Alternative models for understanding pericyclic reactions: aromaticity of transition state structures

 

[4+2], [8+2] and [6+4] thermal cycloadditions are common

 

[2+2], [4+4] and [6+6] cycloadditions are known almost only as photochemically induced reactions

 

(these numbers refer to the number of electrons involved, e.g. the Diels-Alder reaction is a [4+2] cycloaddition, with 4 p-electrons from the diene and 2 from the dienophile)

 

The allowed reactions involve (4n+2) electrons – the same ‘magic’ numbers as for aromatic rings.

 

So we can think of the transition state structures for these reactions as being aromatic and so being stabilized,

 

e.g. the Diels-Alder transition state in some ways resembles benzene.

 

 

Transition state structures with 4n electrons are avoided – except when they have an antarafacial component.

 

The p-orbitals in a cyclic conjugated system like this will be form a Möbius strip if the molecule has a single twist – possible in long, flexible molecules.

 

A Möbius strip

 

Calculations predict that Möbius conjugated systems will be stable (aromatic) with 4n electrons. Möbius strip (twisted) transition states with 4n electrons are formed in some pericyclic reactions.


Recently, the first stable Möbius strip molecule was synthesized:

‘Synthesis of a Möbius aromatic hydrocarbon’ D. Ajami et al. Nature 426, 819-821 (18 December 2003).

 

Next: frontier orbitals