Alder-Ene Reaction
Ene Reaction
he four-electron system including an alkene π-bond and an allylic C-H σ-bond can participate in a pericyclic reaction in which the double bond shifts and new C-H and C-C σ-bonds are formed. This allylic system reacts similarly to a diene in a Diels-Alder Reaction, while in this case the other partner is called an enophile, analogous to the dienophile in the Diels-Alder. The Alder-Ene Reaction requires higher temperatures because of the higher activation energy and stereoelectronic requirement of breaking the allylic C-H σ-bond.
The enophile can also be an aldehyde, ketone or imine, in which case β-hydroxy- or β-aminoolefins are obtained. These compounds may be unstable under the reaction conditions, so that at elevated temperature (>400°C) the reverse reaction takes place - the Retro-Ene Reaction.
Hosomi-Sakurai Reaction
Nozaki-Hiyama Coupling
Nozaki-Hiyama-Kishi Reaction
This coupling between halides and aldehydes is a chromium-induced redox reaction. A key advantage is the high chemoselectivity toward aldehydes. A disadvantage is the use of excess toxic chromium salts.
Newer methods allow the use of catalytic amounts chromium(II), which is regenerated by reduction with manganese or via electrochemical reduction.
[2,3]-Wittig Rearrangement
he [2,3]-Wittig Rearrangement allows the synthesis of homoallylic alcohols by the base-induced rearrangement of allyl ethers at low temperatures.
Mechanism of the [2,3]-Wittig Rearrangement
The [2,3]-Wittig Rearrangement is a [2,3]-sigmatropic reaction, a thermal isomerization that proceeds through a six-electron, five-membered cyclic transition state. A general scheme for [2,3]-sigmatropic reactions is given here:
