Counting electrons in Pericyclic Reaction Mechanisms

If one is to understand the nature of pericyclic processes, it is important to learn how to "arrow push" correctly, and to count the number of electrons involved in the reaction. It is often the case however that there are several different ways of how to "arrow push". Even if this aspect is fully understood, one often has to confront other manifestations of chemical reactivity such as steric strain and molecule conformation and geometry.

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.

Mobius form of [16] annulene Known form of [16] annulene Compare with [14] annulene Photochemical product Photochemical product We think this isomer is actually formed Claimed product of cyclisation Reactions of [16] annulenes

  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;
  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.
  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.

Copyright H. S. Rzepa, 2000, 2001.