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№ 85

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  • Евгений Зиновьевич ГОЛОСМАН
    К 80-летнему юбилею
  • НАУЧНЫЙ СОВЕТ ПО КАТАЛИЗУ ОХНМ РАН
    Отчет о научно-организационной деятельности в 2017 году
  • IV Международная конференция
    “Катализ для переработки возобновляемого сырья:
    топливо, энергия, химические продукты” (CRS-4)
  • За рубежом
  • Приглашения на конференции
  • Памяти Роберта К. Грасселли

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Евгений Зиновьевич ГОЛОСМАН
К 80-летнему юбилею

Переход к элементу

Свернуть/Развернуть


Отчет о научно-организационной деятельности в 2017 году

Переход к элементу

Свернуть/Развернуть


CRS-4

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За рубежом

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Sulfones expand the reach of radical cross-couplings

New reagents offer streamlined synthesis of valuable fluorinated products

This is one of more than 60 examples of radical cross-couplings using new sulfone reagents with unique reactivity.

 

Cross-coupling reactions, wherein a catalyst brings together molecular partners to forge a new bond, are powerful transformations that are among the most used tools in synthesis. For decades, researchers have mined this area of chemistry, developing methods for a litany of coupling partners. Now chemists have found a novel, and valuable, pairing.

A team of researchers led by Phil Baran at Scripps Research Institute, California, has introduced alkylsulfones as coupling partners for radical cross-coupling reactions, providing access to valuable fluorinated structures that would be cumbersome to make with typical alkyl coupling partners (ChemRxiv 2017, DOI: 10.26434/chemrxiv.5715106.v1). Whereas classic cross-couplings conjoin two aryl partners, this type of cross-coupling is particularly well-suited for threading together aryl and alkyl partners.

With the help of a nickel catalyst and pyridine ligand, the reaction couples arylzinc compounds with alkylsulfone reagents—in which the oxidized sulfur atom is connected to a five-membered ring loaded with nitrogens. The researchers demonstrated the scope of the reaction by reacting compounds containing a variety of substitution patterns and by synthesizing known biologically relevant molecules to highlight the shortened synthetic routes enabled by the technique.

Notably, the sulfone reagents let researchers directly install fluorine atoms at the alkyl coupling site, whereas previously chemists would have had to run difficult deoxyfluorination reactions to make the requisite fluorinated coupling partner, says Scott Denmark of the University of Illinois, Urbana-Champaign. “The unique reactivity of the N-phenyltetrazole sulfones is surpassed only by their practicality as bench stable and odorless, crystalline compounds,” he says.

The team produced a handful of the fluorinated sulfone reagents on a large scale. In a tweet, the Baran lab offered up free samples of the compounds, giving interested chemists the opportunity to try out the method before the work is published in a peer-reviewed journal.

Cathleen Crudden of Queen’s University in Ontario and Masakazu Nambo of the Institute of Transformative Bio-Molecules at Nagoya University, who collaborated with each other on a different cross-coupling reaction using phenyl sulfones, also highlight the value of the new sulfone reagents’ ability to incorporate fluorine, especially in medicinal chemistry applications. “There is no doubt that this work is going to be game-changing and illustrates that there is still much to do in the field of cross coupling,” they told C&EN.

 

 

Catalyst treatment could boost exhaust cleanup

Steam treating enhances platinum’s durability and knack for scrubbing CO from engine emissions

The catalysts that clean up automotive emissions typically consist of particles of platinum and other precious metals anchored on oxides. Because only the metal atoms at the particle surfaces come in contact with reactants and catalyze reactions, catalyst manufacturers strive to make these particles as tiny as possible.

But these supported catalysts come with trade-offs. Dispersing the precious metal as finely as possible can go too far, and the catalysts can become unstable: The metal particles diffuse, coalesce, and lose their catalytic oomph. And the catalysts are often inactive when the temperature of the exhaust is low, which is the case when today’s engines start on a cold morning and will regularly be the case with future energy-efficient engines.

A new study on automobile exhaust cleanup describes a way to bypass those problems in a catalytic two-for-one deal. Researchers have shown that a simple procedure can stabilize a platinum-based automotive catalyst and reduce the temperature at which it can thoroughly strip CO from engine exhaust (Science 2017, DOI: 10.1126/science.aao2109).

 

Such a treatment could help clean up emissions from future engines designed to recover energy lost in hot exhaust, which results in lower temperatures of the gas that passes through the catalytic converter.

In the run-up to the new study, a team led by University of New Mexico chemical engineer Abhaya K. Datye took dispersing metal particles on an oxide support to the extreme. In 2016, the group reported that isolated platinum atoms on ceria could convert CO to CO2, a key reaction in engine-emissions cleanup. But the catalyst worked weakly.

