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№ 73

 

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  • Валерий Васильевич Лунин —
    к 75-летию со дня рождения
  • НАУЧНЫЙ СОВЕТ ПО КАТАЛИЗУ ОХНМ РАН
    Отчет о научно-организационной деятельности в 2014 году
  • За рубежом
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Валерий Васильевич ЛУНИН -
К 75-летию со дня рождения

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Carbenes Take On A New Dimension

Asymmetric Synthesis: Pairing ferrocene with an imidazolium ring system results in planar chiral catalysts

Iidazolium-based N-hetero­cyclic carbenes (NHCs) are a versatile set of catalyst molecules. Chemists use them as ligands in transition-metal catalysts, and they use them alone as metal-free organocatalysts. They would also like enantioselective versions of these compounds to prepare chiral molecules, but so far only a few successful versions have been reported.

A research team led by Christopher T. Check and Karl A. Scheidt of Northwestern Uni­versity set out to change that situation by fusing a metal sandwich complex with an NHC framework. In doing so, they created a versatile new class of NHCs with a rigid planar chiral imidazolium ring system that can be tuned to serve as a ligand or organocatalyst by altering the substituents on the imidazolium ring (Angew. Chem. Int. Ed. 2015, DOI: 10.1002/anie.201410118)

The Northwestern chemists first synthesized a ferrocene derivative with a cyclopentadienyl ligand on one side of the sandwich and a cyclopentapyridinyl ligand bearing a pseudoephedrine side chain on the other. After separating the resulting enantiomers, they carried out an additional reaction sequence to finalize formation of the planar chiral imidazolium ring system.

“This new molecule represents a creative translation of the notion of planar chirality to the bond-forming business end of the NHC ligand-catalyst scaffold,” says Scott J. Miller of Yale University, whose group develops catalytic methods for synthesizing stereochemically complex molecules. “In doing so, the researchers have provided new dials to turn for optimizing catalysts. It is very thought-provoking and stimulating work.”

With the ferrocene-based chiral NHC in hand, Scheidt and coworkers carried out a series of reactions. In one example, the team used the new NHC as an enantioselective organocatalyst for aryl homoenolate additions to aryl α-ketoesters. In another example, they paired the NHC with nickel for reductive coupling of phenylpropyne with aldehydes to form allylic alcohols in high regioselectivity and enantiomeric excess—among the best ever reported for this reaction, Scheidt notes. The researchers further used the new NHC to prepare and study copper and rhodium complexes.

“These NHCs are a result of really smart modular design,” comments NHC specialist Frank Glorius of the University of Münster, in Germany. “The researchers convincingly show that these NHCs are incredibly versatile, being applicable as chiral organocatalysts in their own right and as ligands in nickel-and copper-catalyzed asymmetric transformations. It would be great if this family of NHCs could quickly become commercially available.

 

Photocatalyst Converts Nitrogen To Ammonia

Catalysis: Chalcogenide gel mimics nitrogen fixation and photosynthesis

By taking cues from two biological processes, researchers have made a catalytic material that converts nitrogen to ammonia when irradiated by white light (J. Am. Chem. Soc. 2015, DOI: 10.1021/ja512491v). The new strategy may one day help scientists achieve energy savings in various catalytic processes by capitalizing on abundant sunlight to produce valuable chemicals.

Manufacturers worldwide produce some 200 million tons of ammonia annually, mainly for use as fertilizer and for making nitrogen-containing compounds. The standard industrial process, the Haber-Bosch method, involves reacting nitrogen, which is relatively inert, with hydrogen at 400 °C and at a pressure roughly 250 times atmospheric pressure in the presence of an iron-based catalyst. It is, of course, highly energy intensive.

Nature also converts nitrogen to ammonia, albeit far more slowly, through a process known as nitrogen fixation. The reaction, which runs under much milder conditions, occurs in microbes containing nitrogenase enzymes. These catalysts tend to include a reactive cluster of iron, molybdenum, and sulfur.

In an effort to understand nature’s energy-efficient ways, researchers previously made synthetic analogs of these clusters. They found that a few of them can catalyze ammonia production from nitrogen under strongly reducing conditions.

A team of Northwestern University chemists, including Abhishek Banerjee and Mercouri G. Kanatzidis, has now demonstrated a potentially more useful catalyst: one that can be switched on and driven by light to mediate ammonia production at room temperature and ambient pressure. Dubbed a chalcogel, the material, which mimics some aspects of nitrogen fixation and photosynthesis, is a light-absorbing, porous, amorphous solid composed of Mo2Fe6S8 clusters linked by Sn2S6 ligands.

