Elbs reaction

The Elbs reaction is a name reaction in organic chemistry and named after the German chemist Karl Elbs. The reaction was first published in 1884 and it is a pyrolysis in the ortho position of methylated aromatic acyls to a fused aromatic system.

Mechanism

Three mechanisms are possible in principle for the Elbs reaction, in this section two useful mechanisms are described. The first mechanism is established by Fieser and starts with a cyclization of methylierteren, aromatic acyl 1 under the action of heat. Followed by a [1,3] H -shift, so that the joint 3 is formed. After dehydration of the desired polyaromatic created 4

The second mechanism describes Cook. After a rearrangement under the influence of heat and cyclization, created the connection 3 The desired polyaromatic 5 arises by a [1,3 ] -H shift and a subsequent dehydration.

The acyls required for the synthesis are accessible via a Friedel-Crafts acylation with aluminum chloride.

Variants

As shown in the previous sections, anthracene, can be made accessible by dehydration .. But also more aromatic systems, such as pentacene, can be prepared by a Elbs reaction. Here, however, the reaction does not proceed in a single step, but result in a dihydropentacene, which must be dehydrated in a further step, with copper as the catalyst.

Polynuclear heterocyclic compounds can be synthesized by the reaction Elbs. So the Elbs reaction of a derivative of thiophene was published in 1956. Here, however, not the expected linear system was obtained. This is due to a change in reaction mechanism proceeds in accordance with the formation of an intermediate product through a plurality of free radical levels.