The Ene reaction is a type of thermal pericyclic reaction that forms new carbon–carbon (C–C) and carbon–hydrogen (C–H) bonds in a single, concerted step. It involves two components: ^[1-4]

• an ene (an alkene containing an allylic hydrogen atom—one attached to a carbon atom next to a double bond)
• an enophile (a molecule that accepts the hydrogen atom and forms a new sigma bond).

The general form of the reaction is:

Ene (with allylic H) + Enophile → Product (new C–C bond and hydrogen transfer)

This reaction was first reported by Kurt Alder in 1943, the same chemist who co-discovered the famous Diels–Alder reaction. The Ene reaction is an important tool in organic synthesis, as it allows chemists to construct more complex molecules from simple starting materials.

Example

A classic example is the reaction between propene (CH₃–CH=CH₂) and ethene (CH₂=CH₂). When heated, the allylic hydrogen from propene transfers to ethene, and a new carbon–carbon bond forms. The product is pentene (CH₃–CH₂–CH₂–CH=CH₂). ^[1-4]

Mechanism

The Ene reaction proceeds via a concerted mechanism—all bond-breaking and bond-forming events occur simultaneously in a single step. The key steps include: ^[1-4]

• The π bond of the ene breaks.
• The allylic hydrogen from the ene moves to the enophile.
• A new sigma bond forms between the ene and the enophile.

This process occurs through a cyclic transition state, a hallmark of pericyclic reactions. The reaction typically takes place at temperatures ranging from 100°C to 300°C, depending on the reactants involved.

Thermodynamic vs. Kinetic Control

Like many pericyclic reactions, the product distribution in the Ene reaction depends on reaction conditions: ^[1-4]

• At higher temperatures and longer reaction times, the reaction usually yields the thermodynamically stable product.
• Under milder or quenched conditions, a kinetically favored product may form instead.

How is Ene Reaction Different from Diels-Alder Reaction and Cope Rearrangement

The Ene reaction is often compared to other pericyclic reactions, such as the Diels–Alder reaction and the Cope rearrangement: ^[1-4]

While the Diels–Alder reaction and Cope rearrangement focus on carbon–carbon bond formation, the Ene reaction introduces both new carbon–carbon and carbon–hydrogen bonds, often creating greater molecular complexity in a single step.