[Michal Bieleski]

Hi,

I came across a task to classify a rather unusual chemical transformation without considering the mechanism. It resembles a [1,5] sigmatropic rearrangement but lacks one pi-bond and leads to fragmentation. I was thinking about calling it a Grob-like fragmentation. Any suggestions?

cheers,
Mike

[Dan]

No, I don't think Grob fragmentation fits.

My first thought was retro-ene reaction, but it doesn't quite fit. Perhaps the more general term "group transfer reaction" is appropriate?

[pgk]

In my opinion, it’ a [1,5] sigmatropic rearrangement, accompanied by fragmentation. Anyway, it’s not the number of double bonds (π bonds) that counts but rather the number of electron pairs. In this case there are six electrons that are involved or else, equal to [4x1 + 2]π electrons and therefore, it’s a thermal fragmentation. 

[Michal Bieleski]

Thank you for your replays! In addition to your suggestion, Dan, of calling it a group transfer reaction I am also considering calling it a pericyclic fragmentation.

My knowledge of sigmatropic rearrangements is very basic and I think of them as a group migration from one end to the other of a pi-system. pjk your suggestion, however, implies that the definition of sigmatropic rearrangements can be broader. Anyone familiar with literature examples of such non-classical sigmatropic rearrangements?

[pgk]

Indeed, it’s a pericyclic fragmentation, mediated by a [1,5] sigmatropic rearrangement. But these terms are not often met in the literature. So, we have to be very prudent, when adopting terms that they are not yet approved and validated by scholars and academics.

[Doc Oc]

I don't know if it will help you classify, but McLafferty rearrangements work in a similar way as you're describing. Maybe looking into that will help you find better info.

[pgk]

McLafferty rearrangements mainly occur in Mass Spectrometry, under high excitation conditions (generation of radical ions) and rather fail to interpret pericyclic reactions that follow the Woodward–Hoffmann rules.