Pauson–Khand reaction

Under the Pauson -Khand reaction (abbreviated PK reaction or PKR, also: cyclopentenone annealing ) refers to a chemical name reaction and multi- component reaction for the synthesis of substituted cyclopentenones, mainly from the British Jewish chemist Peter Ludwig Pauson ( 1925-2013 ) and by the Indian chemist Ihsan U. Khand (1935-1980) and Graham R. Knox and William E. Watts was developed in 1971 at the University of Strathclyde in Glasgow. It is a [2 2 1 ] cycloaddition to the alkene, an alkyne, and carbon monoxide as well as dicobaltoctacarbonyl are required.

Mechanism

The reaction mechanism of the Pauson -Khand reaction is not fully secured. The following mechanism is postulated as the most likely: First, the cobalt octacarbonyl two CO ligands, for example, oxidatively removed. The subsequent addition of an alkyne provides an Dicobaltatetrahedran (see Nicholas reaction). After dissociation of an additional CO ligand, the addition of the alkene takes place, then CO is complexed to the metal again. By nucleophilic attack at a carbonyl carbon of the carbonyl group is introduced later. The reaction ends with the ring closure to the five-membered ring and the elimination of Dicobalthexacarbonyl.

Products

The Pauson- Khand reaction, a mixture of two racemic regioisomers formed. Shown here is only one enantiomer of the two regioisomers:

Intramolecular reaction

Besides the already introduced intermolecular variant, there is also an intramolecular variant of the Pauson- Khand reaction. The drawn asterisk indicate chiral centers.

Variations

Instead of the conventional cobalt catalyst, other transition metal catalysts, such as the Wilkinson's catalyst can be rhodium-based is used. Other catalyst systems, molybdenum, iron, or iridium have been used successfully.