GREENER METHYLATIONS
Dimethyl carbonate is a “superior” methylating reagent in a high-yield synthesis of industrially important methyl ethers, according to a research team led by Derek J. Irvine and Martyn Poliakoff of the University of Nottingham, in England (Org. Process Res. Dev., DOI: 10.1021/OP900307w). Ethers are traditionally made via the Williamson ether synthesis, which employs an alcohol and sodium hydroxide to form an alkoxide that subsequently couples with an alkyl halide. But that method requires toxic alkyl halides as reagents and generates undesirable inorganic salts as by-products. Building on their expertise in continuous-flow supercritical CO2 reactor technology, the Nottingham researchers showed that solid-acid catalysts, such as γ-alumina, mediate the selective reaction of nontoxic dimethyl carbonate with l-octanol and other primary alcohols to form methyl ethers in supercritical CO2 solvent (170-270 °C, 100 bar). The research was sponsored by specialty chemical manufacturer Croda as part of its effort to identify greener technologies for making ingredients that go into cosmetics, toiletries, and other consumer products.—SR
WWW.CEN-ONLINE.ORG FEBRUARY 15, 2010
Carbenes made easy
Chemists have found a straightforward way to make rhodium azavinyl carbenes, a new family of reactive species for organic synthesis. To complement the existing tool kit of carbenes, which are high-energy intermediates c apable of selective reactions, Valery V. Fokin and coworkers at Scripps Research Institute recently developed tunable azavinyl carbenes with unique synthetic capabilities (J. Am. Chem. Soc. 2009, 131, 18034). But making them involves sul-fonyl azides and diazo compounds, which require very careful handling. Fokin, Neil Grimster, and Li Zhang have now devised a new route that gets around the hazardous compounds. From stable NH-triazoles, the one-pot procedure generates carbenes using triflic anhydride, a pyridine base, and a rhodium catalyst (J. Am. Chem. Soc., DOI: 10.1021/ja910187s). The rhodium azavinyl carbenes react with alkenes, yielding cyclopropanes or dihydropyrroles with good stereoselectivity.—CD
WWW.CEN-ONLINE.ORG FEBRUARY 15, 2010
Amides made efficiently from aldehydes
Copper-catalyzed reaction of aldehydes with amine hydrochloride salts provides an efficient new route to amide bond formation, according to Woo-Jin Yoo and Chao-Jun Li of McGill University, Montreal (J. Am. Chem. Soc., DOI: 10.l021/ja064315b).
The carbonyl-nitrogen amide moiety is ubiquitous in pharmaceuticals and polymers, the researchers note,
and amide bond formation is necessary to couple peptides in protein synthesis. Amides are usually formed
by reacting carboxylic acids with an amine, they add, but this and other approaches are limited by costly
transition-metal catalysts and the number of available substrates. Building on their previous work on
copper-catalyzed oxidative esterification of aldehydes, Yoo and Li reacted various benzaldehydes with alkyl,
aryl, or glycine ester amine hydrochlorides in acetonitrile solvent (shown) in the presence of CuI
with AgIO3 as the catalyst, tert-butyl hydroperoxide as the oxidant,
and CaCO3 as a base. They believe their method is the first example of
producing amides and peptides from aldehydes and simple amines.
WWW.CEN-ONLINE.ORG SEPTEMBER 25, 2006
IMAGING INDUSTRIAL-TYPE CATALYSTS
By using a novel electron microscopy method, an international team has recorded images of industrial-type nanostructured molybdenum disulfide catalyst particles with element specificity and single-atom resolution (An-gew. Chem. Int. Ed. 2010, 49, 2708). Previous studies that yielded atomic resolution details of MoS2, a hydrotreating catalyst used to strip sulfur from petroleum, typically focused on model single-crystal-supported specimens that were prepared under pristine vacuum conditions. In contrast, the current study — led by Stig Helveg of Danish catalyst manufacturer Topsøe , Christian Kisielowski of Lawrence Berkeley National Laboratory, and coworkers — focuses on MoS2 samples synthesized on graphitic carbon powder via the same methods used to manufacture catalysts for oil refineries.
The researchers analyzed the samples with a recently developed transmission electron microscopy technique that enabled them to distinguish between one- and two-layer-thick regions of the particles and determine the chemical identity of the atoms throughout those regions. The results, which the team notes are in “excellent agreement” with computer simulations, could help establish new relationships between the catalyst’s function and atomic structure and could offer possibilities for improving catalyst formulations, they say.-MJ
By combining experimental (left) and simulated microscopy results, researchers pinpoint single- and double-layer regions in MoS2 catalyst particles (right).
WWW.CEN-ONLINE.ORG APRIL 5, 2010
EPA TARGETS BISPHENOLA
REGULATION: Agency will examine levels of plastics chemical in water supply and effects on wildlife
EPA HAS ADDED bisphenol A (BPA) to its list of chemicals targeted for possible regulation. The agency announced its action plan for BPA on March 29, citing concerns about potential environmental impacts of the widely used plastics chemical.
