Total synthesis: Exploiting instability
Science 324, 238–241 (2009)
The fungus-derived natural product (+)-11,11′-dideoxyverticillin A has anticancer activity and is one member of a family of natural products — the epidithiodiketopiperazine alkaloids — known for over 40 years. Its dimeric structure and simple amino-acid-derived substructure make a retrosynthetic analysis look deceptively simple but, until now, none of this family has succumbed to total synthesis. The stereochemical complexity of the molecule and the expected instability of several proposed intermediates offer explanations for this.
Mohammad Movassaghi and co-workers from the Massachusetts Institute of Technology have now achieved the total synthesis of (+)-11,11′-dideoxyverticillin A. The synthetic strategy relies on introducing four masked thiol groups in the penultimate step of the synthesis, which, when freed, undergo immediate disulfide formation. Movassaghi and co-workers also had to invest significant effort into a diastereoselective tetrahydroxylation step, eventually settling on an unusual reagent, bis(pyridine)-silver(I) permanganate, to create the four quaternary stereocentres in the target.
Using the inherent reactivities of unstable intermediates provided access to the target alkaloid in just eight linear steps. The strategy should be applicable to other members of this natural product family for a more thorough investigation of their biological activity, and provides valuable insight into the function of enzymes involved in the biosynthesis of these natural products.
Boron acid: The superacid test
Angew. Chem. Int. Ed. 48, 3491–3493 (2009).
Superacids are defined as being stronger acids than pure sulfuric acid, and can be used to stabilize otherwise reactive cations. Carboranes were recently shown to be superacids and have the added benefit of very low nucleophilicity and redox activity, thus preventing the protonated molecules from decomposing. Unfortunately, these have proved too expensive for widespread use.
Now, Christopher Reed and colleagues from the University of California, Riverside, have discovered that halogenated all-boron acids are also superacids — possibly the strongest yet. Their all-boron starting materials are considerably cheaper than the equivalent carborane, making them more attractive candidates. The compounds, with a general formula H2B12X12, are colourless solids, and are prepared by reacting the triethylsilyl derivatives with hydrochloric acid.
The H2B12X12 superacids can protonate benzene at room temperature — a key test of superacidity that not even triflic (trifluoromethanesulfonic) acid can achieve. Furthermore, infrared measurements showed that the superacid had lost both its hydrogen ions while protonating benzene, confirming that they are the first 'diprotic' superacids. The acid strength is suggested to arise because the halide substituents shield the negative charge that is delocalized within the boron cage.
Graphene nanoribbons: Unzipping nanotubes
Nature 458, 872–876, 877–880 (2009)
Graphene nanoribbons are thin strips of hexagonally arranged carbon atoms with straight edges. To understand and make use of their interesting electronic properties, methods for their large-scale production are required. Now, two different methods have been reported for creating such nanoribbons that both use the same idea of 'unzipping' multiwalled carbon nanotube (MWCNT) side walls.
James Tour and co-workers at Rice University have developed a method that uses sulfuric acid and potassium permanganate. The proposed mechanism relies on the oxidation of C=C double bonds at an undefined point along the wall. This creates a diketone defect within the MWCNT wall that also makes adjacent C=C double bonds more susceptible to oxidation. Sequential cleavage therefore unzips the MWCNTs to create highly oxidized graphene nanoribbons that, once annealed, are metallic conductors.
Hongjie Dai and co-workers at Stanford University use a different 'plasma etching' method to unzip MWCNTs. The MWCNTs are embedded in a polymer film that acts as a mask, partially protecting the MWCNT but allowing the most exposed part of the side wall to be removed with argon plasma. The method can produce single-, bi- or multilayer-nanoribbons depending on the starting MWCNT and the etching time.
Neutron diffraction: Hydrogen monitor
Chem. Commun. 2556–2558 (2009)
With the growing importance of compounds that can absorb or conduct hydrogen (for storage or fuel-cell use), the mechanisms underlying these phenomena are being increasingly studied. However, studying materials interacting with hydrogen in situ is experimentally challenging for a variety of reasons.
Now, Peter Battle of the University of Oxford, Mona Bahout of the University of Rennes and colleagues have followed the reduction of a complex metal oxide under flowing hydrogen. They used high-flux fixed-wavelength neutron diffraction to determine which oxygen atoms were removed from the Ruddlesden–Popper material Pr2Sr2CrNiO8. It has previously been thought that this family of materials did not have the thermal stability to be used in high-temperature ionic devices. They found, however, that once reduction to Pr2Sr2CrNiO7.5 was complete at 400 °C the material was chemically and structurally stable up to 750 °C.
Although the amount of reduction seemed to be governed by the Ni3+ ions changing to Ni2+, a similar material La2Sr2CrNiO8 reduced to La2Sr2CrNiO7. Regardless of whether this promising material finds a use in high-temperature ionic devices, this demonstration of gathering subtle structural information in situ in such an extreme environment should open the possibility for many more systems to be studied.
Energetic materials: Burn baby burn
J. Am. Chem. Soc. 131, 4576–4577 (2009)
Explosives — energetic materials containing an oxidizer and a fuel — can be categorized as either low or high order, examples being gunpowder and TNT respectively. The molecules in high-order explosives contain both an oxidizing part and a fuel part, and performance is usually a trade-off between energy density and reaction rate. The reaction rate of lower explosives can be improved by mixing the oxidizer and fuel as closely as possible.
Now, Nicholas Leventis and colleagues from Missouri University of Science and Technology working with Hongbing Lu from Oklahoma State University have created a mixed aerogel of an inorganic oxidant (CuO) and an organic fuel (a resorcinol–formaldehyde polymer, RF). Aerogels are assemblies of nanoparticles surrounded by space. Leventis and colleagues used a one-pot sol–gel reaction to make the aerogel, and the precursor of one component catalysed the other.
Previous reports have suggested that RF polymer networks can desensitize energetic materials. In this case, the RF–CuO aerogel is the energetic material itself, and is suitable for pyrotechnics. The material burnt rapidly on ignition with a flame, leaving behind only particulate CuO, but aerogels made from RF alone could not sustain the flame.
Kinetic modelling: Locating key states
Microkinetic models describe multistep reactions using all of their component elementary processes and respective rate constants to provide information about the reaction's macroscopic behaviour. Such analysis can be used to find optimal reaction conditions efficiently. These complex systems often have elementary steps that are more important than others in defining overall reaction rate. A method to measure this—by quantifying how changing the rate constant of one step influences the overall reaction rate—has already been developed by Charles Campbell of the University of Washington, Seattle.
Now, Campbell and colleagues from Denmark have extended this method so it can be used to identify the transition states and intermediates that control the overall rate. The method defines a parameter that conveys how the overall reaction rate changes when the free energy of a specific transition state or intermediate is changed—stabilizing or destabilizing the state—and all other reaction free energies are kept constant.
This creates the possibility of adjusting the overall reaction rate by stabilizing the important transition states or destabilizing the key intermediates. Though such changes are difficult to achieve in practice, Campbell suggests that relative stabilities could be altered by changing the solvent or the reactant's molecular structure.
The definitive versions of these Research Highlights first appeared on the Nature Chemistry website, along with other articles that will not appear in print. If citing these articles, please refer to the web version.