SYNTHESIS
New way to make chiral fluorinated arenes Combined chemical and biocatalytic method installs trifluoromethylcyclopropyl groups
two enantiomers for each addition product. The chemical and biocatalytic reactions are not compatible. So the chemists carry out the two steps sequentially in two different pots to keep them separate. Researchers have used myoglobin and other biocatalysts to transfer carbenes to substrates before, but those previous methods all used α-diazoacetate carbene donors. The new technique expands the
A trifluoromethyl-substituted cycloproEd. 2011, DOI: 10.1002/anie.201004269). pane group can play a key role in a drug canFasan notes that medicinal chemists didate. The group’s rigid cyclopropyl ring prefer to use reactions with enantiomeric can increase the compound’s lipophilicity excesses greater than 98% to minimize the and metabolic stability while its fluorines difficulties of purifying away unwanted can boost the candidate’s stability and abilenantiomers. Fasan’s new method proity to permeate membranes. But installing vides enantiomeric excesses between 97 trifluoromethylcyclopropyl groups in a and 99.9% and has much shorter reaction highly enantioselective manner is hard. times and 50 times the catalytic effiThe process uses carbene A team led by Rudi Fasan at the Univerciency of the earlier method. generation and engineered sity of Rochester now reports a combined In the Fasan group’s technique, myoglobins to add trifluorochemical and biocatalytic methylcyclopropyl groups to approach that could make arene substrates. F3C NH2 F3C N2 it easier to add trifluoromethylcyclopropyl Trifluoroethylamine Carbene donor groups to compounds Myoglobin catalyst Ar CF3 enantioselectively for Ar = arene Ar drug design (J. Am. Chem. Trifluorocyclopropyl arene Soc. 2017, DOI: 10.1021/ Vinylarene substrate jacs.7b00768). Erick M. Carreira of the Swiss Fedthe researchers first convert trifluororange of biocatalytic carbene transforeral Institute of Technology (ETH), ethylamine to a carbene donor reagent, mations to diazotrifluoroethane carbene Zurich, and coworkers developed the diazotrifluoroethane. They then mix the donors. only previous method for adding the reagent with bacterial cells expressing an “It is a great piece of work,” Carreira fluorinated groups in a fairly stereoengineered myoglobin, which catalyzes says. “It combines chemical and biocatselective manner. In that technique, carbene addition to aryl olefin substrates, alytic steps to make nontrivial, highly cobalt catalysts generate trans-trifluoroyielding trans-trifluoromethylcyclopropyl enantioselective cyclopropanation methylcyclopropyl derivatives of arene arenes. The scientists created multiple reactions possible. It is a win-win for substrates with enantiomeric excesses versions of the engineered myoglobins that chemistry, biology, and catalysis in between 84 and 94% (Angew. Chem. Int. can catalyze the synthesis of each of the general.”—STU BORMAN
Two steps
ENERGY
CREDIT: COURTESY OF RUDI FASAN (CATALYST)
Quantum dots are superefficient at generating H2 Getting a two-for-one deal is always appealing. That math is particularly beneficial for scientists when it comes to generating electrons from light in solar cells. Using the right materials, researchers can generate two or more electrons for every sufficiently energetic photon of light absorbed, which computes to greater than 100% quantum efficiency. Building on this capability, Yong Yan of New Jersey Institute of Technology, Matthew C. Beard of the National Renewable Energy Laboratory, and coworkers have constructed a superefficient quantum-dot-based photoelectrochemical cell that produces hydrogen (Nat. Energy
2017, DOI: 10.1038/nenergy.2017.52). “As far as we know, this is the first time that hydrogen has been produced photoelectrochemically under visible light with a quantum yield greater than 100%,” Yan says. The new system relies on a process called multiple exciton generation, or MEG. During MEG, two or more electron-hole pairs, known as excitons, are created within quantum dots from the absorption of one high-energy photon. The team’s photoelectrochemical cell includes an anode constructed of a lead sulfide quantum dot layer deposited on a fluorine-doped tin oxide base. The cathode is a platinum mesh. When light strikes the anode, electrons
and holes are generated within the lead sulfide layer. The holes oxidize sulfide in a sodium sulfide solution in the anode compartment of the cell to form sulfur. Meanwhile, the electrons make their way to the platinum cathode in a phosphate-buffered solution in the other compartment, where hydrogen ions are reduced to H2. A salt bridge separating the compartments enables hydrogen ions and sodium ions to migrate from one side to the other. Pushing the efficency threshold over 100% with a photoelectrochemical cell provides new opportunities to capture excess photon energy to produce solar fuels such as H2, the researchers note.—STEVE RITTER APRIL 24, 2017 | CEN.ACS.ORG | C&EN
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