Greener Nylon Synth? Just Add UV and Ozone!

"Any sufficiently-developed technology is indistinguishable from magic" - Arthur C. Clarke

Looks like we'll soon have a more straightforward way to make stockings, zip-ties, and tire belts. 

Adipic acid, a six-carbon diacid representing one of the "sixes" in Nylon 6-6, apparently takes quite a bit of industrial "elbow grease" to make. The current process, starting from cyclohexane, requires cobalt, manganese, copper, and vanadate salts, high pressures of oxygen gas, and hot nitric acid. Out the other side, its responsible for 5-8% of the nitrous oxide we humans spew into the atmosphere each year.

From Science 2014, Hwang and Sagadevan

Now, researchers Hwang and Sagadevan (National Tsing Hua University, Taiwan) believe they have a better method. Reporting in this week's Science, the two disclose a method that sounds so much simpler: flush a sample of cyclohexane with ozone and UV light, and, presto! Solid adipic acid at the bottom of your reactor. No metal salts, no nitrous oxide, no high pressures or temperatures.

Wow, that looks a lot simpler.

The researchers note that zapping ozone produces both singlet oxygen, ¹O₂, and a single singlet oxygen atom O(¹D). The highly reactive single singlet (say that three times fast!) can easily insert into C-H bonds, and, since it seems to prefer insertion next to an already-oxidized carbon, the diol, diketone, and finally diacid products are formed preferentially.

Applause, please: Look at this beautiful pictorial SI!
Twice, in two days.

Just for fun, Hwang and Sagadevan crack open some larger hydrocarbons, and check the selectivity of alkyl-functionalized rings and aromatics. There are tantalizing possibilities here that I'm sure, given the ease of this reaction setup, most organic chemists will already be trying: how do complex natural products* react under these conditions? If anyone tries it this weekend, please drop me a line.

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*For that matter, I wonder if this pathway is operative in human tissues under physiological conditions? Sunlight does have some 300 nm band, and we certainly come into contact with ozone out in the wide world. Hmm.

A Cage-y Déjà Vu

I love finding molecular "diamonds in the rough." Especially when they look, well, a bit like diamonds.

While strolling through the literature, I did a double-take at this polycyclic caged structure reported by Santiago Vázquez  (Universitat de Barcelona) and coworkers. Wow!

Source: ACIEE 2014, Vasquez et. al.

Spawned from a "highly pyramidalized" (nonplanar) olefin dimerization, it features three cyclobutanes, four "envelope" cyclopentanes, and four cyclohexanes locked in their "boat" conformations (Even worse? All those eclipsing methyl groups on each end . . .jeepers). One might expect a thing like that to fall apart in a minute with a little heat, but, amazingly, the compound remains stable up to 500 degrees Celsius.

Model of the "highly-pyramidalized" olefin monomer.
Phil Baran used to say that you could judge the strain based
on how badly your model's bonds bent...

A little reference digging proved that similar caged olefins were first produced almost half a century ago at Smith Kline & French, using a seven-step prep reliant on chloroketals and long-duration sulfuric acid stirs (yuck). A major step forward came in 1970, when Avila and Silva (San Fernando Valley State U) realized that blasting UV light at a double-Diels Alder adduct of acetylene dicarboxylic acid led to a symmetric anhydride precursor.

Vázquez extended this little gem of a starting material towards finding active antiviral analogues of amantadine. Using the imide version of Silva's anhydride, Vázquez produced a caged secondary amine (see right) that exhibits activity against mutant versions of the H1N1 flu virus. His compound also inhibits the influenza A wild-type proton channel; molecular dynamics simulations indicate that it performs a "flip" relative to bound amantadine that plugs up this pore more effectively.

Let's just hope that these new polycycles get an easier-to-pronounce nickname. 
Octopane? Rocketane? Congestane? I'm open to suggestions!