The team bubbled nitrogen through aqueous solutions containing the chalcogel, a proton source (pyridinium hydrochloride), and an electron donor (sodium ascorbate). They detected ammonia shortly after aiming a white light source at the gel and report that during irradiation the chemical’s concentration increased continuously.

Control tests show that solutions lacking the catalyst and those kept in the dark do not produce ammonia. The team acknowledges that the chalcogel evaluated in this study produces ammonia too slowly for industrial use but notes that the material remained stable with no loss of activity during a 72-hour test.

“This is extremely elegant work,” says University of Liverpool chemistry professor Matthew J. Rosseinsky. He notes that the study paves the way for further research projects to determine the catalyst’s active site structure and the role of the Sn2S6ligands. Rosseinsky also wonders whether there is a relationship between the electronic structure of the cluster and a wavelength dependence of the catalytic activity.

 

High-Valent Gold Catalyst Shines

Organometallics: Team makes stable version of hard-to-come-by gold(III) catalyst

More of gold’s charms have been revealed, as chemists report a rare stable gold catalyst in the +3 oxidation state. The high-valent transition-metal catalyst offers molecule makers a shiny new tool that does chemistry distinct from that of gold catalysts in the more common +1 oxidation state.

For more than a decade, gold(I) catalysts have proven to be increasingly useful in the assembly of complex molecules. Gold(III) catalysts, on the other hand, have been harder to come by because it usually takes harsh reagents to prepare them and they tend not to be stable.

F. Dean Toste, Chung-Yeh Wu, Takahiro Horibe, and Christian Borch Jacobsen, chemists at the University of California, Berkeley, have now made a stable gold(III) catalyst by getting a gold(I) catalyst to insert into a carbon-carbon bond—a process known as oxidative addition (Nature 2015, DOI: 10.1038/nature14104).

Toste tells C&EN that the team got the idea while studying the opposite reaction, the reductive elimination of gold(III) to gold(I). Because gold prefers to be in the +1 oxidation state, that reaction is thermodynamically favored. But the Berkeley team reckoned that by adding some strain to the system, they could tip the scales in favor of the +3 oxidation state. Gold(III) turns out to behave differently from gold(I) in several different types of reactions, including Mukaiyama-Michael additions and Diels-Alder reactions. “You’ve changed the electronic nature of the metal, and therefore it’s going to have different reactivity,” Toste explains.

“This work is particularly impressive because it puts together several well-known principles of gold chemistry and employs this knowledge in combination with brilliant new ideas to come up with novel and highly versatile synthetic protocols,” comments Hubert Schmidbaur, an expert in gold chemistry at Germany’s Technical University of Munich.

Steven P. Nolan, an expert in organometallic catalysis at the University of St. Andrews, in Scotland, adds, “This contribution will provide access to up-to-now only proposed entities that will enable a vast landscape of reactivity to be developed.”


Gold catalysts in different oxidation states behave differently in this Mukaiyama-Michael addition.

 

Classic Addition Reaction Gets A Makeover

Chemical Synthesis: New catalyst expands stereochemical repertoire
of alkene dichlorination

In conventional alkene anti-dichlorination (top), Cl+ and Cl− attack opposite faces of the carbon-carbon double bond. In the new syn reaction (bottom), two Cl− ions attack the same face, giving a stereoisomeric product.

In a development that could revise organic chemistry textbooks, a new catalytic version of alkene dichlorination makes the reaction more versatile. Adding Cl2 to double-bonded carbons to give saturated dichlorinated products is a fundamental reaction taught early in the organic chemistry curriculum. It mostly proceeds in just one way: via an “anti” mechanism, in which Cl+ and Cl− ions attack opposite faces of the double bond.

Now, researchers have devised the first catalytic alkene dichlorination that proceeds by the alternative “syn” route, in which two Cl− ions attack the same face of the double bond. This approach provides a direct route to stereoisomers of anti-dichlorination products.

Chemists have reported alkene syn-dichlorinations before, using antimony and molybdenum chloride reagents, but these reactions were not catalytic and their applicability was severely restricted, primarily to nonsubstituted alkene substrates. The only other way to get alkene syn-dichlorination products has been to use multiple steps.

