The last two weeks have seen me secure a postdoc in the US, pass my PhD viva and catch a nasty cold, leaving little time in my life for writing. Now that I've finally got a few spare hours I'm completely out of ideas for a post, so I'm just going to write about some nice oldschool mechanistic detective work from the 1970's that I came across recently. Enjoy!
Here's a tricky little problem that was set at the start of a postgraduate research symposium I attended back in July. It was printed on the programmes given out at the door with a deadline at the end of the two day conference and a cash prize up for grabs. So, all that was required for the glory and spoils was a mechanism that rationalised the following experimental outcome:
Once you've drawn a few things down, you'll quickly realise that the difficulty lies not in drawing a mechanism for formation of the product, but in getting those pesky labels in the right place. Give it a go, and then read on for more information and the solution!
In actual fact, the title of this post is a bit misleading as although the final product was hexradialene, the synthesis of this elusive molecule wasn't the aim of these studies at the outset. In fact, the triyne was synthesised (by Vollhardt and co-workers; J. Am. Chem. Soc, 1978, 98, 2667) in order to investigate its relationship with the corresponding tricyclic [2 + 2 + 2] cycloaddion product. It turned out that this reaction, although perfectly okay according to Woodward and Hoffmann, turned out to not be thermodynamically favourable, with a considerable barrier present. It was was eventually found that application of forcing conditions to the triyne in the presence of dimethyl maleate lead to the expected triple [4 + 2] adduct, and *careful* FVP of the triyne on its own, followed by immediately cooling the product in liquid nitrogen, allowed to isolation of the hexaradialene intermediate. Interestingly, when a suitably 13C labelled triyne was later synthesised it turned out that the reaction did not proceed via the expected [2 + 2 + 2] product at all (as you'll probably have found out by now, it puts the labelled atoms in the wrong place), but was actually the result of three consecutive sigmatropic rearrangements:
1. If you're thinking that you might have heard of Vollhardt, you've probably seen his famous steroid synthesis:
There's a free review on this strategy by Vollhardt (with some more optimised conditions) in Pure Appl. Chem. here.
N.B. This is not the same Vollhardt as in the Hell-Volhard-Zelinsky reaction.