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Amide Umpolung aller Werte

30 June 2010 - Organic chemistry

AmideByUmpolungShen2010  Amides are typically made from a nitrogen nucleophile (amine) and an activated electrophilic carbonyl compound (for example acid chloride) in the Schotten-Baumann reaction. This functional group is relevant to biochemistry as peptides and chemists are always on the lookout for new synthetic methods. A novel approach by Shen et al. reverses the parts played by nucleophile and electrophile in what is known in organic chemistry as umpolung (DOI).

In the new scheme alpha bromo nitroalkane 1 forms a nucleophilic synthon after deprotonation by potassium carbonate to nitronate salt 2. At the same time 1-phenylethylamine 3 exchanges a proton for an iodine atom with NIS to N-iodiamine 4. The nucleophilic attack by the nitronate is similar to what can be seen in electrophilic amination by an enolate. Intermediate 5 after hydrolysis as in the Nef reaction gives amide 6.

Invisible ink

20 June 2010 - In nanotech

reductionGrapheneOxideAFM.jpgAccording to Wikipedia an invisible ink is a substance used for writing, which is invisible either on application or soon thereafter, and which later on can be made visible by some means. Invisible ink is a theme in two recent nanotech ventures: Walz et al. (DOI) write with iron and Wei et al. write on graphene oxide (DOI).

In regular electron beam-induced deposition , iron pentacarbonyl gas is used to write iron nanostructures on an SiOx monolayer. The electron beam of an scanning electron microscope decomposes the gas and iron particles land at the targeted area. In an ultrahigh not high vacuum environment the iron deposition efficiency can be as high as 95%. Writing a single line of 45 micron length and 95 nm width takes 3 hours. Walz et al. discovered that it is also possible to first write the message using only the electron beam and then expose the silicon oxide surface to the iron pentacarbonyl. Appearantly the SiO surface becomes activated for Fe(CO)5 to catalytically decompose (the exposed surface is visible as a dark line in a regular EM image). Walz reassures her readers that other applications are pursued than just writing secret nanomessages.

In the second invisible ink nanotech application Wei et al. use the heated tip (330°C) of an atomic force microscope to write patterns on a layer of nonconducting graphene oxide (GO) by converting it to conducting reduced GO which is very similar to graphene. The surface height is reduced by 2 to 5 nm by combination of two effects: loss of oxygen atoms and conversion of rippling sp3 carbons to flat sp2 carbon atoms. A typical writing speed is 2 micron per second and the rGO trench has a 25 nm width.

Shvo again

14 June 2010 - Catalysis

Last month's blog was about indole and hexanol condensation using the Shvo catalyst and this month Takahashi et al. present a novel way to synthesize n-alkanols (such as hexanol) using the same Shvo catalyst (DOI). The new method describes catalysis of the reaction of a terminal alkene with syngas which is a mixture of CO and hydrogen. The industrial production of n-alcohols relies on an alkene hydroformylation step and a hydrogenation step and rolling them into one should be advantageous. Issues to tackle in academic research thus far are branching and alkane formation. The new Takahashi method (according to Takahashi of course) should provide a breakthrough.

The formylation part is taken care of by (Acetylacetonato)dicarbonylrhodium / xantphos and Svho (with or without phosphine assistance) tackles the hydrogenation. And how does Svho reduce the polar C=O bond while ignoring the apolar reactant C=C bond?: Takahashi reminds the reader that Shvo generates a proton and a hydride ion which simultaneously act with the C=O bond with a polar transition state. Polar aprotic solvent N,N-dimethylacetamide works best.
Alkene hydroformylation hydrogenation Takahashi 2010

Asymmetric hypoiodite catalysis

12 June 2010 - Metal-free catalysis

enantioselective oxidative cycloetherification Uyanik 2010  It is always a good idea to do away with metals in catalysis as in organocatalysis. In a new strategy Uyanik et al. (DOI) use catalytic amounts of an ammonium hypoiodite (R4N+IO-) in an oxidative Cycloetherification. The hypoiodite is continuously regenerated from stoichiometric hydrogen peroxide as sacrificial catalyst and the only by-product is water. The ammonium ion is chiral and based on the BINOL framework making the reaction asymmetric. Not that the road towards this result was easy: for the initial screening of a m-CPBA / iodobenzene system a messy yield is reported (frank but not very scientific description), the chiral catalyst has to be huge (Ar = 3,5-(3,5-(CF3)2C6H3)C6H3) in order to be stereo selective (common issue in asymmetric synthesis) and the reaction will not work unless the imidazoline group is present (for anchoring the hypiodite group) further reducing the reaction scope.

