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The undecabromide ion

29 April 2013 - Chemical Zoo

In the zoo that is chemical space some strange creatures can be found wandering around. Take for example the undecabromide ion (Br11-) as very recently reported by Haller et al. (DOI) It is a member of a group of polyhalogen ions that already contains obscure species such as linear Br5- and ring-shaped Br102-.
Want some? Get a quantity of bis(triphenylphosphine)iminium chloride (PPNCl), replace the chloride for bromine (mix in water with excess KBr, collect crystals), then add elemental bromine and wait several days for red-brown crystals of (PPN)(Br11·Br2) to form. Judging from X-ray crystallography in this compound 5 bromine molecules are coordinated to a central bromine ion. One arm is extended with an additional bromine unit, linking the anions into chains.

Stereochemistry assignment can be a challenge

27 April 2013 - Drug development

Mefloquine (Lariam) is an antimalarial drug and commercially available as the racemate. The (-) enantiomer is believed to be responsible for some undesirable neuropsychiatric symptoms so an improved product would only involve the (+) enantiomer. In a surprising report Muller et al. (DOI) state that several previous assignments of the absolute configuration have been wrong. Notable three total synthesis efforts (from 2008, 2011 and 2012) erroneously assigned (+) to 11R,12S where it should have been 11S,12R. Total synthesis is usually a reliable tool for configuration elucidation and any attempt at producing the (+) enantiomer requires the correct absolute stereochemistry information.
The Muller group used X-ray crystallography of the Mosher's acid adducts for their assignment. As to the why of the three erroneous total syntheses: one of them could have overlooked an epimerization and another one only disclosed the absolute configuration of an intermediate.

Zinc dihydride - NHC marriage

21 April 2013 - Orgo

In the category of binary compounds of hydrogen a large group of metals is conspicuously absent or are unwilling partners. Zinc dihydride is one of them: dimeric in nature and very prone to decomposition. Luckily Rit et al. have recently demonstrated a novel way to tame the compound: with an N-heterocyclic carbene (DOI).

All it takes is combining solid zinc dihydride with 1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene in THF for the complex the crystallize. In this complex each zinc atom in the dimer is stabilized by a NHC ligand. The crystals only decompose with temperatures in excess of 170°C. In toluene above 40°C the molecule is fluxional of an uncertain origin.
Whereas zinc dihydride itself does not react with carbon dioxide, the corresponding complex is converted to the formate. The complex is also found the catalyse the reaction of phenylsilane and methanol to trimethoxyphenylsilane and hydrogen.

Introducing the fluoronium ion

07 April 2013 - Orgo

fluorium ion Struble 2013  Struble et al. have presented their evidence for the existence of an organo fluoronium ion. (DOI)
Onium compounds containing the other halogens are known and some can even be isolated. The high electronegativity on fluorine makes synthesis or even detection a challenge.

The experimental setup is all about compound 1 in which the triflate group can be replaced by an alcohol group via hydrolysis. But is intermediate 2 involved? Computational data suggest the symmetrical fluoronium ion is stable. The experimental crystal structure suggests the two methylene bridges repel each other. After labelling the compound and replacing water with trifluoroethanol the new product is a 1:1 mixture of 3 (NMR) , consistent with the fluoronium ion.