Cope rearrangement

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.

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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.

Examples

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 a cyclobutane ring to a 1,5-cyclooctadiene ring:

In this case, the reaction must pass through the boat transition state to produce the two cis double bonds. A trans double bond in the ring would be too strained. The reaction occurs under thermal conditions. The driving force of the reaction is the loss of strain from the cyclobutane ring.

Variations

Variations of the Cope rearrangement are the Oxy-Cope rearrangement and Heteroatom Cope reactions such as the Aza-Cope rearrangement. Another widely studied [3, 3] sigmatropic rearrangement is the Claisen rearrangement.

References

• Cope, A. C.; et al.; J. Am. Chem. Soc. 1940, 62, 441.
• Rhoads, S. J.; Raulins, N. R.; Org. React. 1975, 22, 1-252. (Review)
• Hill, R. K.; Comp. Org. Syn. 1991, 5, 785-826.
• Wilson, S. R.; Org. React. 1993, 43, 93-250. (Review)