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Версия для печати | Главная > Центр > Научные советы > Научный совет по катализу > ... > 2014 год > № 69

№ 69



  • Илья Иосифович Моисеев
    — к 85-летию со дня рождения
    Отчет о научно-организационной деятельности в 2013 году:
  • За рубежом
  • Приглашения на конференции
  • Памяти П. Путановой и В.С. Соболевского

Илья Иосифович Моисеев
К 85-летию со дня рождения




За рубежом

Mixing and Matching Metals

Organometallics: Chemists create an inorganic Grignard reagent to forge metal-metal bonds

An international research team has reported some surprising new metal-metal-bonded complexes—sought after as catalysts and enzyme mimics — including an unprecedented Mn(0)–Mg(II) species that serves as an inorganic version of a Grignard reagent.

The team, led by Cameron Jones of Monash University, in Mel-bourne, Australia, initially was attempting to prepare an Mn–Mn complex using a Mg(I) complex as a reducing reagent. But the researchers discovered they had made the mixed-metal LMn–MgL′ instead, where L and L′ are enormous amide ligands that defy naming (J. Am. Chem. Soc. 2014, DOI: 10.1021/ja5021348). Jones and his colleagues then used the deep-blue Mn–Mg species to make Mn(I)–Mn(I) and Mn(II)–Cr(0) complexes.

Normal Grignard reagents are organomagnesium compounds widely used in organic synthesis to transfer an organic group from magnesium to an organic molecule to form a new C–C bond. In an analogous way, an inorganic Grignard reagent transfers a metal and its ligand from magnesium to another metal complex, resulting in a new metal-metal bond in a bimetallic compound. The concept of inorganic Grignard reagents was developed in the 1970s, but few examples have been reported and the reagents and products were never well-defined.

“Controlled access to heterometallics, especially when the metal ions are electronically similar, can be quite challenging,” says Connie C. Lu of the University of Minnesota, Twin Cities, whose group studies multiply bonded bimetallic complexes. “Using inorganic Grignard reagents is a neat strategy that’s currently not being exploited in the field of metal-metal bonding.”

Of interest to inorganic chemists, the Mn–Mg species is the first example of a two-coordinate Mn(0) complex. Most transition metals typically require coordination via four or more bonds for stability. The bulky amide ligands restrict access to metal coordination sites, forcing the metals to stabilize in lower oxidation states than usual based on their complement of valence electrons and therefore with fewer ligands. In addition, preliminary reactions with O2, N2O, and a carbodiimide suggest that the new Mn(0)–Mg(II) complex can serve as a strong reducing agent for organic synthesis.


Constructing Crowded Carbons

Organic Synthesis: Chemists use Mizoroki-Heck reaction to tackle tough-to-make motif

In an advance that could help chemists build complex natural products and pharmaceuticals, researchers have found a new way to make quaternary carbon centers. The reaction is enantioselective, giving scientists a tool to make just one enantiomer of a chiral compound.

Quaternary carbon centers often present a stumbling block for chemists trying to make complex molecules. There simply aren’t many ways to create this crowded chemical motif, in which a carbon makes four bonds to other carbon atoms. Until now, to create quaternary carbons chemists had to use a functional group handle, such as a carbonyl, on the carbon adjacent to the quaternary carbon construction site. But removing that handle later can be difficult.

University of Utah chemists Matthew S. Sigman, Tian-Sheng Mei, and Harshkumar H. Patel found they could enantioselectively create quaternary carbons without using a nearby functional group handle. To do this, they use a palladium catalyst to perform a Mizoroki-Heck-type reaction that adds an aryl group from an aryl boronic acid to a trisubstituted alkenyl alcohol (Nature 2014, DOI: 10.1038/nature13231).

It was a fairly simple idea, Sigman tells C&EN, but the group had doubts. They wondered whether the trisubstituted alkenyl alcohol would bind well enough to the catalyst. If it did, would the aryl group add to the more substituted carbon to make the quaternary center?

The addition took place as they had hoped, and the alcohol in the substrate acts as an escape route for the palladium catalyst by undergoing oxidation. Without the alcohol, the catalyst would wander along the substrate’s alkyl chain with no way to leave.

“It is likely that this process and the building blocks prepared by this method will see great use in the synthetic community,” comments Brian M. Stoltz, an expert in organic synthesis at Caltech. A particularly useful aspect to this chemistry, Stoltz notes, is that preexisting stereocenters in the substrates aren’t affected by the reaction.

At the moment, the reaction can be used only with aryl boronic acids, but Sigman says his group is working to expand its scope so that alkyl groups can be added to the substrates as well.


Self-Assembled Makeover

Surface Chemistry: N-Heterocyclic carbenes offer an alternative to thiols for modifying metal surfaces

N-Heterocyclic carbenes, which have a high affinity for gold, displace didodecyl sulfide groups to create NHC-based self-assembled monolayers.

