Thursday, December 18, 2014

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.

--
*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.

3 comments:

  1. Wow, that looks so close to winning the Barton challenge (http://pubs.acs.org/doi/abs/10.1021/cen-v077n001.p024)!
    I'm amazied by the "reagentlessness" of this reaction and that noone observed it before.

    ReplyDelete
  2. I'm also enjoying the pictorial guides in SI, hopefully this trend continues. (I'll have to make sure I do it next time...)

    ReplyDelete
  3. bubbling pure oxygen into cyclohexane (not mentioning ozone) is a huge safety risk on scale, and static electricity buildup is always problem with cyclohexane. Maybe they can develop a flow-based system, but the precipitate formation may complicates things, and so can the a presence of organic peroxides in the product. Also, generating ozone by UV irradiation of oxygen is quite energy intensive, it may not pay for a commodity chemical like adipic aid.

    As far as total synthesis is concerned, I bet that the only reason why this works is that cyclohexane is a symmetric substrate (used in excess, as a reagent and a solvent) and the product precipitates from the reaction mix. There are alternative, much more gentle systems for generating singlet oxygen

    ReplyDelete