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A novel annulene

A novel annulene
27 April 2021 - Chemical Zoo

The laboratory of Michel Gravel (University of Saskatchewan) has reported the synthesis of a novel annulene (Karnjit Parmar et al., DOI). Annulenes are cyclic hydrocarbons with alternating double or triple bond and of some interest in researching topics in physical chemistry such as aromaticity. For such a system to be aromatic, Hückel's rule has to be obeyed but the strain that comes with a planar geometry must also be accommodated. Benzene ticks all the boxes and is aromatic, the compound next up, cyclooctatetraene should be anti-aromatic but is too floppy for proper alignment of the p-orbitals. Cyclooctadecanonaen is again aromatic.

The new annulene is a 10-membered annulene. The parent compound suffers from too much strain (painful CCC bonds) for it to have a planar geometry. However Gravel notes that back in 1996 computational chemists Schleyer and Schaefer had predicted that by decorating the ring with cyclopropane rings this strain could be lifted (DOI). Therefore an annulene with one cyclopropane ring was adopted as the target. Additionally one double bond was replaced by an acetylene group.

Synthesis was not without challenges with double bonds unexpectedly resistant to ozonolysis and with collapsing rings in transannulation. In one critical step the entire venture was rescued by a double Mitsunobu inversion. The final step with just 20 mg on the clock was a base elimination reaction that only worked at -20 degrees and even then the target was found present only in trace amounts, the main product the result of a chain expansion. It did also not help that the target turned out to be very volatile.

On the upside, a minute quantity of the new annulene was isolated by TLC and its structure and aromaticity confirmed by proton NMR. Perfumers take note! The compound is stable and Gravel observes it has a "very distinct and pleasant smell". What lies in the future? "Eau de L'Annulene" by Dior?

Just one nagging thing. the original target proposed by Schleyer had two cyclopropane rings fitted whereas the new annulene has only one of those and one acetylene group. In fact, for various reasons Schleyer warns against the triple bond. It is clear the Gravel project ran out of material with less than 0.5 mg left in an NMR tube but obviously one final synthetic step is missing, conversion of the alkyne to the second cyclopropane ring. To be continued?
10annulene Gravel 2021.PNG