Tandem Wittig

09 June 2010 - Organic chemistry

In an earlier blog the catalytic Wittig was introduced as an interesting way to remove the presence of a large amount of triphenylphosphine oxide (TPPO) waste-product from a regular Wittig reaction. In a novel approach to the same problem (increasing atom efficiency) Jun-Jie Cao et al. (DOI) use TPPO as a catalyst for a secondary reaction. The second part in this tandem reaction is a conjugate reduction with reagent trichlorosilane.

The investigators agree that as TPPO is present in an stoichiometric amount , the label catalyst is dubious but on the other hand the reduction will not take place at all without the oxide. They are less clear about the possibility of reduction of TPPO back to the phosphine by the silane which is a well known reaction. And the atom efficiency does not really improve: the value for the Wittig reaction alone is about 45% and due to the introduction of two equivalents of silane the tandem total (despite the free catalyst) drops to 35%.
Tandem wittig Conjugate reduction Cao 2010

What killed Phar Lap?

07 June 2010 - Forensic chemistry

Phar lap XANES.jpgThe investigation into the 1932 death of Phar Lap is still ongoing. The famous Australian racehorse died suspiciously while on tour in the United States and there is no shortage of theories. In a recent publication (DOI) Ivan Kempson and Dermot Henry report on an 2008 investigation of hairs from the horses preserved hide (kept at Museum Victoria) for arsenic: poisoning is one of those popular theories. A synchrotron X-ray fluorescence microprobe allowed them to detect the element for each 10 micrometer section of hair and a single intense band was indeed found. This shows an arsenic distribution and chemistry consistent with ingestion of a large dose of arsenic just prior to death according to the investigators and not some sort of chronic exposure to arsenic from herbicides in feed or performance-enhancing arsenic-based tonics. Even the arsenic formulations used in the taxidermy process are ruled out.

But what form of arsenic? Some additional work was done but nothing conclusive. XANES is another X-ray technique that was used in the investigation. It can discriminate between different forms of arsenic two of which were found: As(III) and As(V) arsenate. The As(III) fraction is bound to thiol possibly from the presence of pheomelanin (the horse was a chestnut) or from arsenic as ingested. In a hair cross-section As(III) was also found to build up in the cysteine-rich parts. Analysis gets even more complicated because As(V) can be rapidly photoreduced to As(III) during measurements.

Ammonia + diborane + sodium?

03 June 2010 - Random chemistry

What do you get when you add together ammonia , diborane and sodium?. No university chemistry laboratory is complete without these chemicals and it may come as a surprise that it took until 2010 before this question was answered. Limiting the mix to just two components gives ammonia borane (add diborane as the monomeric THF adduct and ammonia), the ammonia/sodium electride solution or sodium triborohydride (NaB3H8, related to sodium borohydride) by reaction of diborane with sodium metal. The order in which to add the chemicals is therefore important.

Daly et al (DOI) of the University of Illinois at Urbana-Champaign decided to add sodium metal to a solution of ammonia borane in THF. At room temperature the first stop is the salt Na(NH2-BH3), then at reflux the formation of the aminodiboranate Na(BH3-NH2-BH3) with evolution of hydrogen is reported. This compound refuses to form crystals but by adding erbium chloride the aminodiboranate (isoelectronic to propane) could be studied as a ligand to erbium. According to the X-ray analysis 4 of the 6 boron hydrogen atoms chelate to the erbium center. Next question: what do you get when you add ammonia, diborane and sodium together all at once?