Reppe carbonylation

What is Reppe carbonylation?

The Reppe carbonylation, a three-component reaction discovered by Reppe in 1953, transforms unsaturated hydrocarbons, such as alkenes, alkynes, conjugated dienes, and styrene, with carbon monoxide (CO) into a variety of saturated or unsaturated acids, esters, or anhydrides.

Reppe carbonylation - general reaction scheme - Reppe process - Reppe chemistry
Reppe carbonylation

The reaction uses a CO complex of group VIII transition metals as a catalyst with water, alcohol, or acid as a nucleophile, respectively. This reaction is part of the Reppe chemistry and is sometimes referred to as the Reppe carbonylation or Reppe process. Depending on the nucleophile used, the reaction may be called hydrocarboxylation, hydroesterification, hydroalkoxycarbonylation, or hydrocarbalkoxylation.

The most active catalysts are the cobalt and palladium complexes, although complexes of all group VIII transition metals are active.

Reppe carbonylation is versatile and can tolerate a wide range of functional groups. The reaction rate generally follows the order of:

aliphatic alcohols ≈ acids > water > phenols

for the carbonylation of alkenes, whereas among the alcoholic nucleophiles, the reactivity order is:

primary > secondary > tertiary alcohols

although aliphatic alcohols with a carbon chain length between one and six have no apparent effect on the reaction rate. Steric bulkiness has no effect in the presence of hydrogen.

For the carbonylation of alkynes, water increases the reaction rate and especially the rate for the formation of linear acids. In alkaline solution of an iron-carbon monoxide complex, the alkenes are converted into aldehydes and finally alcohols of one more carbon atom.

The iron complex of carbon monoxide CO is binuclear under these conditions, whereas the mononuclear complex is labile at temperatures above 140 ºC. The nature of the base plays an important role in controlling the reaction path, either to produce aldehyde (e.g., KOH) or alcohol (e.g., alkyl amines). The formation of aldehyde is known as Reppe hydroformylation or the Reppe reaction.