Not long ago, I described a cyclic carbene in which elevating the carbene lone pair into a π-system transformed it from a formally 4n-antiaromatic π-cycle into a 4n+2 aromatic π-cycle. From an entirely different area of chemistry, another example of this behaviour emerges; Schreiner’s[1] trapping and reactions of t-butyl-hydroxycarbene, as described on Steve Bachrach’s blog. A point I often make is that chemistry is all about connections, and so here I will discuss such a connection.
The essence of Schreiner’s[1] work is that once generated, t-butyl hydroxycarbene could rearrange in three different ways:
As pericyclic reactions, all three would be four electron thermal processes, and again all would require an antarafacial component to be present somewhere. So I pose the question here: are they indeed true pericyclic reactions, and if so are they true four-electron ones (with an implied antarafacial component somewhere)?
I will show a computed IRC for each (ωB97XD/6-311G(d,p)). First, the [1,2] hydrogen migration.[2] This emerges as a proton transfer, with a base (the σ-carbene) abstracting a proton from an acid (the σ-O-H bond). Crucially, the C=O bond is hardly involved in the process. Put simply, the non-involvement of that C=O means the process is not pericyclic. So there is no need for an antarafacial mode.
Next, the [1,2] methyl migration (if a true 4-electron pericyclic, one might imagine it would migrate with inversion of configuration). Here, one can see two distinct phases to the migration:
Now for the third mode, the insertion of the carbene into a C-H bond.[3] This too occurs in two phases:
.
So in the end, all three apparently pericyclic thermal transformations of t-butyl hydroxycarbene avoid 4-electron cyclic antiaromaticity by either becoming acyclic, or by timing the development of the two electron pairs so that they occur sequentially and not concurrently. None of the three is a true pericyclic!
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