Cope rearrangement

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The Cope rearrangement is an extensively studied organic reaction involving the [3,3]-sigmatropic rearrangement of 1,5-dienes [1][2][3][4]. 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 of 3-methyl-1,5-hexadiene

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

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[edit] 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.

Image:Cope.png

[edit] 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:

Image:3,3copeexpansion.svg

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.

[edit] Oxy-Cope rearrangement

In the Oxy-Cope rearrangement a hydroxyl group is added at C3 forming a enal or enone after Keto-enol tautomerism of the intermediate enol [5] [6]:

Oxy-Cope rearrangement

for instance in this reaction:

Oxy-Cope Applic Berson 1964

[edit] Variations

Another variation of the Cope rearrangement is the heteroatom Cope reactions such as the Aza-Cope rearrangement. Another widely studied [3, 3] sigmatropic rearrangement is the Claisen rearrangement.

[edit] References

  1. ^ Arthur C. Cope; et al.; J. Am. Chem. Soc. 1940, 62, 441.
  2. ^ Rhoads, S. J.; Raulins, N. R.; Org. React. 1975, 22, 1-252. (Review)
  3. ^ Hill, R. K.; Comp. Org. Syn. 1991, 5, 785-826.
  4. ^ Wilson, S. R.; Org. React. 1993, 43, 93-250. (Review)
  5. ^ A Synthesis of Ketones by the Thermal Isomerization of 3-Hydroxy-1,5-hexadienes. The Oxy-Cope Rearrangement Jerome A. Berson, Maitland Jones, , Jr. J. Am. Chem. Soc. 1964; 86(22); 5019-5020. doi:10.1021/ja01076a067
  6. ^ Stepwise Mechanisms in the Oxy-Cope Rearrangement Jerome A. Berson and Maitland Jones pp 5017 - 5018; J. Am. Chem. Soc. 1964; doi:10.1021/ja01076a066