A change is in the works for self-assembled monolayers (SAMs). This 30-year-old technology used for surface protection, sensing, microelectronics, and drug delivery has relied almost exclusively on modifying gold surfaces with a uniform single layer of long-chain alkanethiols.

But now a Canadian research team has come up with a more stable alternative: N-heterocyclic carbenes (NHCs). These cyclic amines, which dramatically increase the lifetime of SAMs, could enable new applications of SAM-modified surfaces.

Crudden’s team attached an N-heterocyclic carbene on a gold surface, then chemically modified the carbene with a redox-ready ferrocene-tipped group for electrochemical applications.

In alkanethiol-based SAMs, a lone pair of electrons on sulfur binds to the gold surface and the alkane tail is left free. But thiol-based SAMs aren’t perfect. The sulfur groups start detaching from the gold surface after a week or two or slowly degrade with extended exposure to air, light, and water. The monolayers break down more quickly when heated or exposed to common solvents, such as tetrahydrofuran. This instability has been an impediment to their broader commercial use.

Chemists have tried a host of alternative molecules for preparing SAMs but continue to experience similar stability problems or have been unable to achieve good control over monolayer deposition. As Cathleen M. Cruddenand J. Hugh Horton of Queen’s University in Kingston, Ontario, and their colleagues have now found, NHCs are proving to be different.

NHCs are already popular ligands that bind and stabilize transition-metal catalysts through a lone pair of electrons on the central carbon atom. Although NHCs are more reactive than thiols, Crudden notes, they have the advantage of interacting more strongly with gold and remaining inert for longer periods in air, water, high- and low-pH conditions, and even boiling tetrahydrofuran (Nat. Chem. 2014, DOI: 10.1038/nchem.1891 ).

One secret to successfully using NHCs could be starting with a thiolmodified surface, says Tobias Weidner of the Max Planck Institute for Polymer Research, in Mainz, Germany, whose team was the first to report NHC monolayers on gold in 2011. Thiol groups can displace molecular contaminants from gold to create uniformly modified surfaces — a self-cleaning process, Weidner explains. NHCs tend to bind to contaminants instead of displacing them, which may have been a limiting factor in the quality of the NHC-based SAMs reported in the past.

But the Crudden team found that the stronger-binding NHCs displace sulfur groups, leading to modified surfaces. Crudden adds that less bulky NHCs also seem to be critical.

“This work represents an important step forward for NHC-based SAMs,” Weidner says. “What is really exciting about the paper by the Crudden lab is that they avoided the contaminants by using thiolprotected gold surfaces as clean substrates — a very elegant solution.”

The Canadian team has further modified the NHCs with electroac-tive groups once they are on gold. “We are already getting really good results on biosensing and in protecting reactive metals from corrosion in automotive applications,” Crudden says. The researchers are also making NHC-modified nanoparticles, which could be useful for drug delivery


Natural Gas Gets An Upgrade

Fuels & Chemicals: Main-group metals selectively oxidize alkanes to make commodity alcohol esters

The U.S. natural gas production boom is presenting a wealth of new research opportunities for chemists. The scientists have been getting creative in designing new methods for converting plentiful natural gas into fuels and chemicals that can better compete economically as alternatives to petroleum-derived products.

This energy diagram shows how a thallium trifluoroacetate complex coordinates methane and then rips apart a C–H bond on the way to forming a methanol trifluoroacetate ester.

In one of these developments, a research team including Brian G. Hashiguchi and Roy A. Periana of Scripps Research Institute Florida and Daniel H. Ess of Brigham Young University has discovered that inexpensive main-group thallium and lead complexes work well at converting the typically unreactive alkanes in natural gas into alcohol esters (Science 2014, DOI:10.1126/science.1249357). The new chemistry operates more selectively and at much lower temperatures than conventional natural gas reforming methods that operate at about 900 °C.

“This is a highly novel piece of work that opens the way to upgrading of natural gas to useful chemicals with simple materials and moderate conditions,” says Yale University chemistry professor Robert H. Crabtree, whose group studies catalytic C–H activation reactions.

Current commercial catalysts for oxidizing hydrocarbons such as propylene are based on solid metal oxides such as molybdenum oxide that can’t be used to process natural gas to make alcohols, Periana explains. These commercial catalysts cleave C–H bonds by reactions involving radicals and react more rapidly with the alcohol product than the starting alkane, generating undesired carbon dioxide.

In the new chemistry, soluble thallium(III) or lead(IV) salts in trifluoroacetic acid solvent operate stoichiometrically at only 180 °C, which in an optimized commercial catalytic process could lead to substantial energy savings. Using computational and experimental studies, the team found that thallium and lead trifluoroacetate complexes activate C–H bonds without radical formation to generate metal alkyl intermediates, which selectively leads to the alcohol trifluoroacetate esters.

