Mechanism
Although the Cope rearrangement is concerted and pericyclic, it can also be considered to go via a transition state that is energetically and structurally equivalent to a diradical. This is an alternative explanation which remains faithful to the uncharged nature of the Cope transition state, while preserving the principles of orbital symmetry. This also explains the high energy requirement to perform a Cope rearrangement. Although illustrated in the chair conformation, the Cope can also occur with cyclohexadienes in the "boat" conformation.
The above description of the transition state is not quite correct. It is currently generally accepted that the Cope rearrangement follows an allowed concerted route through a homoaromatic transition state and not a diradical. That is unless the potential energy surface is perturbed to favor the diradical.
Read more about this topic: Cope Rearrangement
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