Alexander J. Cresswell, Stanley T.-C. Eey, and synthetic organic chemist Scott E. Denmark at the University of Illinois, Urbana-Champaign, have now designed a selenium(IV) reagent that catalyzes alkene syn-dichlorinations in one step (Nat. Chem. 2015, DOI: 10.1038/nchem.2141). They report 27 examples of cyclic and acyclic dichlorinated products synthesized using the strategy, including dichlorocyclohexane.

In the reaction, Se4+ gets reduced to Se2+. According to synthetic chemist Ross Denton of the University of Nottingham, in En­gland, the key to making the process catalytic was “identifying a suitable oxidant that would convert Se2+ back to Se4+ in the presence of the alkene and not interfere with the dichlorination process itself.” Denmark’s group did that, Denton says.

In a classic anti-dichlorination, Cl+ forms a cyclic intermediate on one side of the alkene. The ring is opened when Cl− attacks the other side. In the syn-reaction, the Se4+ reagent forms a cyclic intermediate on one side of the alkene that is opened by Cl−on the opposite side. The Se4+ reagent then gets displaced by a second Cl− attacking on the same side as the first.

“The magic comes about from a deep-seated appreciation of reaction mechanism that follows from analytical thinking about the individual steps that constitute the process,” says synthetic organic chemist Erick M. Carreira of ETH Zurich. “The simplicity and availability of catalyst and reagents ensure that the method—and more generally, the concepts—will be widely adopted.”

Synthetic organic chemist Takehiko Yoshimitsu of Osaka University says the reaction “is a major breakthrough that could provide easier access to polychlorinated compounds, such as chlorosulfolipid natural products.”

 

Nickel Shines In Ammonia Coupling Reactions

Organic Synthesis: Chemists use nickel to replace palladium in an important catalytic reaction for making aryl amines


AMMONIA AMINATION

The addition of a single aryl group to ammonia using a nickel catalyst enables the synthesis of a variety of aryl and heteroaryl amines, such as in the reaction shown.

 

By developing a stable nickel catalyst system, a Canadian research team has fulfilled a quest to move beyond using expensive palladium catalysts in cross-coupling reactions to make aryl amines from ammonia. The approach taken by Andrey Borzenko, Mark Stradiotto, and coworkers of Dalhousie University, in Halifax, Nova Scotia, is expected to find quick adoption in the pharmaceutical industry, where it would be used to help synthesize complex drug candidates.

Ammonia is the simplest and most abundant N–H source in chemistry—virtually all synthetic nitrogen-containing compounds originate from the inexpensive feedstock. Industrial syntheses of amines from ammonia, however, typically require heterogeneous catalysts at relatively high temperatures and pressures, resulting in modest product selectivity. Only recently have chemists devised homogeneous palladium catalysts for the task, which allow for more selective ammonia couplings under milder conditions.

The Dalhousie team’s method is the first example of nickel-catalyzed arylation of ammonia to make amines. The researchers used Ni(cyclooctadiene)2 or NiCl2(dimethoxyethane) with a ferrocenyl phosphine ligand known as JosiPhos to link up a range of substituted aryl and heteroaryl bromides, chlorides, and tosylates with ammonia to make diverse aryl and heteroaryl amines (Angew. Chem. Int. Ed. 2015, DOI: 10.1002/anie.201410875).

“Catalysis with nonprecious metals is one of our primary goals in the pharmaceutical industry,” says Robert A. Singer, a process chemist at Pfizer. There are a number of attributes to like about the new work, Singer notes. Nickel is much more abundant and less expensive to use than palladium. The new catalyst is also air stable and less susceptible to ammonia deactivation or ligand dissociation.

The method does require more catalyst than might be ideal, however, which could pose a challenge in purifying final products, Singer notes. And the specialty phosphorus ligand is costly, which might require a replacement. But process chemists in pharma “will find this specific work interesting in that Stradiotto’s group demonstrates that nickel-catalyzed arylations of ammonia can be competitive with palladium-catalyzed technology,” Singer says.

 

Radical Reaction Forms Previously Inaccessible Alkene Coupling Products

Organic Synthesis: New iron-based catalyst links reactants, avoids heteroatom byproducts


RADICAL REACTION

A new reaction uses an iron-based catalyst, a reducing agent, and a weak base to combine heteroatom-containing alkene “donors” with “acceptor” alkenes bearing electron-withdrawing groups.A new reaction uses an iron-based catalyst, a reducing agent, and a weak base to combine heteroatom-containing alkene “donors” with “acceptor” alkenes bearing electron-withdrawing groups.