Much of the new supply of shale gas has substantial amounts of ethane and propane mixed with methane, Periana notes. Unlike prior systems based on precious metals such as platinum and gold that reacted only with methane, thallium and lead work on all three alkanes, separately or as mixtures. A complete one-pot reaction can therefore controllably produce methanol, ethanol, propanol, ethylene glycol, isopropyl alcohol, and propylene glycol, he says.

Periana says the team is in discussions with several companies and entrepreneurs and would ideally like to jointly develop the technology with a petrochemical company or spin off a start-up company. “Initial targets would be higher-value, lower-volume commodity chemicals such as propylene glycol or isopropyl alcohol directly from propane,” Periana says. “The next targets after that could be to develop lower-temperature processes for higher-volume chemicals, such as converting methane to methanol and ethane to ethanol or ethylene as inexpensive sources of fuels and plastics.”


X-Rays Probe Single Molecules

Molecular Structure: Method could yield precise information on reaction dynamics

When chemists want atomic-level structural information about chemical compounds, they often turn to X-ray crystallography. But the technique requires an ensemble of molecules in crystalline form. Researchers have now taken a first step toward using X-rays to obtain precise structures and observe the reaction dynamics of individual gas-phase molecules, reports an international team (Phys. Rev. Lett.2014, DOI: 10.1103/physrevlett.112.083002).

“This is proof-of-principle work,” says Stephen H. Southworth, leader of the atomic, molecular, and optical physics group in the X-ray Science Division of Argonne National Laboratory. With additional development, he says, the method could be used to probe chemical processes and see structures more directly than is possible with other techniques. Southworth was not involved in the newly reported research.

The technique was developed by a team led by Jochen Küpper, leader of the controlled molecule imaging group at Germany’s Centre for Free-Electron Laser Science, which is affiliated with the DESY synchrotron accelerator center.

Küpper and colleagues took advantage of established methods to align molecular beams using an electric field: When a polarizable molecule interacts with the electric field of a type of laser radiation, the molecules line up to minimize their energy.

The research team then intersected that molecular beam with high-energy, short-duration X-ray pulses produced at SLAC National Accelerator Laboratory’s Linac Coherent Light Source.

In the reported experiments, the researchers looked at X-ray diffraction of 2,5-diiodobenzonitrile. The compound’s iodine atoms strongly scatter X-rays, yielding a two-center interference pattern similar to that from a classic double-slit experiment. The research-ers determined that the molecule has an iodine-iodine distance of 800 pm, longer than the expected value of 700 pm.

Küpper’s team is now trying to improve the resolution of the experiment. The X-ray pulses they used to interrogate the molecular beam had a wavelength of 620 pm, similar to the distance between the iodine atoms. To get better resolution — and to narrow in on additional structural detail — scientists need shorter wavelengths. Instrumentation advances since the 2,5-diiodobenzonitrile meas-urements were completed now allow for wavelengths down to 100 pm, Küpper says. Researchers are also working on creating shorter pulses with faster repetition to minimize radiation damage prior to diffraction.

Shorter pulses will also enable time-resolved experiments, such as following a photochemical reaction in real time, Küpper says. Experiments to create molecular movies showing reaction dynamics would require pulses of just a few femtoseconds, compared with 100-fs pulses the team used to study 2,5-diiodobenzonitrile


Simple Catalyst Pair Transforms Excess Glycerol into Useful Compounds

Green Chemistry: A one-pot, two-catalyst reaction turns leftover glycerol from biodiesel production into chemicals of value

One-Pot Wonder

A two-catalyst system transforms glycerol into 1,3-diace¬tylglycerol at yields as high as 98%. First, the glycerol reacts with a lanthanum montmorillonite catalyst (La3+-mont) and acetic acid (AcOH) to give a mixture of acetylated products (top). These compounds are oxidized and isomerized by a copper-nanoparticle-embedded aluminum oxide catalyst (CuNP@AlOx) in air. Finally the oxidized compounds go through a hydrogenation reaction to give the final diacetylglycerol product (bottom).

Biodiesel may be a green fuel, but its production has a waste problem. Every year, producers of the alternative fuel create over a million tons of glycerol worldwide, much of which goes to waste. Turning this side product into something useful and salable would help transform biodiesel into a more profitable commodity. Some scientists have suggested creating acetylglycerols from the unwanted glycerol, since the compounds are used in many consumer products. In a new study, chemists report a set of reactions that produces acetylglycerols in high yields using low-cost and abundant materials (ACS Sustainable Chem. Eng. 2014, DOI:10.1021/sc500006b).

