19.22 : Amines to Alkenes: Cope Elimination

Cope elimination, much like Hofmann elimination, produces the less-substituted alkene as the major product through eliminating N,N–dimethyl hydroxylamine from an amine oxide instead of a tertiary amine from a quaternary ammonium salt.

The amine oxide is formed from the oxidation of a tertiary amine by hydrogen peroxide.

The full positive charge on the nitrogen atom promotes Cope elimination, which is a concerted elimination process that follows syn stereochemistry under thermal conditions.

The amine oxide intramolecularly abstracts the less hindered β proton, followed by a cyclic flow of electrons that ultimately leads to the loss of the leaving group and the formation of the less-substituted alkene.

The participating atoms involved in the transition state have a nearly planar arrangement, with the β hydrogen and the leaving group oriented in a syn manner.

Cope elimination, unlike Hofmann elimination, does not require an external base as the amine oxide functions as the base in the reaction.

Cope elimination reaction involves the conversion of tertiary amines to alkene using hydrogen peroxide under thermal conditions, as depicted in figure 1.

Figure 1. The Cope elimination reaction

The mechanism of the reaction is illustrated in Figure 2. Here, the oxidation of tertiary amines by hydrogen peroxide yields amine oxide. Subsequently, the thermally induced elimination of the N,N-dimethyl hydroxylamine as leaving group occurs. However, it produces a less substituted alkene as the major product. In this process, amine oxide, in the role of a base, follows the syn stereochemistry and proceeds through a cyclic transition state.

Figure 2. The mechanism of the Cope elimination reaction

Cope elimination is helpful for the synthesis of reactive/sensitive alkenes because the reaction occurs under mild conditions.

Tags

Cope Elimination Tertiary Amines Alkenes Hydrogen Peroxide Thermal Conditions Amine Oxide N N dimethyl Hydroxylamine Syn Stereochemistry Cyclic Transition State Alkene Synthesis Mild Conditions

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