A modern take on pericyclic sigmatropic migrations.
Another common type of pericyclic reaction is the migration of hydrogen or carbon along a conjugated chain, as in the [1,3] migration of a carbon as shown below. As before, I explore the stereochemistry of the thermal and photochemical reactions.
The reaction is known to proceed thermally with inversion of configuration at the migrating carbon, and constitutes a 4n-electron reaction involving one antarafacial component (at that carbon). As usual, the thermal reaction can be rationalised by its transition state and an accompanying IRC.
IRC for the thermal migration of a carbon. Click for 3D.
The photochemical reaction (not hitherto studied as far as I know) has a conical intersection between the S1 and S0 states which clearly displays migration of the carbon with retention (suprafacially).
Photochemically induced 1,3 migration with retention of configuration. Click for 3D
I conclude this trio of pericyclic reactions with a re-affirmation of the general rules for such reactions, which state that changing the reaction conditions from heat to light is likely invert one suprafacial or antarafacial stereochemical component, located either on the transition state (for the thermal reaction) or the conical intersection (for the photochemical equivalent). A caveat is necessary, in that the pathways for photochemical reactions are generally more complex than thermal ones, and so exceptions to this broad statement may well be easy to find.
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[cite]10.1039/C2SC21555A[/cite] trapping and reactions of t-butyl-hydroxycarbene, as…
As my previous post hints, I am performing my annual spring-clean of lecture notes on pericyclic reactions. Such reactions, and their stereochemistry, are described by a set of selection rules. I am always on the lookout for a simple example which can most concisely summarise these rules. The (hypothetical) one…
Most representational chemistry generated on a computer requires the viewer to achieve a remarkably subtle transformation in their mind from two to three dimensions (we are not quite yet in the era of the 3D iPad!). The Cahn-Ingold-Prelog convention was a masterwork (which won the Nobel prize). It is shown…
This entry was posted on Tuesday, November 29th, 2011 at 7:16 am and is filed under Uncategorised. You can follow any responses to this entry through the RSS 2.0 feed.
You can leave a response, or trackback from your own site.
One Response to “A modern take on pericyclic sigmatropic migrations.”
[…] Henry Rzepa Chemistry with a twist « A modern take on pericyclic sigmatropic migrations. […]