Ene reaction
The ene reaction, also called Alder-ene reaction is a chemical reaction in the field of organic chemistry. Is a pericyclic reaction, in which an alkene carrying an allylic hydrogen in position, is reacted with a compound having a multiple bond. The alkene component is briefly as En, denotes the second component enophile. The reaction was first described in 1943 by Kurt Alder.
S reactions need to activate a higher reaction temperature, or a Lewis acid. Electron-poor enophiles promote the reaction, since they facilitate the attack of the Ens on enophile (Compare with diene and a dienophile in a Diels -Alder reaction). A good enophile example, maleic anhydride, whose double bond by the electron-withdrawing effects of two carbonyl is electron-deficient.
General
At the ene reaction, four π - electrons are involved. Formally, a π - bond in favor of a σ - bond is broken. It is favored by electron-rich and electron-poor Ene enophiles. It takes a worse the higher the degree of substitution in the allylic position, carries the transferred proton.
Intramolecular ene reactions run smoother than intermolecular in most cases. Can classic Lewis acids such as aluminum chloride or boron trifluoride may be used. The reaction itself then runs still concerted because the Lewis acid serves only the activation of the enophile.
Enophiles are not limited to, the use of alkenes. Alkynes or heteroatom- bearing groups that have a multiple bond can be used.
Mechanism
The ene reaction is concerted. There is a bond formation between a carbon atom of the double bond and the enophile with the allylic hydrogen transfer to an enophile.
Variants
Hetero -ene reactions
The enophiles also groups can serve, carry the heteroatom. The main factors include carbonyls, but also reactions with functional groups that carry multiple fixed nitrogen or sulfur atoms, are described. If carbonyls, one also speaks of a carbonyl -ene reaction.
Retro-ene reaction
The retro -ene reaction usually occurs as an intramolecular reaction. It extends inverse of the ene reaction and usually occurs when high hoop stresses, for example by Cycplopropylringe are present, which can be dismantled.
Ene reaction with inverse electron demand
Similarly, the Diels -Alder reaction, ene reactions with inverse electron demand may occur. This means that an electron-deficient ene is reacted with an electron-rich enophile. Unlike the Diels -Alder reaction, these ene reactions, however, are very rare.
Schenck ene reaction
The Schenck ene reaction, or even short Schenck reaction, named after Otto Günther Schenck, is a variant of the ene reaction in which singlet oxygen is used as enophile. This allows the synthesis of hydroperoxides.