Cope rearrangement

Cope rearrangement
Named after	Arthur C. Cope
Reaction type	Rearrangement reaction
Identifiers
Organic Chemistry Portal	cope-rearrangement
RSC ontology ID	RXNO:0000028

The Cope rearrangement is an extensively studied organic reaction involving the [3,3]-sigmatropic rearrangement of 1,5-dienes. It was developed by Arthur C. Cope. For example, 3-methyl-1,5-hexadiene heated to 300 °C yields 1,5-heptadiene.

The Cope rearrangement causes the fluxional states of the molecules in the bullvalene family.

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.

The rearrangement is widely used in organic synthesis. It is symmetry-allowed when it is suprafacial on all components. The transition state of the molecule passes through a boat or chair like transition state. An example of the Cope rearrangement is the expansion of a cyclobutane ring to a 1,5-cyclooctadiene ring:

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