Examples Of Electrocyclic Reactions.

  1. Opening of the cyclopropyl cation involves only one electron arrow, and hence the number of cyclically conjugated electrons is 2 (4n+2, n=0). Under thermal conditions, this will go via a Hückel aromatic transition state involving a suprafacial component, which has the effect of rotating both methyl groups outwards in opposite directions (disrotation) and in the process preserving a plane of symmetry.
    Click here for animation of the IRC for ring opening of cyclopropyl cation Click here for animation of the IRC for ring closing of cyclopropyl cation

    Note that in the transition state model, the leaving group (protonated water in this model) has not entirely departed before the electrocyclic ring opening starts. In reality, the SN1 solvolysis and the electrocyclic pericyclic may in fact be synchronous with each other, rather than comprising two separate and distinct steps (i.e. both steps together constitute a single concerted process). This could be thought of as a solvolytically-assisted pericyclic reaction.

  2. Both the reactions below involve two electron arrows and hence are 4n systems (n=1).

    Click here for animation of the
    IRC for ring opening of cyclobutene    		 Click here for animation of the
    IRC for ring closing of cyclobutene

    Thermally, they will proceed via a Möbius aromatic transition state involving one antarafacial component. Both methyl groups will rotate in the same direction (conrotation) leaving one endocyclic and one exocyclic relative to the new ring. The process in this case does not preserve an axis of symmetry exactly, but only approximately. This can also be (optionally) illustrated via the Frontier orbital (HOMO) approach.

    In order to form a new sigma bond, the HOMO of the butadiene fragment must rotate in the sense shown, one node coming from above the plane of the molecule, the other from below (ie antarafacial).
  3. Under photochemical conditions, a 4n reaction is predicted to proceed via a Hückel aromatic transition state with suprafacial bond formation and a (presumed) plane of symmetry, as for example;

    The sense of this can again be seen from the frontier orbital (normally the LUMO but in the excited state occupied by a single electron, and hence effectively the HOMO).

    A more modern approach to understanding photochemical reactions is to locate the conical intersection rather than the transition state.

    conical intersection
    Conical intersection

    This reveals that the suprafacial specificity is retained but that the plane of symmetry is not.

  4. Where heteroatoms are involved, the lone pair counts as two electrons. Thus the ring opening of aziridine is a 4 electron process (=4n) with one antarafacial component, proceeding via a Möbius aromatic transition state with both methyl groups rotating in the same direction (conrotation) preserving an (approximate) axis of symmetry
    Click here for animation of the IRC
    for ring opening of aziridine
  5. 4n+2 (n=1) examples of steroidal reactions which could only be rationalised on the basis of the selection rules;

  6. One important aspect when counting electrons is to include only the cyclically conjugated system. In the example below (DOI: 10.1002/anie.196708701) for one resonance isomer of the [16] annulene shown, the arrow pushing gives rise to two independently cyclic six electron (4n+2) reactions, and the electrons in the central double bonds are NOT counted in the process, since they are apparently uninvolved in the reaction;

    10 4+4 cycloaddition The Mobius form of [16] annulene The 6+6 form of [16] annulene 4+2 or twin electrocyclic udud uddu uuuu uudd


    If the (thermally antiaromatic) [16] annulene is drawn as a different valence bond isomer, something else happens. But if the (thermally aromatic) [14]annulene is drawn in either resonance form, the same reaction happens. From this observation, science progressed (see blog entry).

    The [16] annulene system The [14] annulene system
    Animation of the conrotatory/antarafacial electrocyclic + concurrent antarafacial π4a+π4s Diels Alder reaction Animation of the conrotatory/antarafacial electrocyclic + concurrent suprafacial π2s+π4s Diels Alder reaction
    Click here for animation of the intrinsic reaction path for disrotatory hexatriene ring closure/Diels Alder reaction The anti-aromaticity for the antarafacial conrotatory hexatriene ring closure/Aromaticity of the Suprafacial Diels Alder reaction

  7. Carbanions contribute 2 electrons to the total count, the reaction proceeding suprafacially (DOI:10.1016/0040-4020(78)80226-9:)

    Click here for animation of the intrinsic reaction path for ring closing of pentadienyl anion
  8. Recently, an example has been proposed which would demonstate a double-twist Möbius transition state exhibiting two antarafacial components (a double "S" = "figure 8" transition state is aromatic for 4n+2 electrons, see DOI: 10.1039/b510508k).
  9. Finally, as a conundrum, think about the following ring openings, and whether they proceed via a Hückel or a Möbius transition state?
    These last two examples contain logical contradictions. Can you spot them? For an analysis, go see the article here.