So the team, which includes Yong Wang of the Pacific Northwest National Laboratory, searched for chemical and physical treatments that would boost the Pt-CeO2catalyst’s activity without causing it to fail quickly, a common occurrence during catalyst development.

Eventually the group found that heating the catalyst to 750 °C in steam drastically improves its ability to mediate CO oxidation. Specifically, in contrast to the untreated catalyst, which needs to be heated to roughly 210 °C to begin oxidizing CO and achieves 100% CO conversion at 320 °C, the treated catalyst begins working at just 60 °C and reaches 100% conversion at 148 °C. Furthermore, the treatment makes the catalyst durable: It showed no signs of deactivation even after 300 hours of testing.

Microscopy and spectroscopy analyses indicate that the steam enhances CO-oxidation performance by creating catalytically active sites featuring ceria-bound Pt-OH groups.

“This discovery could help advance the technology for vehicle exhaust conversion,” remarks Bruce C. Gates, a catalysis specialist at the University of California, Davis. “The authors’ catalyst characterizations provide deep insights and point the way forward.” The characterization work also raises intriguing questions for further study, he adds. For example, Gates proposes that researchers should examine the nature of the sites on ceria at which platinum bonds and determine if they are defects. He also wonders if a metal cheaper than platinum would work similarly. 

 

 

A light approach to installing heavy hydrogens

Photoredox catalysis shines in a reaction that swaps out hydrogen for deuterium or tritium

To follow the fate of a drug or a drug metabolite as it wends its way through the body, scientists will often label the compound by switching out a hydrogen atom for one of its heavier isotopes—deuterium or tritium. This molecular switcheroo sounds simple, but it can actually be a lot of work, requiring chemists to resynthesize a molecule so it has an atom, usually a halogen, or unsaturated bond where the heavy hydrogen can be swapped in or added. The process can take months.

Chemists at Princeton University and Merck & Co. have now come up with a way to switch hydrogen out for deuterium or tritium in a matter of moments. The reaction uses photoredox-mediated hydrogen-atom transfer to replace the hydrogens at C–H bonds adjacent to amines with deuterium or tritium from D2O or T2O (Science 2017, DOI: 10.1126/science.aap9674).

Amines are common motifs in biologically active molecules, and the chemists, led by Princeton’s David W. C. MacMillan, show the reaction works on myriad drug molecules (examples shown). For the tritiation reactions, the chemists also figured out how to make T2O, which isn’t commercially available, during the reaction using T2 gas and platinum oxide.

The new labeling methodology could “open the door to earlier and expanded use of isotopic labeling in drug discovery, significantly enhancing our ability to study drug candidates on a deeper level and across a range of applications,” says Jennifer Lafontaine, senior director of synthesis and analytical chemistry in Pfizer’s oncology medicinal chemistry group.

These pharmaceuticals were tritiated (indicated with a red dot) using
a photoredox-catalyzed exchange reaction.

Ian Davies, a coauthor on the paper who recently moved from Merck to Princeton, points out that the reaction could also help chemists studying the biology of complex natural products. “You don’t have to resynthesize the natural product,” he explains. As long as chemists have isolated the molecule, this reaction can deuterate or tritiate it.

Paul Greenspan, senior director of discovery chemistry at Takeda Pharmaceuticals, says the work “provides yet another impressive example of the power and versatility of photoredox catalysis, which continues to surprise us with mild, controllable, and highly practical transformations that would have been previously unattainable.”

MacMillan’s lab has been working in photoredox catalysis for several years. He says that the idea for the isotopic labeling reaction came about when he was visiting Merck, and Davies had set up a meeting for him with the company’s isotopic labeling group. “I said, ‘Why am I meeting with these people? What’s the point?’ ” MacMillan recalls. Davies’s reply was: “I don’t know.” But he assured MacMillan that there was value to be found in the meeting between scientists who don’t often interact.

MacMillan says that only hours after the meeting chemists in his lab were running experiments to test whether photoredox chemistry could help with isotopic labeling. “By that evening we knew we were onto something pretty interesting,” he says.

Despite deuterium and tritium’s rich history in physical organic chemistry and biochemistry, the isotopes have been underused in recent decades, notes Jacob M. Hooker, a molecular imaging expert at Massachusetts General Hospital. MacMillan and Merck’s photoredox method “is likely to draw attention to and revitalize the field,” he says. “This will spark additional innovative methods that drive quantitative chemical biology and pharmacology once again.”

 

Chemical & Engineering News


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