Current strategies for turning glycerol into acetylglycerols require harsh conditions and produce low yields. For the new reactions, Kiyotomi Kaneda, a catalysis chemist from Osaka University, in Japan, and colleagues used a one-pot method to transform glycerol into 1,3-diacetylglycerol, which is used in pharmaceuticals, cosmetics, polymers, and food additives. The transformations required two catalysts: a silicate clay called montmorillonite that they embedded with lanthanum ions, and aluminum oxide embedded with copper nanoparticles. The lanthanum montmorillonite catalyst acetylated the glycerol, and the copper catalyst oxidized, isomerized, and hydrogenated the acetylated product.

The chemists churned out diacetylglycerol at yields as high as 98%. Previous methods produced mixtures of diacetylglycerol isomers, but this one was selective, with the target compound 1,3-diacetylglycerol accounting for 99% of the product.

The reactions run for 24 hours at 120 °C, a mild temperature relative to those used in previous methods. The catalysts are relatively easy to obtain as well. Montmorillonite clays are often used in industry, and copper is much cheaper than commonly used precious-metal catalysts such as palladium or platinum. Besides the catalysts and the starting material, the reactions require only air, acetic acid, hydrogen gas, and toluene as the solvent. Also, the chemists can easily filter out the catalysts so they don’t contaminate the products.

One of the problems with the glut of glycerol from biodiesel is that it is impure, says Adam F. Lee, a catalysis chemist from the University of Warwick, in England. Most biodiesel production uses a liquid catalyst that’s difficult to remove from the glycerol by-product, so it can’t be used directly for food or cosmetics. “So there’s a desperate search for something to do with all this glycerol, and there have been very few low-temperature, efficient processes to transform it into useful chemicals,” he says.

This new process is important both because of the mild conditions and the inexpensive, plentiful catalytic materials they use, Lee says. Most chemists would design a process that uses one catalyst and one reaction at a time, so putting together two catalysts that have different roles is unusual, he says, but “nice and straightforward.” However, for these reactions to be truly useful in an industrial setting, they would have to be engineered into a solventless, continuous-flow type of process, he says.


Chemists Present Innovative Methods for Reducing Alkenes and Coupling Them Directly

Organic Chemistry: First-row transition-metal-catalyzed processes build motifs in medicinally active molecules

Alkenes are incredibly common in the molecular world, so researchers constantly seek more ways to use them in chemical transformations. Now, two independent teams have uncovered new alkene reactivity after activating olefins with first-row transition metals. The methods — a selective alkene reduction and a carbon-carbon bond formation — each solve a problem chemists commonly face.

The advances come from the groups of Ryan A. Shenviand Phil S. Baran, both at Scripps Research Institute California. Both chemistries convert an alkene starting material to a reactive species, explains Erick M. Carreira of ETH Zurich, who has also worked in this area. But from there, he adds, “each brilliantly utilizes the ensuing reactive intermediates in different and innovative ways.”

Shenvi’s group developed an alkene reduction that leads to alkane products with thermodynamically favorable configurations (J. Am. Chem. Soc. 2014, DOI: 10.1021/ja412342g). Alkenes are most often reduced through catalytic hydrogenation, which typically involves delivery of H2 to the same face of the alkene. The resulting products may be more accessible, but they’re not necessarily the more stable reaction product nor the desired one, points out organic chemist Michael J. Krische of the University of Texas, Austin. Reductions with a metal such as lithium dissolved in liquid ammonia provide thermodynamic products but obliterate nearby functional groups.

To get thermodynamic control without the mess, Shenvi’s team avoided the unstable intermediate that dissolving metal reductions produce — radical anions. “If you can in essence add a hydrogen atom to an alkene, your intermediate becomes a carbon radical, which is more stable than a radical anion,” Shenvi explains. To reduce the alkene, his group used a combination of phenylsilane and a manganese catalyst. They added tertbutyl hydroperoxide to their reaction to regenerate the catalyst. The method works on heterocycles frequently seen in drug molecules and also leaves nearby halogen atoms intact. Shenvi’s group is working on an asymmetric version of the chemistry and investigating its mechanism.

Meanwhile, Baran and his group developed their own variation on the alkene activation theme. They coupled unactivated alkenes to electronpoor alkenes directly (J. Am. Chem. Soc. 2014, DOI: 10.1021/ja4117632). The reaction typically takes place in less than one hour in an open flask and can be performed on gram scales. The chemistry generates crowded bicyclic molecules and quaternary centers, which are otherwise hard to construct. “The fact that a simple iron catalyst can be used to promote these transformations makes this method especially attractive,” Krische says.

Baran notes that both his work and that of Shenvi, his former student, build on a rich legacy of olefin functionalization from multiple teams. This field, he says, is poised for a renaissance.

Baran also speculates that the flavor and fragrance industry might be interested in the chemistry. “Some of our products smell really good,” he says.

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Памяти Паулы Путановой (1925 – 2014)


Памяти В.С. Соболевского (1935 – 2014)


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