We'd like to understand how you use our websites in order to improve them. Register your interest. Oxidation and reduction are two important transformations in organic chemistry. Although several oxidation and reduction reactions are studied in practical organic chemistry, Birch reduction as an experiment, is generally avoided.
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The Birch reduction is pretty interesting to run, especially the first time you do it. Liquid ammonia is not a typical reaction solvent, and condensing it off a cold finger always looks a bit like a magic trick. Sodium metal itself looks like a grey lump or cylinder under those conditions, but as you dig into it the stuff has a consistency in between cold butter and a hard salami , the fresh surface is brilliantly shiny and metallic, as if somebody had invented spreadable chrome for car fenders.
But only for an instant; it tarnishes while you watch. Or any ammonia. As you add your alkali metal to the solution, you get a startling blue color that forms around each piece of dissolving metal — actual solvated electrons — and at least at first, the blue probably dissipates as it heads out into the solution and meets your substrate. Eventually, though, it holds on, and you have a frosty blue reaction flask that looks like a very refreshing drink for an alien on a hot day.
Note: more laboratory blue candidates here. The final part of the workup is simple, anyway: you just pull the flask out of the cold bath and come back in a while when all the ammonia has boiled away. The reason you go to all this trouble, of course, is that the electron-dunk provides you with transformations and structures that can be hard to realize any other way.
But no one wants to do this on large scale, which can present a real impasse. There have been many reports of this sort of thing, naturally, going back to Arthur Birch himself, but electrochemistry itself has generally been an unattractive proposition for synthetic organic chemists, with its own reputation for wonky unscalability.
The paper really shows you how tricky such optimization can be. Along the way, they noticed a metallic substance plating out on the cathode, and realized that this was lithium metal itself. The proton source for the reaction needed optimization dimethylurea worked the best , as did the material of the anode magnesium instead of aluminum , and the cathode itself needed to be made physically smaller to increase the current density.
Overall, the reaction looks like a single-electron reduction, then a protonation, then both steps again. Synthetically, this reaction seems to behave very similarly to the classic Birch, but with no ammonia and no alkali metals, of course. Very interestingly, it appears that a range of other synthetic transformations that depend on dissolving-metal conditions ketone reduction, McMurry coupling, reductive cyclizations and ring openings of various kinds can also run under the same conditions — which is not true of the Birch reaction itself, nor of any of its alternatives.
And finally, the whole thing seems to be easily scaled up. The paper demonstrates this by just stacking more electrochemical modules together and running them in either batch or flow mode. This take it from milligram scale, to 10 grams, to gram scale with virtually identical yields, which makes me think that a commercial Birchomatic machine Baranomatic?
They actually keep the lithium from depositing by use of the TPPA additive — the lithium comes in in the form of lithium bromide added to the mixture. What are solvated electrons? You only get Cerenkov radiation with very high speed electrons that is, electrons that are moving faster than the speed of light in that particular medium.
Paywall on the Baran paper. For me, the rate limiting step in doing electrochem on my synthesis projects was PI resistance, way off the megaohms scale on my PI resistance meter. When it comes to synthesis, I am generally in favor of whatever works, be it Birch or electrochem. Now, if you start to need platinum electrodes, then, yes, that will cost arm, leg and kidney.
One of my favorite activities in the college lab was to cut up sodium pieces from a larger piece for use in all kinds of reactions including Birch reductions. One of the TAs in grad school started a small fire in undergrad lab when he threw the ethanol with cut-up sodium pieces in it from a Birch setup in the waste jug — someone noticed when flames were coming out the top.
I think the Birch reductions worked, though, other than that. Ah, yes. Always time for a minor fire. An undergrad researcher in the lab when I was in grad school opened a brand new can of sodium metal and proceeded to start cutting into it to get fresh slices for a Bouveault-Blanc reduction.
The knife slipped, the chunk flipped away, and a tennis ball sized lump of sodium tumbled into the nearby sink! Fortunately for all in the lab the sink was dry. Fresh underwear all around that day. Back to the balance to get the mass.
Depending on the reaction, the metal reacts slowly from the open end of the capillary or aggressive stirring or jabbing with a glass rod or spatula can break the capillary glass and release the metal entirely. He went off to a teaching position at a small college. Worked well. Mixed feelings myself. Otherwise the iron contaminants can blow into the flask with the ammonia and it catalyzes the formation of lithium amide, which serves as a base rather than a reducing agent.
Birch reduction was one of the reactions I used to love to do, the colour was great. Also like Sandmeyer reaction, beautiful yellow crystals emerging from black mixture. This electron generator is operated simply by illumination with a frequency-doubled Nd:YAG laser nm running at its normal repetition rate.
From large scale ozonolysis, stoichiometric osmolyations to the infamous Sitosterol pile, that place had serious old school chemistry credibility…. Barry is right. Relatively speaking, sodium and ammonia are dirt-cheap, At a plant where I was worked as a process chemist in the s, in a ferrocene process, I watched old plant hands picking up 5-lb blocks of sodium and slinging them into the reactor manhole.
A colleague in uni did a birch on a 1L scale and the whole department came to see the beautiful blue colour — now, after 20 years of scale-up I can only think that the liter of liquid ammonia could have easily killed us all… How did we survive university? The Process and Manufacturing groups at Smith-Klein ran the BouvealtBlanc reduction of an ester to an alcohol for cimetidine on tons of material in hundreds if not thousands of liters of ammonia for years.
That ammonia was distilled out of one reactor upon completion into a second. And back and forth, year after year. Ammonia is cheap, and biodegradeable. Clean chemistry, cheap, unproblematic waste-stream. Not in need of re-invention. We reductively removed a benzyl ether in the presence of an alkene in route to a total synthesis of discodermolide J.
Classic Birch with Na or Li in NH3 resulted in the over reduction of the alkene to the corresponding alkane. In a set of reaction conditions similar to the Benkeser reaction, but with Li metal instead of Ca metal, and no sand required. It was a performed on any scale we tried and very mild.
Slow enough to TLC and monitor. Aqueous ammonium chloride work up. Hundreds of pounds of liquid ammonia are routinely used in big refrigeration units.
Thousands of pounds of liquid ammonia are routinely injected into farmlands where it presumably kills nematodes as well as providing fixed nitrogen to the crops to follow. Sure, the farmhands doing that application should have more training and more protective equipment to satisfy a chemist.
Efforts to avoid using ammonia are efforts that could be better spent. All rights Reserved. Or Ammonia. Or Metals. Derek Lowe says:.
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Birch and Benkeser Reductions
Birch Reduction Without Tears. Or Ammonia. Or Metals.