The epoxidation of an alkene to give an oxirane is taught in introductory organic chemistry. Formulating an analogous mechanism for such reaction of an alkyne sounds straightforward, but one gradually realises that it requires raiding knowledge from several other areas of (perhaps slightly more advanced) chemistry to achieve a joined up approach to the problem. I had indeed hinted in a previous post that the mechanism for oxidation of acetylene to ketene might be an interesting arrow pushing challenge to set a bright tutorial group, and it was that self-hint that has led me to here. I now explore how my “arrow pushing” intuition stands up to a computational examination.
The products of the reaction of an acetylene with mCPBA (m-chloroperbenzoic acid) are set out[1] and the primary step of the mechanism[1] (this article declines to discuss the subsequent steps). The primary product is the formation of oxirene 1, a 4π-annulene which then undergoes an electrocyclic pericyclic ring opening to give the formyl carbene 4 (or 2 or 3). This presumed intermediate is also presumed to be highly reactive, and likely to undergo a variety of reactions (for example it will insert into a C-H bond if given one). I concentrate initially on just one path, the Wolff rearrangement (a [1,2] sigmatropic pericyclic reaction) to give the ketene 7. The scheme above constitutes one of those mandatory mechanistic challenges that organic chemists, almost without exception, cannot resist trying to solve, in the same way that some people may be addicted to Sudoko puzzles! So now for the reality check.
Well, that was a complicated story, and we have learnt a lot about (anti)aromaticity and hydrogen bonds in the process. There is one fly in the ointment however. If you look that the barriers that have to be overcome for the sequence 1-3-6-7 to occur, they are all quite large. In fact, the original report (for di-t-butyl acetylene[1]) does say very little ketene is actually formed. The next step would be to find lower energy pathways for reaction (such as possibly involving 2 or 4). But I will save that for a future posta future post.
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