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Novel bismuth 3-5 catalytic cycle

24 January 2020 - Catalysis

organobismuth planas wang 2020 The research group of Josep Cornellà at the Max-Planck-Institut für Kohlenforschung has reported on a new use of bismuth in catalysis (Planas, Wang DOI). Cornellà has a longstanding interest in coupling reactions and it will come as no surprise the particular field of catalysis concerns fluorination of arylboronic acids. The work is extensive with a 228 page supplement. The basic idea: a Bi(III) / Bi(IV) cycle in a coupling reaction instead of the usual palladium, platinum, nickel or copper.

As a reminder the Suzuki reaction (typical coupling reaction) works by Ar1X (X = halide) oxidative addition to pd(0) to a Ar1-Pd(II)-X complex. In a transmetallation step the aryl group in Ar2B-(OR)3 is exchanged with the alkoxy group in Ar1-Pd(II)-OR complex to a Ar1-Pd(II)-Ar2 complex. The reductive elimination step then liberates Ar1-Ar2 and regenerates Pd(0).

The new bismuth catalyst was directly accessible from triphenylbismuth and bismuthtribromide, the authors write that the particular framework constrains the central atom to be reactive. The sulfone group is an additional ligand. In the proposed catalytic cycle the phenyl group in phenylboronic acid pinacol ester is exchanged for a tetrafluoroborate group in transmetallation. Next step is oxidative addition of 1-fluoro-2,6-dichloropyridinium tetraborate. The pyridinium ligand is then replaced by the internal bridging NCF3 ligand. In the reductive elimination step fluorobenzene is pushed out.

The catalyst loading was 10%, reported yield 90%. The reaction requires a sizeable amount of sodium fluoride, introduced as base and not additional fluoride source. Other non fluorine containing bases work but less efficient. The mechanistic role is unexplained or considered obvious.

New in Grignards

19 January 2020 - Industrial chemistry

Researchers from the Fraunhofer IMM institute in Germany have devised a metal reactor for the continuous delivery of Grignard reagents as reported in OPR&D (DOI). The lab-scale synthesis of these reagents is always a challenge let alone their use in industry. The authors note the dependence on the type of halide, the type of activation (classic reagent is iodine) and the exothermicity of the reaction once the organomagnesium compound has formed. The novel reactor was built using 3D selective laser melting, not an entirely surprising choice since the technique was invented by Fraunhofer in 1995. The article or the patent do not disclose what metal was selected (secret, obvious, should read more carefully or should have been mentioned).

At the heart of the device is a reactor filled with magnesium turnings. Next are an inlet for halide and a outlet for product, 4 temperature readers and a ATR-IR spectrometer. The magnesium turnings are activated in a mechanical grinding process via vibration, rubbing the turnings together removed the inactive outer layers. Magnesium is also present in a large up to 30-fold excess with respect to the halide. All reagents and solvents are used as is from the supplier. Reported yields are 90% and up with molar range 1 -2 mole/L and residence time up to 30 minutes.

Industrial scale calixarenes

05 January 2020 - Orgo

It is one of these classic Organic Syntheses preps: para-t-butyl-8-calixarene (DOI). Just take para-tert-butylphenol, paraformaldehyde, sodium hydroxide and xylene, stir and reflux and out comes the wonderfully vase shaped molecule. In a recent JOC contribution Cornelius Haase has been re-investigating this reaction and claims to have found the industrial scale recipe with top yields and more importantly top purities (DOI). To be fair the 1990 Gutsche prep is pretty basic involving no purification whatsoever. The authors note that the melting point is not consistent and the product riddled with impurities. What is Haase doing differently? Less excess of formaldehyde, xylene solvent replaced by diethylether / xylene (bringing down the boiling point, xylene sticking around for azeotropic water removal), tetraethylammonium hydroxide catalyst added, concentration increased (as long as the mixture can stir) and reaction time considerably increased. The Gutsche prep has 4 reflux hours and the new prep has 12 hours of reflux and then another 10 hours of distillation. According to Haase the first phase grows all the linears and the second phase the cycles. And the results: 81.2% yield with 98% HPLC purity versus 62-65% and unknown purity. Note: a 100 gram scale is hardly industrial, expecting 1000 Kg follow-up in Organic Process Research & Development.

2D ice growth revealed

04 January 2020 - Physical chemistry

klik hier voor de afbeelding op ware grootteEver wondered how a two-dimensional nanosheet of ice grows on the edges? With a good sense of public relations (submitted to Nature in May but published online January 1 2020, first research article in a new decade) Runze Ma et al. (DOI) have tried to find out. They grew a 2D hexagonal bilayer of ice at 120 Kelvin on a layer of gold inside an STM. Nothing new thus far but with improved resolution of the STM setup it was possible for the first time to visualize the edges.

They were found to exist as equal parts zigzag and armchair. For comparison carbon nanotubes are also hexagonal and can also be of an armchair or a zigzag type. At 120 Kelvin the edges continue to grow but by further freezing to 5K it was possible to observe the dynamics of edge growth. Zigzag edges grow by adding pentagons which then mature into new hexagons. Armchair edges first morph into pentagon-heptagon-pentagon-hexagon structures. They also have more kinks and defects. The authors also speculate on expansion of the results to 3D ice but warn that imaging 3D ice growth this way would be exceedingly difficult.