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.

'March Madness' - Cobalt Catalysis?

Catalysis fads come in waves. This shouldn't surprise - when one group finds almost-too-good-to-be-true reactivity, everyone jumps on to ensure rapid publication and novelty for that dusty grant submission.

For precious metals, King Palladium still reigns supreme, though some serious recent coups came courtesy of gold, iron, rhodium, and iridium. But a new challenger now looms on the horizon:
cheap, plentiful COBALT.

Within about a week of one another, the Dong, Chirik, and Yoshikai groups have disclosed some really neat-o transformations that run on the Co(I)-Co(III)* redox engine. Dong's group discovered a new diene hydroacylation. Chirik's installed B(pin) onto heteroaromatics with low loading and no solvent. Yoshikai offers a mini-career-retrospective on his group's efforts to develop several (new to me!) Co reactions, among them hydroarylation, zinc insertion, and acylation.

So, why the sudden upswing in cobalt? For one, relatively new ligand architectures (pincers, NHCs) have allowed access to stable architectures earlier chemists could only dream of. Also, the concept of "reductive" coupling, still in its infancy compared to the oxidation behavior most folks affiliate with top-row metals, has lured younger groups fighting to carve out a career niche.

Let's see how swollen the cobalt catalysis field becomes in the next five years. The next "gold rush?"
Only time will tell.

*As @Organometallica (rightly) points out, these reactions may yet be more complex - say, for instance, a fast Co(I)-Co(II) to Co(II)-Co(III). This could have mechanistic ramifications later down the road (radicals, anyone?), or be responsible for various off-cycle activity / resting states.

Highly Active, Barely Seen

Bench chemists know it's tough enough to control the multiple variables that go into any one reaction. But what about the ones you never saw coming?

The literature abounds with cautionary tales: Trace nickel (II) in the NHK reaction. Trace phosphate in the GFAJ "arsenic life" saga. "Metal-free" couplings found to rely upon parts-per-billion levels of Pd or Fe contaminants in "pure" sodium carbonate.

In yesterday's post, a volcanic mudpot-dwelling bacterium flourished in lab culture, but only when its growth media was doped with a rare earth element (REE). The authors had quite a bit of trouble eliminating residual metals from the growth media:

"When testing REE dependency (salts > 99% pure), it was observed that standard serum bottles resulted in a highly variable growth. . . Sand is one of the major raw materials of glass and may contain considerable amounts of REE, and Ce may be used as an additive during glass manufacturing. It was concluded that REEs in glass are extractable, at least partly, by the acidic media used."

Whoa! I confess, I've stirred hundreds of acidic solutions in glassware of all shapes and sizes, and never once have I assayed the rare earth content! And the glass wasn't the only cause for concern:

"Contact of the acidic medium with needles used for sampling was minimized as the metal seems to release REE as well. For these experiments, concentrations of trace elements were (in μM): NiCl2, 1; CoCl2, 1; Na2MoO4, 1; ZnSO4, 1; FeSO4, 5; and CuSO4, 10."

For those playing at home, some of these trace metals guest star at the ppm level in this media. Due to the materials used in glass and disposable needle manufacture, I guess there will always be a baseline of (potentially active) metal contaminants in acidic solution.

Want to take bets that one or more play roles in our favorite cross-coupling reactions?