Alkyne metathesis


Alkyne metathesis is an organic reaction that entails the redistribution of alkyne chemical bonds. The reaction requires metal catalysts. Mechanistic studies show that the conversion proceeds via the intermediacy of metal alkylidyne complexes. The reaction is related to olefin metathesis.

History

Metal-catalyzed alkyne metathesis was first described in 1968 by Bailey, et al. The Bailey system utilized a mixture of tungsten and silicon oxides at temperatures as high as 450 °C. In 1974 Mortreux reported the use of a homogeneous catalyst—molybdenum hexacarbonyl at 160 °C—to observe an alkyne scrambling phenomenon, in which an unsymmetrical alkyne equilibrates with its two symmetrical derivatives.
The Mortreux system consists of the molybdenum precatalyst molybdenum hexacarbonyl Mo6 and resorcinol cocatalyst. In 1975 T.J. Katz proposed a metal carbyne and a metallacyclobutadiene as intermediates. In 1981 R.R. Schrock characterized several metallacyclobutadiene complexes that were catalytically active.
In 2001, Alois Fürstner reported a new molybdenum catalyst replacing alkoxide with aniline-derived ligands.

Catalysts

The Schrock catalyst is tristungsten. Commercially available, it is prepared from W26, which undergoes alcoholysis with tert-butanol.

Ring closing alkyne metathesis

Alkyne metathesis can be used in ring-closing operations and RCAM stands for ring closing alkyne metathesis. The olfactory molecule civetone can be synthesised from a di-alkyne. After ring closure the new triple bond is stereoselectively reduced with hydrogen and the Lindlar catalyst in order to obtain the Z-alkene. An important driving force for this type of reaction is the expulsion of small gaseous molecules such as acetylene or 2-butyne.
The same two-step procedure was used in the synthesis of the naturally occurring cyclophane turriane.
Trisamidomolybdenum alkylidyne complexes catalyze alkyne metathesis.

Nitrile-alkyne cross-metathesis

By replacing a tungsten alkylidyne by a tungsten nitride and introducing a nitrile Nitrile-Alkyne Cross-Metathesis or NACM couples two nitrile groups together to a new alkyne. Nitrogen is collected by use of a sacrificial alkyne :