In the plan, EPA will use its authority under the Toxic Substances Control Act (TSCA) to gather information on the concentrations of BPA on surface, ground, and drinking water. The agency will also require manufacturers to provide data on the reproductive and developmental effects of BPA on aquatic organisms and wildlife.
EPA estimates that in the U.S., the amount of BPA released into the environment exceeds 1 million lb annually. The proposed rules are expected this fall.
In January, FDA announced that it has “some concern” about the potential health effects of BPA on infants and children (C&EN, Jan. 25, page 8). FDA is currently studying the health impacts of BPA from food packaging and ways to reduce exposure to the chemical.
“We share FDA’s concern about the potential health impacts from BPA,” Steve Owens, assistant administrator of EPA’s Office of Prevention, Pesticides & Toxic Substances, said in a statement. EPA will also examine ways to reduce exposures to BPA and assess the safety of alternatives. EPA does not, however, intend to initiate rulemaking under TSCA on the basis of risks to human health at this time.
EPA’s decision comes amid pressure from environmental activists and members of Congress to expedite regulatory action on BPA. In early March, Sen. Charles E. Schumer (D-N.Y.) sent a letter to EPA Administrator Lisa P. Jackson urging EPA to take immediate action against BPA as part of the agency’s efforts to reform its chemicals management program.
The American Chemistry Council, a trade group that represents the chemical industry including several BPA manufacturers, emphasized in a statement that EPA is not proposing any regulations because of concerns about human health. The group said it looks forward to working with EPA on modernizing TSCA “in a way that allows EPA to better prioritize chemicals for review.’— BRITT ERICKSON
WWW.CEN-ONLINE.ORG APRIL 5, 2010
SURVIVAL OF THE WEAKEST
CATALYST DISCOVERY: Least stable intermediates can lead to the best catalysts
Evolution may be based on “survival of the fittest,” but when it comes to selecting the best catalysts, that dictum doesn’t always hold.
A research team has shown that one can select the best catalysts from combinatorial libraries of candidates by instead using the principle of “survival of the weakest” — that is, the most unstable catalytic intermediates make for the best catalysts. The approach could make it possible to discover catalysts more quickly for syntheses of drugs and other products.
It has been difficult to find selection methods to separate the cream of the crop (the fastest catalysts) from the dross when screening libraries to identify highly active catalysts. Now, theoretical chemist F. Matthias Bickelhaupt of Free University of Amsterdam, catalysis specialist Joost N. H. Reek of the University of Amsterdam, and coworkers have developed an innovative selection strategy for such experiments based on survival of the weakest intermediates (Nat Chem., DOI: 10.1038/nchem.614).
A Pd-ligand-substrate complex that is energetically less stable catalyzes allylic alkylation of a substrate more efficiently — that is, with a lower activation
energy (Ea) — than a more stable complex. L = ligand, R = functional group, Nu = nucleophile.
The researchers mix limited amounts of palladium — bound to the substrate of an allylic alkylation reaction — with an overabundance of several organic ligands. In the equilibrium mixture, different ligands bind to Pd, forming catalysts that vary in their ability to accelerate alkylation of the substrate. The ligands that form the most stable Pd-ligand-substrate complexes are most abundant in solution, and those that form unstable complexes are scarce.
Using electrospray mass spectrometry (ESI/MS) to determine the least abundant and therefore least stable complexes, the researchers identified ligands that form the most active catalysts. The study thus evaluates catalysts not in the conventional way — by determining their ability to lower the activation barrier, the energy required to make the reaction go — but instead by finding the least stable intermediates, which are closer energetically to the top of the barrier and can therefore more easily traverse it.
The work “highlights the value of smart combinatorial approaches for the development of highly efficient catalysts,” says catalysis specialist Helma Wennemers of the University of Basel, in Switzerland.
“This is a clever demonstration of successful catalyst screening using simple ESI/MS methodology,” says polymer chemist Krzysztof Matyjaszewski of Carnegie Mellon University.
Groups such as Matyjaszewski’s and that of catalyst screening specialist Andreas Pfaltz of the University of Basel have identified good catalysts from mixtures before. But the new study represents one of the first screening examples in which “catalyst systems are in equilibrium and communicate,” Reek says. “It’s a Darwinian selection in which the weakest survive, whereas most others have monitored the properties of mixtures of catalysts that are not communicating.”
Dynamic combinatorial chemistry expert Sijbren Otto of the University of Groningen, in the Netherlands, says, "We understand initial states much better than transition states,” which are transient species at the tops of activation barriers. Manipulating the energies of reactant intermediates“ may, for selected systems, be a much easier way to find catalysts than attempting to interact selectively with much more elusive transition states,” Otto says.
Using the new technique to detect least abundant species in large libraries and screen substantially different ligand classes in the same mixture, however, could be difficult, Otto adds. Nevertheless, he says, “the approach holds considerable promise for catalyst development and discovery.” — STU BORMAN
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