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Версия для печати | Главная > Центр > Научные советы > Научный совет по катализу > ... > 2023 год > № 107

№ 107

 

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  • 8-й Азиатский симпозиум по современным материалам
  • За рубежом
  • Приглашения на конференции
  • Памяти Евгения Зиновьевича ГОЛОСМАНА



8-й Азиатский симпозиум по современным материалам ASAM-8

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Photocatalytic reaction creates cyclopropanes from unactivated alkenes

The oxygen-mediated redox pathway makes carbon triangles without relying on hazardous reagents

Cyclopropanes are a common motif in medicinal chemistry. Three-carbon rings show up in drug structures for conditions including asthma, psoriasis, and COVID-19.


This new reaction makes it easier and safer to add cyclopropanes to complex molecules.

But despite its utility, cyclopropane is not easy to make compared with larger rings. There are a few established chemical methods for coaxing these highly strained molecular triangles into existence, but they require extra prefunctionalization steps or hazardous reagents such as diazo compounds.

Now, a team from Pennsylvania State University led by Ramesh Giri has developed a photocatalytic method for making cyclopropanes by combining alkenes and activated methylene derivatives such as 1,3-dicarbonyl compounds (Science 2023, DOI: 10.1126/science.adg3209). The reaction uses bench-stable reagents and isn’t air-sensitive—in fact, oxygen participates in it as an intermediate oxidant.

Rather than using a carbene or metal carbenoid as the most popular established methods do, this reaction was designed to go through a series of single-electron transfer steps initiated by blue light. “Carbenes are basically two radicals on one carbon. So instead of generating them in one step, we thought we would generate [radicals] stepwise,” Giri says. In addition to the starting materials, photocatalyst, and oxygen, the reaction also requires a catalytic amount of iodine to regenerate the catalyst.

The researchers found that the reaction works with a variety of mono- or disubstituted alkenes—though not dienes or styrene. It also works with multiple combinations of electron-withdrawing groups on methylene. Those groups also provide handles for further functionalization on the cyclopropane ring, which opens up synthetic possibilities that other methods can’t access.

Giri says he and his team are working on expanding the scope of alkenes that can be used and on scaling up the reaction. They also want to explore how to use this catalytic process for other alkene functionalization reactions. Giri also says he’s looking into incorporating the reaction into an undergraduate lab course. “This is how convenient it is. You couldn’t do that with any of the other reactions that you use to make cyclopropanes.”

 

Stereoenrichment with a single catalyst

Multitasking titanium catalyst erases and rewrites alcohols’ stereochemical information using two different mechanisms

Sometimes conformity is a good thing. Particularly when it comes to stereochemistry. While stereocenters arise in mirror-image enantiomer pairs when left to their own devices, the enantiomers can have very different biological activity. So synthetic and medicinal chemists would prefer to make only one of them at a time.

Researchers from the Shanghai Institute of Organic Chemistry, led by Zhiwei Zuo, have now added a new reaction to the stereoselective toolbox. They have devised a clever method for turning mixtures of alcohol isomers into an enantiomerically pure product using a single catalyst (Science 2023, DOI: 10.1126/science.adj0040).

Once a mixture of stereoisomers is created, it’s very difficult to amend that without separating the isomers from each other and throwing out the unwanted one. This is because physics isn’t on chemists’ side in this matter. Mixtures have higher entropy, which the universe prefers. And something called the principle of microscopic reversibility says that any mechanistic step that flips an unwanted enantiomer into its twin can also flip it back. Fixing a mixture requires at least two distinct steps: one to erase stereochemical information from the unwanted isomer, and another to rewrite it to match the desired form.

Chemists developing deracemization reactions have previously accomplished this by using different catalysts for each step. “We saw a more straightforward way to do that,” Zuo says—with a single titanium catalyst that can play two different mechanistic roles, bound to a chiral phosphoric acid ligand.

Light provides the energy to kick an electron from the ligand to the metal center through a process called ligand-to-metal charge transfer. The reduced metal center can then facilitate a radical reaction to break a carbon-carbon bond adjacent to the alcohol, creating an achiral intermediate. The titanium changes roles to act like a typical transition metal catalyst to coordinate with the intermediate and stitch the molecule back together. Throughout the reaction, the ligand helps direct the stereochemistry, making it so that bonds are preferentially broken in the unwanted isomer and ensuring that the molecules are reassembled with the correct orientation.

