Alkyne metathesis

At the alkyne metathesis is an organic reaction, to be replaced in the substituents on a alkinischen triple bonds. The reaction is closely related to the alkene metathesis.

The metal -catalyzed alkyne metathesis was first described by Bailey et al. Described in 1968. The working group used as a catalyst a mixture of tungsten and silicon dioxide at temperatures up to 450 ° C. Published in 1974 Mortreux results indicate that a homogeneous catalyst ( molybdenum hexacarbonyl at 160 ° C) was also able to exchange alkinische triple bonds; a non- symmetrically substituted alkyne react in an equilibrium reaction with the two symmetrically substituted alkynes.

History

Mortreux, the system consisting of the catalyst molybdenum hexacarbonyl Mo (CO ) 6, and a resorcinol -catalyst. 1975 hit TJ Katz before a Ethinylmetall and a metallacyclobutadiene as intermediate. Richard R. Schrock characterized 1981 various catalytically active metallacyclobutadiene complexes.

Tris (tert- butoxy) ( neopentylidin ) tungsten (VI ) is an unreactive alkenes over Schrock catalyst. ( Fischer carbenes are not to be used for alkyne and Alkenmetathesereaktionen. )

The Schrock catalyst is commercially available. It is prepared by adding to W2 ( NMe2) 6 is amidated at the first step, tungsten ( IV) chloride with lithium dimethylamide. The resulting compound is then reacted with tert- butanol. Finally, a metathesis reaction of the complex with Neohexin is carried out for the final product.

2001 reported Alois Fürstner a new molybdenum catalyst, wherein the alkoxide ligands are replaced by aniline.

Ring closures by alkyne metathesis

Alkyne metathesis is often used to carry out ring closure reactions. The fragrance civetone can be represented, for example, from a deen. After the ring closure, the triple bond is stereoselectively hydrogenated with hydrogen over a Lindlar catalyst in order to obtain only the Z-isomer. ( The E isomer could be obtained by reduction with an alkali metal in liquid ammonia and a weak acid such as ethanol. ) An important driving force for such reactions is the outgassing of small molecules such as acetylene or 2 -butyne.

The same procedure was used in the synthesis of the naturally occurring cyclophane Turrian.

Cross - metathesis with nitrile and alkyne

In this cross - metathesis, a tungsten nitride is used instead of a Alkylidinwolfram connection. In this way two nitriles can be coupled, wherein an alkyne obtained. The nitrogen goes beyond a victim -alkyne, so there will be no elemental nitrogen: