Tag Archives: Diels Alder reaction

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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A new wrinkle in an old reaction

Just when you thought we knew everything there was to know about the Diels Alder reaction, cometh Nature vol. 532 pp. 484 – 488 ’16 in which triple bonds are used in both the diene and the dienophile. Naturally they are all put in the same molecule so they can’t get away from each other. I can’t draw the structure in this post, but it’s worth a look, particularly since a benzyne intermediate is formed in one, and an even more bizarre (and labile) intermediate (a diradical with the unpaired electrons, each on an atom, separated by two more carbons) is formed in the other. It’s sort of chemical bonsai. Enjoy

Paul Schleyer 1930 – 2014, A remembrance

Thanks Peter for your stories and thoughts about Dr. Schleyer (I never had the temerity to even think of him as Paul). Hopefully budding chemists will read it, so they realize that even department chairs and full profs were once cowed undergraduates.

He was a marvelous undergraduate advisor, only 7 years out from his own Princeton degree when we first came in contact with him and a formidable physical and intellectual presence even then. His favorite opera recording, which he somehow found a way to get into the lab, was don Giovanni’s scream as he realized he was to descend into Hell. I never had the courage to ask him if the scars on his face were from dueling.

We’d work late in the lab, then go out for pizza. In later years, I ran into a few Merck chemists who found him a marvelous consultant. However, back in the 50’s, we’d be working late, and he’d make some crack about industrial chemists being at home while we were working, the high point of their day being mowing their lawn.

I particularly enjoyed reading his papers when they came out in Science. To my mind he finally settled things about the nonclassical nature of the norbornyl cation — here it is, with the crusher being the very long C – C bond lengths

Science vol. 341 pp. 62 – 64 ’13 contains a truly definitive answer (hopefully) along with a lot of historical background should you be interested. An Xray crystallographic structure of a norbornyl cation (complexed with a Al2Br7- anion) at 40 Kelvin shows symmetrical disposition of the 3 carbons of the nonclassical cation. It was tricky, because the cation is so symmetric that it rotates within crystals at higher temperatures. The bond lengths between the 3 carbons are 1.78 to 1.83 Angstroms — far longer than the classic length of 1.54 Angstroms of a C – C single bond.

I earlier wrote a post on why I don’t read novels, the coincidences being so extreme that if you put them in a novel, no one would believe them and throw away the book — it involves the Princeton chemistry department and my later field of neurology — here’s the link https://luysii.wordpress.com/2014/11/13/its-why-i-dont-read-novels/

Here’s yet another. Who would have thought, that years later I’d be using a molecule Paul had synthesized to treat Parkinson’s disease as a neurologist. He did an incredibly elegant synthesis of adamantane using only the product of a Diels Alder reaction, hydrogenating it with a palladium catalyst and adding AlCl3. An amazing synthesis and an amazing coincidence.

As Peter noted, he was an extremely productive chemist and theoretician. He should have been elected to the National Academy of Sciences, but never was. It has been speculated that his wars with H. C. Brown made him some powerful enemies. I’ve heard through the grapevine that it rankled him greatly. But virtue is its own reward, and he had plenty of that.

R. I. P. Dr. Schleyer