all blogs

A triply twisted Möbius annulene

26 June 2014 - Compound of Interest

mobius2.PNGThere is Möbius aromaticity news. A new compound having such aromaticity has been reported as synthesised by Schaller et al. in Nature Chemistry (DOI). The challenge: the creation of a large pi-ring (as in a regular aromatic compound) but then with one or more twists in it. As in a Möbius ring. The gain in aromaticity should offset the gain in strain. That is the theory but the authors confess that initially the project looked "fairly hopeless". They then found inspiration from a simple observation that telephone cords tend to wind around themselves in order to release strain, something called projection of twist into writhe. DNA supercoiling has something to do with it as well. The solution would then be to synthesise a compound that has writhes built in that then would convert back to twists. Schaller et al. ultimately came up with a compound that can be viewed as a cyclotrimer of three helicene units fitted with diacetylenic arms. This unusual compound called for unusual reaction conditions and the final ring-closing reaction (an Eglington coupling of two terminal alkynes) took three weeks for a 10% yield.

Suzuki without metal

22 June 2014 - Orgo

aggerwal 2014 boron cross coupling  The Aggerwal group has recently described an alternative to the well-known Suzuki carbon - carbon coupling reaction that still utilises a boronic acid but does away with the platinum catalyst (Bonet et al. DOI). The novel method is applicable to aromatics such as furan, thiophene and electron-rich arenes. The metal is replaced by NBS acting as an electrophile. This novel research was triggered by a reinvestigation of some very old chemistry dating from the 1970's (DOI, DOI, DOI) with the key difference being that the Aggerwal boron shuttles are now chiral.

Chiral boron compound A (described in a 2007 publication) was synthesised from a carbamate of phenylpropanol that via its alkyllithium was stereoselectively methylated with pinacol methylboronate using sparteine. Compound A was then reacted with lithiumfuran to first give a boronate complex which was then brominated with NBS triggering a 1,2-shift (akin a Matteson homologization) followed by an elimination reaction with rearomatization to the chiral alkylated furan as final product.

Interestingly in the Aggerwal publication the final elimination is triggered by a nucleophile at least according to the published reaction scheme. Details on what the nucleophile might be however are lacking. In their Nature Chemistry editorial on the same topic Ho-Yan Sun and Dennis G. Hall must have struggled with the same problem and in their infographic the nucleophile was simply scrapped.

Update 23-06: the image did contain some errors. Corrected.

The B30 anion

10 June 2014 - Chemical zoo

Boron 30 cluster.PNGAs highlighted here a B18 dianion was recently postulated (computation only) to have a rotating inner ring. A B30 anion reported here is also flat but has a hexagonal hole. Li et al. as described in Angewandte pointed a laser at a piece of isotopically enriched boron, collected the debris in a molecular beam , isolated B30 anionic clusters via a time-of-flight mass spectrometer and identified them using photoelectron spectroscopy. The theoretical part of the work involved the calculation of energies and simulated PES spectra of 3600 B30 isomers. Two isomers were found to be located on global minima. Both of them were found to have these holes and they were also each others enantiomers. The simulated PES spectra matched the experimental ones. It was not possible to do chemical bonding calculations on the B30 anion but if the neutral B30 cluster is to give an indication, the peripheral bonds are regular sigma bonds and the remaining pi bonds (4c-2e, 5c-2e(huh?)) add up to an aromatic 4N+2 number

Something blew up in Moerdijk again

05 June 2014 - Chemical industry

And another chemical explosion hit the Dutch town of Moerdijk this week, home to a considerable amount of chemical industry. This time the culprit was Shell with a facility located by coincidence in the same industrial park as the now infamous Chemie-Pack as reported about earlier. A series of explosions were heard seen for miles around and late at night on 3 June (NRC). Luckily no one died.
According to this source the facility operates a steam cracker producing large quantities of ethylene, propene and benzene. In addition to that it produces propene oxide and its derivative ethylene glycol and also styrene. This operation takes place at the end of a pipeline that starts from the oil refineries of Shell Pernis in port-city Rotterdam.
The main cause of the explosion was the ignition of a large quantity of ethylbenzene. The presence of this compound makes sense as it is produced from ethylene and benzene and is the raw material for the production of styrene. The cause of the ignition is not clear. Apparently the incident started with reactor that was out of commission due to maintenance. For some reason this reactor collapsed, releasing a quantity of an unknown liquid (possibly benzene or propylene oxide) which then caught fire (NRC).

Methane activation by Gallium

05 June 2014 - Catalysis

klik hier voor de afbeelding op ware grootteA previous episode highlighted the conversion of methane to ethylene via iron. Li et al. report here on the conversion of the same methane to benzene now catalysed by gallium. The lede explains that the odds are not in favour of this transition. The reaction 6CH4 -> C6H6 + 9H2 requires 434 KJ/mole. This reaction can take place assisted by heterogeneous catalysis but only with temperatures in excess of 500°C. The new strategy reported about involves photocatalysis and gallium nitride (GaN). It helps that GaN is employed as a nanowire (800 nm length, 6 m2/g surface) that when deposited on silicon orientates itself perpendicular to the surface.
The results look impressive. With a 300 W xenon lamp conversion now takes place at 5°C and benzene and hydrogen are produced in the expected 1 to 9 ratio. The reaction is characterised by an induction period suggesting that it takes time for the methane to absorb to the GaN surface. On one of the GaN surfaces Ga3+ and N3- ions alternate and this arrangement can polarize the C-H bonds in methane. Because the Ga to N distance is longer the C-H bond can be weakened even further. Methyl radicals form first and ethane is the initial reaction product.
Reported conversion rate is 325 micromole h-1 g-1 with 35 mg of catalyst in a 36 hour run. Reported quantum efficiency 0.72%. But hold on. The reported conversion is stated as 0.6 micromole at best. But based on what amount of starting material? This number can only be found in the supplementary info and is: 150 micromole. That puts a dent in the performance. It also does not help (again from the supplementary info) that in order to desorb the reaction product from the catalyst it is heated at 200°C for one hour. Reviewer do something.

S-Ibuprofen purification by MagLev

01 June 2014 - Applied physics

ibuprofen maglev.PNGThe Whitesides lab has earlier adopted a MagLev for peanut butter analyis here and now have moved on to enantiopurification as described here. Case at hand: the purification of crystalline S-ibuprofen by removing a 1.3% contamination of racemic crystals. Magnetic levitation takes place in a manganese chloride solution, between two big magnets and at an angle. Even though the difference in density is small (1.093 vs 1.110 cubic centimeter), both populations levitate at different heights and can be separated. Increase in ee: from 98.7 to 99.8. Even better: on successive purifications a 50% ee sample could be upgraded to a 99.2 ee sample with 99.5% yield. Disclaimers: milligram scale experiments, multihour maglev sessions. Is this sort of contamination really an issue in industry?