organic reaction mechanisms: 1

Theory and Defination :

The Cope rearrangement is an organic reaction where a 1,5-diene, under thermal conditions, is converted to another 1,5-diene structural isomer. This reaction belongs to a class of reactions termed "sigma tropic rearrangements" and it is a concerted process where bonds are forming and breaking at the same time. When substituents are present in the initial diene starting material, the stereochemistry of this reaction can be predicted by drawing the molecule in a chair-like conformation. Placing the substituents in the equatorial position minimizes the steric interactions and leads to the major product. The Cope rearrangement is an equilibrium reaction and the equilibrium position is determined by the overall stability of the starting material and product.

General Reaction :

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.

Mechanism:

The Cope rearrangement is a pericyclic reaction, as shown below

Transition state is formed when heat is applied.

The Oxy-Cope Rearrangement Mechanism is shown below

Examples and Application :

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:
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.

Oxy-Cope rearrangement:

In the Oxy-Cope rearrangement a hydroxyl group is added at C3 forming an enol or enone after Keto-enol tautomerism of the intermediate enol.

for instance in this reaction:

In 1975, Evans and Golob showed that deprotonation of oxy-Cope substrates to form the corresponding alkali metal alkoxides resulted in rate accelerations of 10¹⁰ to 10¹⁷ for the oxy-Cope rearrangement. Typically potassium hydride and 18-crown-6 are employed in order to generate a fully dissociated potassium alkoxid.

It is noteworthy that the anion-accelerated oxy-Cope reaction can proceed with high efficiency even in systems that do not permit good orbital overlap, as seen in this example from Schreiber's synthesis periplanone B

Benzilic Acid Rearrangement

Theory and Defination :

Benzilic Acid Rearrangement is the rearrangement reactions of 1, 2-diketones to give alpha hydroxy carboxylic acids. 1, 2-Diketones can be converted into the salt of an alpha hydroxy caboxylic acid upon treatment with alkali hydroxide after acidic workup, the free alpha hydroxy carboxylic acid is obtained.
A well-known example is the rearrangement of benzil into 2-hydroxy-2, 2-diphenyl acetic acid. The substituent should not bear hydrogen to the carbonyl group, in order to avoid competitive reactions.
The conversion of benzil (α-diketone) into the salt of α-hydroxy acid by means of base treatment is generally referred to as the benzilic acid rearrangement or benzil-benzilic acid rearrangement. This rearrangement is normally carried out in the favored solvents of water and aqueous ethanol, and also in other aqueous organic solvents, such as in aqueous dioxane or even in solid state. This rearrangement has been reported to complete within a few hours under refluxing condition. The counterion of the base affects the reaction rate of the rearrangement when the reaction is carried out in aqueous organic solvents. The coordination of metal cation also helps this rearrangement.

General Reaction :

Illustration as below ,

Mechanism:

Reaction is induced by nucleophilic addition of the hydroxide anion to one of the two carbonyl groups.
The aryl substituent migrates with the bonding electrons to the adjacent carbon atom.
Electrons excess at the center is avoided by the release of a pair of $\pi$-electrons from the carbonyl group to the oxygen.

Finally, a proton transfer leads to the formation of carboxylate anion. The benzilic acid rearrangement of cyclic diketones are of particular interest, since these reactions leads to ring contraction.

Example and Application:

1) The reaction is general one and can take place with aromatic, heterocyclic, alicyclic, and aliphatic 1, 2 – diketones as also 1,2 quinones.

2) Doering extended the reaction to the formation of the corresponding ester by replacing the normal alkali by alkoxides. Thus benzil may directly be converted into alkyl benzilate by treatment with sodium alkoxide.

3) The reaction may be used for the preparation of αα-hydroxy acids from the easily accessible starting materials.