which “leads to addition faster than isomerization,” Hartwig says. The team hopes that this new synthesis will lead to adding O–H and C–H groups in the same way.

The ultimate goal is to use ammonia as the amine source, but the researchers started out with the aminopyridine as a stand-in. “This approach is promising because it not only tunes the catalyst but includes an ammo-nia surrogate,” says Vy Dong, an organic chemist at the University of California, Irvine. “It highlights the power of designing reagents and using mechanistic insights to achieve otherwise impossible reactivity.”

 

One-pot method turns polyethylene into valuable detergent precursor

Pt-catalyzed upcycling process doesn’t use solvents or relatively high temperatures

Though polyethylene (PE) is recyclable, the high-temperature, energy-intensive process yields only low-value products. As a result, industries don’t earn enough money to make recycling PE worth their effort. Now Susannah Scott and coworkers from the University of California, Santa Barbara; the University of Illinois; and Cornell University have developed a catalytic process to upcycle PE into something of higher value: long-chain alkylaromatics (Science 2020, DOI: 10.1126/science.abc5441). Scientists can sulfonate these compounds to make detergents and surfactants. The market for one of the alkylaromatics, linear alkylbenzenes, is about $9 billion per year. To upcycle PE, the researchers seal either high- or low-density PE and an alumina-supported platinum catalyst in a reaction vessel. They then heat the materials to 280 °C for 24 h. The polymer melts and then reacts with the catalyst, releasing H2. This hydrogenolysizes the PE, and these smaller bits of polymer then aromatize to give the higher-value products in 80% yield. This method “couples a thermodynamically unfavorable reaction, the aromatization, with a thermodynamically favorable reaction, the hydrogenolysis,” Scott says, a key step to getting the temperature below 300 °C.

Chemical & Engineering News


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