Chemical groups containing oxygen, nitrogen, silicon, and other heteroatoms often react during alkene coupling reactions, generating undesirable product mixtures. But a new, iron-catalyzed radical reaction, developed by Phil S. Baran and coworkers at Scripps Research Institute, La Jolla, Calif., couples heteroatom-containing alkenes without all the messy side products (Nature 2014, DOI: 10.1038/nature14006). The heteroatom group remains unmodified while a carbon-carbon linkage forms in a highly controlled and predictable way.

About a year ago, Baran and coworkers developed a radical reaction in which an iron-based agent catalyzed the coupling of all-carbon alkene substrates (J. Am. Chem. Soc. 2014, DOI: 10.1021/ja4117632). Although this method is useful and practical, the products it generates can also be obtained—albeit with difficulty, in some cases—from known reactions of nonalkene substrates such as alkyl halides, alcohols, and carboxylic acids. The researchers have now identified a new iron-based catalyst that extends the scope of the reaction to heteroatom-containing alkene substrates. The paper shows 60 products, “90% of which have never been made before,” Baran says.

The extended reaction uses the new catalyst, a reducing agent, and a weak base to form C–C bonds that link heteroatom-substituted olefin “donor” substrates to “acceptor” alkenes bearing electron-withdrawing groups. The catalyst removes an electron from the donor, producing a radical that then combines with the acceptor.

A variety of readily available heteroatom-containing alkene donors can be used, including enol ethers, enamides, vinyl boronates, vinyl thioethers, vinyl silanes, and vinyl halides. The reactions work at ambient pressure, and no special precautions need to be taken to exclude air and moisture. To prove just how versatile the reactions are, Baran and coworkers showed that they could be run in a variety of commercial alcoholic beverages as solvents.

“It is surprising that the substrate scope of the reaction can be extended to such a wide variety of heteroatom-substituted olefins,” comments senior synthetic chemist Jonas Brånalt of AstraZeneca R&D, in Mölndal, Sweden, who uses the Baran group’s original all-carbon alkene reaction in his lab. “The new methodology will allow synthetic chemists to access novel structures that would be either difficult or even impossible to make using traditional synthetic routes.”

The approach “has the potential to shift the retrosynthetic paradigm,” says catalytic reaction specialist Michael J. Krische of the University of Texas, Austin. “It unlocks new possibilities for the construction of C–C bonds that are otherwise difficult to achieve, and its operational simplicity—the use of an inexpensive iron catalyst in ethanol solvent—makes it especially attractive.” According to Baran, his team has some exciting follow-up plans: “Let me just say that this is the tip of the iceberg. There’s much more to come.”

 

Pressure-Based Rates By Theory Alone

Chemical Kinetics: Method reveals how pressure affects reaction rates without need for experimental data

Researchers have devised a new theoretical method for calculating the speediness of reactions whose rates depend strongly on pressure. In the past, such calculations have always needed at least a bit of empirical help, but the new technique does the job purely theoretically, with no experimental data needed.

PRESSURE PROCESS

Reaction coordinate shows changes in potential energy during pressure-dependent radical dissociation of methane. Theoretical rate calculations on the system model not only these changes in energy but also changes in angular momentum (not shown).

 

The speed of some reactions depends mostly on temperature, and rates of those reactions could in many cases already be determined theoretically without experimental assistance. But rates of some reactions depend primarily on pressure, and calculating them has proven to be much more difficult.

The new technique could make it easier to obtain such information, especially for reactions for which experimental kinetics data are unavailable. Pressure-dependent reaction rates are important because researchers can use them to optimize reactions for industrial use, understand chemical reactions in space, and estimate environmental and other effects of combustion and atmospheric processes.

The new approach was developed by Stephen J. Klippenstein of Argonne National Laboratory, Ahren W. Jasper of Sandia National Laboratories, and coworkers (Science 2014, DOI: 10.1126/science.1260856). In each of the three reactions they studied, their theoretical rate predictions were quantitatively accurate—within about 20% of previous experimental data.

The work is “the first completely a priori, first-principles calculation of pressure-dependent reaction rates,” comments chemical kinetics modeling expert William H. Green of MIT. “All the other pressure-dependent rate calculations in the literature contain at least one adjustable parameter tuned to match experiments. Pressure-dependent reaction rates are important in almost all high-temperature situations above about 500 °C, including all of combustion and much of industrial chemistry, and also at low pressures, such as in interstellar space.”

