Counting electrons in Pericyclic Reaction Mechanisms

The following problem illustrates some of the issues involved, and its worth considering them in a little detail. click on each 2D structure on the diagram below to see the 3D structure of the molecule.

1. The Reactant. Valence bond isomers vs Resonance Isomers.

   Consider the so-called [16] annulene. Because it conforms to the Huckel "4n" rule, it woudl be predicted to be anti-aromatic as a planar hydrocarbon. To avoid this anti-aromaticity, the system buckles and becomes non planar. In this form, the two ways of arranging the alternating double and single bonds in the ring no longer give rise to the same geometries, and indeed the two bond "localised" isomers each form distinct minima in the potential surface. Such a system has quite different properties from a "delocalised" system such as benzene. Thus;
   • In benzene ([6] annulene) the two predominant Kekule structures are known as resonance isomers, and the actual geometry of the molecule is approximately the mean of the two.
   • In [16]annulene, the two structures shown above are known as valence isomers, and each represents an energy minimum for the molecule. The geometric "mean" now represents a transition state for the interconversion of valence isomers rather than an energy minimum.
   • Although valence isomers are much rarer than resonance isomers, there are many examples known. It is important to realise that each valence isomer can have its own unique properties and reactivity.
   • Inspect the 3D structure of the two forms of [16] annulene. The first has a saucer shaped geometry containing two hexatriene units joined by two alkene groups (the "66" geometry) whilst the second has a twisted aspect reminiscent of a Mobius strip and having two central butadiene groups facing eachother (the "8" geometry). Notice the steric requirements of each isomer are quite different. Thus in the "66" form, there are four inner hydrogens pointing equatorially inside the ring. In the "8" form, these hydrogens adopt axial positions with much more space. Thus replacing these groups with larger substituents would be expected to have a dramatic effect on the relative stability of the two forms!

2. Reaction of the "66" form of [16] Annulene

   This valence bond isomer of the [16] annulene is really best regarded as comprising two more or less independent hexatriene units, and arrow pushing within these gives rise to two independently cyclic six electron (4n+2 = "66") electrocyclic reactions. The electrons in the central double bonds are NOT counted in the process, since they are apparently uninvolved in the reaction.
   This reaction however is full of subtle complexities.
   • The original report of the thermal reaction implied the product was the one marked as uudd. Each hexatriene reacts by forming a new sigma bond suprafacially (the uu or the dd). Unfortunately, the most stable conformation of the [16] annulene is set up for an antarafacial bond to form (see the 3D model). This has to equilibrate with a less stable form (see 3D model) to set up the correct suprafacial bond formation In fact, another isomer, uuuu is also allowed by the selection rules, and molecular modelling suggests this isomer is in fact significantly more stable (by about 12 kJ/mol).
   • Similarly, the photochemical reaction, which is predicted to follow a Mobius transition state, can give rise to both uddu and udud via two independent antarafacial (ud, du, or ud, ud) electrocyclic reactions. The udud form is again predicted the more stable by molecular modelling by about 12 kj/mol.
   • Because two (independent) pericyclic processes are involved, this implies in each case the intermediacy of molecules in which only one C-C bond has been formed. TWO pericyclic transition states are thus involved in this reaction. Molecular modelling implies the first bond formation has a much higher energy then the subsequent second one (by about 78 kJ/mol).

3. Reaction of the "8" form of [16] Annulene

   The alternative valence bond isomer of the [16]annulene adopts an entirely different shape from the other isomer. With this new shape an alternative way of forming the two bonds can be drawn, which involves pushing only one set of four arrows, and thus constitutes a single pericyclic 8 electron (4n = "8") process formally equivalent to a 4+4 cycloaddition reaction. The selection rules predicting a quite different stereochemical outcome, with one butadiene fragment predicted to react suprafacially, the other antarafacially. Such 4+4 cycloadditions are quite rare, and this reaction in this example thus far unobserved.

   With the [16] annulene system therefore, we can have EITHER two separate and sequential electrocyclic reactions occurring, OR one cycloaddition reaction, but each with different predicted stereochemical outcomes.

4. Reaction of a [14] Annulene

   Much of the ambiguity of the [16] annulene reaction occured because being a Huckel 4n electron system, its planar form was formally anti-aromatic. If one double bond is taken out of the system to produce a [14] annulene, the consequences are quite dramatic! Now, the system is a Huckel 4n+2 system, and both the individual electrocyclic reactions and the central 2+4 cycloaddtion should all go with purely suprafacial components (see 3D model). In fact, molecular modelling predicts BOTH sigma bonds form concurently, and hence the reaction is classified as BOTH two synchronous electrocyclic reactions AND a concerted cycloaddition reaction. Unlike the [16] annulene system, there is no conflict between the two.

5. A Note on the Molecular Modelling

   All the predicted 3D models were calculated using a molecular orbital based method, using the AM1 Hamiltonian. Similar results can also be obtained using classical Molecular Mechanics methods, or computationally more rigorous ab initio methods. The transition states located must be modelled using MO based methods.

6. Further Reading

   A formal article has been published on the above, which goes into further detail:
   Huckel and Mobius Aromaticity and Trimerous transition state behaviour in the Pericyclic Reactions of [10], [14], [16] and [18] Annulenes. Sonsoles Martên-Santamarêa, Balasundaram Lavan and Henry S. Rzepa, J. Chem. Soc., Perkin Trans 2 , 2000, 1415.

   The original report of the reactions of [16] annulene was by G. Schroder, W. Martin and J. F. M. Oth, Angew Chemie, 1967, 79, 861.