Carbynes ! ! !

An article on carbynes brought back memories of the Spring of 1961 when I convinced Woodward to let me work on an original idea about carbenes for my PhD thesis.  Back then you had to pass 8 cumulative exams (given monthly) before you could start such work.  It took me 9.

At the time, carbenes were a rather speculative idea, but it seemed to me that they could be generated by photolysis of a diazocarbonyl compound. I thought they might be involved in the Wolff rearrangement

One of the joys of organic chemistry back then (and hopefully now) is that if you have an idea, just build a molecule to test it.

So here’s the idea the great man bought.

l. Condense acrylic acid with cytopentadiene by a Diels Alder reaction.  Because of steric effects the acid will point below the ring system

2. Form the acyl chloride

3. React with diazoMethane to form the diazocarbonyl — there will be no change in the orientation of the carbonyl relative to the ring system

4. Photolyze — a a carbene is formed it would be in perfect position to form a cyclopropane on the other side of the ring system, pretty much proving its existence.

Malheureusement, having the worst lab technique in the world and being very frightened by what I’d heard about diazoMethane, I couldn’t get the idea to work.

However the idea was good, and a friend who kept on in chemistry becoming a department head told me that I was right.

Which brings us to the current article [ Nature vol. 554 pp. 36 – 38, 86 – 91 ’18 ] http://www.nature.com/magazine-assets/d41586-018-01308-7/d41586-018-01308-7.pdf.

A carbyne is basically R – C where the carbon has 3 electrons not forming covalent bonds (two are paired).  As you might imagine, carbynes are quite reactive.  However both articles talk about a carbyne equivalent which is R – C = N2, which IMHO is not a carbyne at all.  It is intrguing that it would be if the N2 were photolyzed off a la 1961, but that isn’t what happens in the paper.  It remains as the intermediate performs all sorts of interesting chemistry, forming an Aryl – C (R) = N2 moiety etc. etc.

One interesting aside is that carbynes were one of the first molecules found in interstellar space.

Can anyone out there enlighten me as to why R – C = N2 is a carbyne equivalent.  Neither paper provides an explanation.

 

 

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Comments

  • Peter S. Shenkin  On February 4, 2018 at 8:21 pm

    (It wasn’t clear to me whether my comment was posted. I’m trying again. If it appears twice, apologies.)

    I read the News & Views overview is as follows. I think this is at least approximately right.

    Look at Figure 1b, p. 37. The two substituents he labels “Masks” – a red one double-bonded and a blue one single-bonded to the “carbyne” carbon, are really leaving groups, except that the connecting bonds are cleaved symmetrically.

    When the bond to the blue mask is cleaved by light, its single bond breaks homolytically, forming a free radical. When the bond to the red mask is cleaved by catalyst, it also cleaves homolytically, so that two additional unpaired electrons are created, forming a carbyne, which can then react.

    The entire reaction is said to take place “as if” it were performed by a carbyne, though it’s not clear to me from a quick reading if they are claiming that a free carbyne ever actually exists for some small increment of time. If it does, I’d personally go for the term “carbyne precursor” rather than “carbyne equivalent.” Perhaps they call it the latter because they want to be cautious and avoid the claim that a carbyne actually exists independently at some point; so it would be an “equivalent” in the sense that the overall process proceeds “as if” a carbyne were actually produced.

  • Slava Bernat  On February 4, 2018 at 11:35 pm

    I think authors meant a retrosynthetic equivalent – or synthone in Corey’s terms, – definitely not physicochemical analog of carbynes. Described reactions allow simultaneous disconnections of three single bonds from sp3-carbon, so it’s sort of a retrosynthetic carbyne.

  • luysii  On February 5, 2018 at 10:13 am

    Thanks, Peter and Slava. That’s probably what they mean by a carbyne equivalent, but it still seems like a cheat. Suppose the =N2 were replaced by =CH2. Would it still be a carbyne equivalent?

  • andyextance  On February 6, 2018 at 10:55 am

    Probably not, because you can use light to get rid of the N2 but not the CH2. Here’s my story on the paper: https://www.chemistryworld.com/news/carbyne-equivalents-fire-up-carboncarbon-bond-formation/3008615.article

    • luysii  On February 6, 2018 at 11:04 pm

      Andy — thanks for sending the link. I had no idea that you did this sort of thing. “Although there may be concerns about diazomethyl groups being explosive, the team’s research indicates that they are safe at room temperature.”

      We were all scared as hell of diazomethane back in the day, including fellow grad student Tom Lowry (of “Mechanism and Theory in Organic Chemistry”) who had to use it in his work with Corey.

  • Curious Wavefunction  On February 12, 2018 at 1:05 pm

    There was some neat work while ago where they used nitrenes to label residues in proteins. You would add an inert diazo compound to a protein and the shine light on it and, zap, the reactive nitrene finds the most nucleophilic sites in the protein, allow you to get some insights into its structure.

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