The Argonne and Sandia scientists demonstrated the technique on hydrocarbon radical dissociation reactions, such as the pressure-dependent radical dissociation of methane. But they note that it could be used to model a wide range of other pressure-dependent reactions as well. In future work, they hope to demonstrate its applicability to complex multistep reactions, particularly those occurring in combustion processes.

Dissociating molecules in pressure-dependent reactions have two characteristic dynamic properties, total internal energy and angular momentum. The new procedure simulates pressure-dependent changes that occur in these two parameters as molecules collide and react. It then uses mathematical models to convert the resulting data into conventional chemical reaction rates, which depend on both pressure and temperature.

“Putting together the combination of calculations that are needed for this is a complicated business,” says theoretical chemistry specialist George C. Schatz of Northwestern University. “It is great to see that when this is implemented it is possible to calculate rates that accurately agree with careful measurements. This connection of theory and experiment has been long sought after.”

 

Chiral Catalyst Leads To New Stereopolymer

Polymer Science: Process marries right- and left-handed polymer chains to create a stereocomplex material with commodity potential

Taking advantage of a chiral cobalt catalyst, Geoffrey W. Coates and coworkers at Cornell University have copolymerized propylene oxide enantiomers and succinic anhydride to form poly(propylene succinate), the first member of a new class of thermoplastics (J. Am. Chem. Soc. 2014, DOI: 10.1021/ja509440g).

A new chiral cobalt catalyst can take (S)- or (R)-propylene oxide and copolymerize it with succinic anhydride to make (S)- or (R)-poly(propylene succinate). The two polymers, when combined, form a stereocomplex material with improved thermal properties.

The team’s new polymer forms a semicrystalline stereocomplex, meaning it is a material made from combining right- and left-handed polymer chains. In addition to starting from commodity and biobased monomers and being inherently biodegradable, the polymer has an ability to form a stereocomplex, providing it with a melting point comparable to that of low-density polyethylene. These properties could one day make the new polymer competitive to polyethylene and isotactic polypropylene, the two most widely produced polymers in the world.

“Stereocomplexes in general spark the interest of chemists who enjoy structure,” says Kenneth B. Wagener, senior member of the George & Josephine Butler Polymer Research Laboratory at the University of Florida. “This new work is typical of the Coates group—making something new of chemistry well-known, epoxides and anhydrides in this case.”

Coates and graduate students Julie M. Longo and Angela M. DiCiccio first designed a chiral cobalt catalyst featuring N,N′-bis(salicylidene)cyclohexane­diimine as a ligand. Using either the (R,R) or (S,S) version of the catalyst, the Cornell researchers copolymerized (R)- or (S)-propylene oxide with succinic anhydride to produce (R)- or (S)-poly(propylene ­succinate).

When right- and left-handed polymers are combined, they typically mix to form a random amorphous material or form a semicrystalline material with segregated right- and left-handed regions. The two polymer chains could also crystallize together in ways they can’t do alone, such as forming paired helices or interdigitated sheets to make a stereocomplex. This structural feature gives polymer chemists better control over the thermal properties and biodegradability of polymers. Stereocomplexes are exceedingly rare, however, with only about a dozen examples known.

When the Cornell team mixed right- and left-handed poly(propylene succinate), they found that the chains snuggled together to form a stereocomplex, the first known example for a an epoxide-anhydride copolymer. The stereocomplex has a melting point of about 120 °C, which is 40 °C higher than either of the polymers ­individually.

With the wide range of epoxides and cyclic anhydrides available, chemists should be able to create a broad class of new polymers, Coates notes. Currently, his team uses enantiopure propylene oxides, which are pretty expensive. “We are actively looking for a catalyst that will make the stereocomplex from racemic propylene oxide, which is considerably cheaper,” he says. Potential uses for poly(propylene succinate) include biomedical applications and large-scale packaging applications where biodegradability is needed. Cornell has patented the technology but has not yet licensed it for commercial development.

“This development bears the stamp of thorough expertise in homogeneous polymerization catalysis,” says Eric P. Wasserman, a senior research scientist at Dow Chemical. “It is relevant to the world of industrial polymers because it addresses issues with biodegradation, renewable raw materials, and the demands placed on modern plastics. In this case, that is the ability to crystallize quickly from the melt and have a melting point above 100 °C. In principle, this discovery could be a keystone of a new line of thermoplastic polymers.”

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