name reaction e

Eglinton Reaction

 

The Eglinton Reaction is an oxidative coupling of terminal alkynes, and allows the synthesis of symmetric or cyclic bisacetylenes via reaction of the terminal alkyne with a stoichiometric amount of a copper(II) salt in pyridine.

---

Mechanism of the Eglinton Reaction

Alder-Ene Reaction
Ene Reaction

The 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.

While mechanistically different, the Ene reaction can produce a result similar to the Prins Reaction.

---

Mechanism of the Alder-Ene Reaction

Also like the Diels-Alder, some Ene Reactions can be catalyzed by Lewis Acids. Lewis-Acid catalyzed Ene Reactions are not necessarily concerted (for example: Iron(III) Chloride Catalysis of the Acetal-Ene Reaction).

Enyne Metathesis

The Enyne Metathesis is a ruthenium-catalyzed bond reorganization reaction between alkynes and alkenes to produce 1,3-dienes. The intermolecular process is called Cross-Enyne Metathesis, whereas intramolecular reactions are referred as Ring-Closing Enyne Metathesis (RCEYM).

---

Mechanism of the Enyne Metathesis

Enyne metathesis, or the so-called cycloisomerization reactions, were first reported using palladium(II) and platinum(II) salts and are mechanistically distinct from metal carbene-mediated pathways. As ruthenium carbenes are catalyst of choice in alkene metathesis and currently also in enyne bond reorganizations, we will focus on this family of catalysts. Ruthenium carbenes are commercially available, tolerate many functional groups and new catalysts are constantly being developed.
In the initiation step, the stable catalyst undergoes cycloaddition to the substrate forming a ruthenacylcobutane. Subsequent cycloelimination releases a stable styrene derivative, which does not interfere in subsequent steps. The catalyst is then bound to the substrate in form of a metal carbene, which reacts intramolecularly with the triple bond to yield a vinyl carbene. More details are shown in the scheme for the catalytic cycle.

In the catalytic cycle (upper left), this vinyl carbene first adds to the double bond of the substrate forming a ruthenacyclobutane. Cycloelimination at this stage gives a ruthenium carbene under release of the product (lower right). Subsequent intramolecular cycloaddition with the alkyne gives a ruthenacyclobutene. After cycloelimination, the vinyl carbene is regenerated; this reacts with another substrate molecule to give the product via methylene transfer, and the chain is carried forward.

The driving force of the reaction is the formation of a thermodynamically stable, conjugated 1,3-diene.
As such reactions are conducted under conditions of dilution that favor the RCEYM over competing cross-alkene metathesis or cross-enyne metathesis, the availability of the methylene is the rate-limiting step. In addition, the vinyl carbene is quite stable. Using less reactive catalysts, Mori has developed a system under an atmosphere of ethylene. In the presence of excess ethylene, there is a much better opportunity for catalyst regeneration to occur:

Eschweiler-Clarke Reaction

This reaction allows the preparation of tertiary methylamines from secondary amines via treatment with formaldehyde in the presence of formic acid. The formate anion acts as hydride donor to reduce the imine or iminium salt, so that the overall process is a reductive amination. The formation of quaternary amines is not possible.

---

Mechanism of the Eschweiler-Clarke Reaction

Ester Pyrolysis

Ester Pyrolysis is a syn-elimination yielding an alkene, similar to the Cope Elimination, for which ß-hydrogens are needed. The carboxylic acid corresponding to the ester is a byproduct.
The cyclic transition state can only be achieved if the steric environment is not too demandin

Fischer Esterification
Fischer-Speier Esterification

The Lewis or Brønstedt acid-catalyzed esterification of carboxylic acids with alcohols to give esters is a typical reaction in which the products and reactants are in equilibrium.
The equilibrium may be influenced by either removing one product from the reaction mixture (for example, removal of the water by azeotropic distillation or absorption by molecular sieves) or by employing an excess of one reactant.

---

Mechanism of the Fischer Esterification

Addition of a proton (e.g.: p-TsOH, H₂SO₄) or a Lewis acid leads to a more reactive electrophile. Nucleophilic attack of the alcohol gives a tetrahedral intermediate in which there are two equivalent hydroxyl groups. One of these hydroxyl groups is eliminated after a proton shift (tautomerism) to give water and the ester.

Alternative reactions employ coupling reagents such as DCC (Steglich Esterification), preformed esters (transesterification), carboxylic acid chlorides or anhydrides (see overview). These reactions avoid the production of water. Another pathway for the production of esters is the formation of a carboxylate anion, which then reacts as a nucleophile with an electrophile (similar reactions can be found here). Esters may also be produced by oxidations, namely by the Baeyer-Villiger oxidation and oxidative esterifications.