Individually, each step is only moderately stereoselective but they amplify each other to give nearly complete conversion to a single isomer. “We can get a synergy effect,” says Zuo.

The reaction works on cyclic secondary and tertiary alcohols with ring sizes between 4 and 7 carbons as well as some acyclic amino alcohols. The ideal ligand is different for each type of ring, and the molecule has to be able to stabilize the radical intermediate for the transformation to work, Zuo says, but he and his team are far from finished working on this reaction. They are developing another paper that describes more mechanistic explorations and an expanded substrate scope, he says.

In an email, Alison Wendlandt of the Massachusetts Institute of Technology called the work “spectacular” and “literally amazing,” praising it for its elegance and simplicity. She said the method “significantly expands” the scope of molecules that can be deracemized, opening up new routes to making useful chiral building blocks.

Also in an email, Robert Knowles of Princeton University called the paper “an exciting addition to the literature on light-driven deracemization” and said it could pave the way for exciting future advances.

Zuo says he envisions improving the reaction to edit the stereochemistry of alcohols—or other key functional groups—on more complex molecules, as a form of molecular editing.

 

A new catalyst for breaking down nylon 6

Researchers turn commercial plastic into reusable monomer without solvent or extreme temperatures

Researchers at Northwestern University have taken a step toward making it easier to chemically recycle nylon 6. The polymer is used in products including carpets, textiles, packaging—and fishing nets, which are a major contributor to the Great Pacific Garbage Patch.

The team, headed up by catalysis chemist Tobin J. Marks, has designed a process that converts nylon 6 back into its starting material, caprolactam (Chem 2023, DOI: 10.1016/j.chempr.2023.10.022). This depolymerization process doesn’t require a solvent and takes less energy to run than other methods for chemically breaking down nylons.

There are already industrial-scale processes for depolymerizing nylon trash and then using the caprolactam to make new nylon for textiles. Several brands, including Patagonia, use recycled nylon in some of their products. But the status quo chemical recycling process for nylon is time- and energy-intensive, making it quite expensive, says Yosi Kratish, who co-led the project. He says the team wanted to make nylon recycling easier, greener, and more cost-effective.

The polymers in plastic waste are simply huge molecules with bonds that can be manipulated like any other molecules, says Kratish. “It’s not a black box. We can zoom in on the weak spots and attack them.”

The catalyst system that the researchers designed is an improved version of one that they reported last year (Angew. Chem. Int. Ed. 2022, DOI: 10.1002/anie.202212543). It uses yttrium or lanthanum metal and a “sandwich” ligand that helps stabilize and protect the metal as it hops between polymer strands, breaking amide bonds.

The researchers did the reaction in air- and water-free conditions just above the polymer’s melting point, using only nylon and the catalyst, which eliminates the added energy and waste typically associated with using a solvent. “The greenest solvent of all is no solvent,” says Marks.

Even with a very small amount of catalyst in the reactor, it took only a few hours to completely break down 1 g of polymer. The researchers simulated a continuous process by periodically adding new material to keep the reaction going and got nearly complete conversion across six batches without adding any extra catalyst. The caprolactam they produced was high-quality enough to polymerize into new nylon that’s virtually indistinguishable in quality from the original material.

The researchers used their catalyst to break down a selection of chopped-up nylon products, including fishing net, yarn, carpet, and a t-shirt. They also found that it works to remove the nylon from mixed plastics, including medical gloves made of a nylon blend.

Liwei Ye, the postdoctoral scholar who designed the catalyst and did many of the hands-on experiments, explains that the catalyst works on mixed waste because it’s very selective for amide bonds. “If the catalyst meets a polyolefin, it doesn’t care, it will not bind to it. The moment it meets a polyamide, it sits on it, and it does its job,” he says.

Geoffrey Coates of Cornell University, who was not involved in the work, says, in an email, that the catalyst is impressively productive and shows “great potential” for chemically recycling nylon 6. “The obvious next challenge will be to see if the catalyst works in the real world” where there are many more substances that could poison the catalyst, he adds.

The team is now working on improving the catalyst and extending the polymerization to other nylons. Kratish is leading efforts to commercialize the depolymerization technology. “This is the dream . . . to develop something and then make it real,” he says.

Chemical & Engineering News


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