Wagner–Meerwein rearrangement

In the Wagner- Meerwein rearrangement is a name reaction in organic chemistry. It was discovered in 1899 in Kazan (Russia) by Georg Wagner ( Егор Егорович Вагнер ) and examined from 1914 by Hans Meerwein. It is an intramolecular rearrangement of atoms or molecular groups in carbenium ions.

• 3.1 terpene chemistry
• 3.2 Miscellaneous

Overview reaction

It is catalyzed by acids, nucleophilic [1,2 ] rearrangement of the carbon skeleton via carbenium ions. This type of rearrangement is also called anionotrope rearrangement. Since the reaction moves a σ - bond, it is a sigmatropic process. Here R1 and R2 are organyl or hydrogen atoms, and R3 are different organyl.

Mechanism

The rearrangement step depicted in the overview reaction can not be carried out separately. He has always been a part of reaction sequences. Reactions which take place this surroundings, consist essentially of three steps:

The driving force of such rearrangements is the formation of more stable carbenium ions. With the stabilization of carbenium ions, the positive charges are better stabilized by more alkyl radicals in the vicinity (see hyperconjugation ). Ie tertiary carbenium ions are more stable than secondary or primary.

Requirements

This rearrangement can take place, there must be attached to a carbon atom which is itself bound to a higher substituted carbon atom is a good leaving group (eg, which can be protonated hydroxyl or halide ).

Rearrangement of an alkyl group

The mechanism of the rearrangement of an alkyl group at the example of an elimination reaction:

Due to the elimination of a halide of a haloalkane 1 a secondary carbenium ion 2 is generated. Since this is less stable compared to the tertiary carbenium ions, an alkyl group superimposed ( here in blue) around. Thus, the intermediate 3 is formed in the last step of the tertiary carbenium ion 3 is deprotonated and the alkene is formed 4

Hydride

In the following example ( isomerization of a haloalkane ) is illustrated [ 1.2 ] rearrangement of a hydride ion. In the reaction is a nucleophilic substitution in which a primary responding to a secondary alkyl halide. This is also called isomerization.

The hydride basically proceeds analogously to the rearrangement of an alkyl group (see above). By the use of aluminum bromide in a catalytic amount, the leaving group is cleaved (in this case, the bromide ) and the bromoalkane 1 is a primary carbenium ion 2erzeugt. In the positively charged carbon atom a substituted higher carbon atom is bonded, namely, a secondary. Thus, instead find a Wagner -Meerwein rearrangement. The carbenium ion 2 reacts to the intermediate 3 by a hydride ion ( here in blue) is rearranged. This creates a secondary carbenium ion 3, which is attacked by the nucleophilic leaving group and the isomer of the starting compound, a secondary bromoalkane 4 further reacts.

Use

The synthetic potential of the Wagner- Meerwein rearrangement is naturally very low and is otherwise in the laboratory often rather an undesirable side reactions - for example, in elimination reactions. However, it has a special meaning in the terpene chemistry.

Terpene chemistry

The Wagner- Meerwein rearrangement is useful for the dehydration of tetrahydrofurfuryl alcohol to dihydropyran. Furthermore, occur Wagner -Meerwein rearrangement in the biosynthesis of cholesterol.

It also plays a major role in the production of Camphenen. Camphene is an intermediate in the synthesis of flavors and odors.

It is an acid-catalyzed dehydration of isoborneol Camphenen (cf. the Elimierungsreaktion for the rearrangement of alkyl groups, see above). It should be noted that the product was made in the form shown here in comparison to the starting material for greater clarity in the room. Note here the numbering of the carbon atoms. The blue marked bonds between two carbon atoms represents the migration of the organyl

The mechanism is as follows:

First, the isoborneol (1) is dehydrated, whereby the carbenium ion 2 is generated. This secondary cation is less stable than the tertiary cation 3 This is the reaction sequence takes place in the Wagner -Meerwein rearrangement. To the ring system is broken and a new link between two carbon atoms is formed - an alkyl group is rearranged. Compound 3 is shown here in two modes of representation. Through rotations in three-dimensional space, the right structure has arisen from the left.

To the stabilized carbenium ion 3 is a hydrogen atom is eliminated and results in the camphene (4) having the double bond.

Others

The Wagner- Meerwein rearrangement is similar to the pinacol rearrangement. This is also a anionotrope skeletal rearrangement of the carbenium ion and specially treated glycols, which react by the elimination of water to ketones. Thus one can understand the pinacol rearrangement as a special form of the Wagner -Meerwein rearrangements.