Editing Cope rearrangement (section)

==Mechanism==

The Cope rearrangement is the prototypical example of a concerted sigmatropic rearrangement. It is classified as a [3,3]-sigmatropic rearrangement with the Woodward–Hoffmann symbol [<sub>π</sub>2<sub>s</sub>+<sub>σ</sub>2<sub>s</sub>+<sub>π</sub>2<sub>s</sub>] and is therefore thermally allowed. It is sometimes useful to think of it as going through a [[transition state]] energetically and structurally equivalent to a [[radical (chemistry)|diradical]], although the diradical is not usually a true intermediate (potential energy minimum).<ref>Michael B. Smith & Jerry March: March's Advanced Organic Chemistry, pp. 1659-1673. John Wiley & Sons, 2007 {{ISBN|978-0-471-72091-1}}</ref> The chair transition state illustrated here is preferred in open-chain systems (as shown by the Doering-Roth experiments). However, conformationally constrained systems like ''cis''-1,2-divinyl cyclopropanes can undergo the rearrangement in the boat conformation.

[[File:Cope.png]]

It is currently generally accepted that most Cope rearrangements follow an allowed concerted route through a Hückel aromatic transition state and that a diradical intermediate is not formed. However, the concerted reaction can often be asynchronous and electronically perturbed systems may have considerable diradical character at the transition state.<ref>Williams, R. V., Chem. Rev. 2001, 101 (5), 1185–1204.</ref> A representative illustration of the transition state of the Cope rearrangement of the electronically neutral [[hexa-1,5-diene]] is presented below. Here one can see that the two π-bonds are breaking while two new π-bonds are forming, and simultaneously the σ-bond is breaking while a new σ-bond is forming. In contrast to the [[Claisen rearrangement]], Cope rearrangements without strain release or electronic perturbation are often close to thermally neutral, and may therefore reach only partial conversion due to an insufficiently favorable [[equilibrium constant]]. In the case of hexa-1,5-diene, the rearrangement is degenerate (the product is identical to the starting material), so ''K'' = 1 by necessity.

[[File:Cope Transition State.png|176x176px]]

In asymmetric dienes one often needs to consider the stereochemistry, which in the case of pericyclic reactions, such as the Cope rearrangement, can be predicted with the [[Woodward–Hoffmann rules]] and consideration of the preference for the chair transition state geometry.