Alkyne metathesis

File:Alkyne montreux.svg

The Mortreux system consists of molybdenum hexacarbonyl resorcinol catalyst system. The phenyl and p-methylphenyl substituents on the alkyne group are scrambled

Alkyne metathesis is an organic reaction involving the redistribution of alkyne chemical bonds. This reaction is closely related to olefin metathesis. 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.

History

The Mortreux system consists of the molybdenum catalyst molybdenum hexacarbonyl Mo(CO)₆ and resorcinol cocatalyst. In 1975 T.J. Katz proposed a metal carbyne and a metallacyclobutadiene as an intermediate and in 1981 R.R. Schrock characterized several metallacyclobutadiene complexes that were catalytically active.

Alkyne metathesis mechanism through a metallacyclobutadiene intermediate

The Schrock catalyst system tris(t-butoxy)(2,2-dimethylpropylidyne)tungsten(VI) is unreactive towards alkenes. On the other hand Fischer carbenes have no value in alkyne or alkene metathesis.

Alkyne metathesis of 2-hexyne with Schrock catalyst, equilibrium after 5 minutes reaction

The Schrock catalyst is commercially available and is prepared by amidation of tungsten tetrachloride with lithium dimethylamide to a W₂(NMe₂)₆ which undergoes alcoholysis by tert-butoxy groups with tert-butanol.

Synthesis of Schrock catalyst starting from tetrachloro tungsten

This alkylidyne complex undergoes a metathesis with neoheptyne to give the final product. In 2001, Fürstner reported a new molybdenum catalyst replacing alkoxide with aniline ligands.

A. Fürstner developed a new molybdenum catalyst replacing alkoxy with aryl ligands

Ring closing alkyne metathesis

Alkyne metathesis is extensively 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 (cyclic E-alkenes are available through the Birch reduction). An important driving force for this type of reaction is the expulsion of small gaseous molecules such as acetylene or 2-butyne.

Synthesis of civetone. Step 1 alkyne metathesis, step 2 lindlar reduction

The same two-step procedure was used in the synthesis of the naturally occurring cyclophane turriane.

Turriane synthesis. Step 1 alkyne metathesis, step 2 Lindlar reduction, PMB = para-methoxybenzyl protecting group. Microwave assisted reaction takes reaction time down from 6 hours to 5 minutes

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 (elemental N₂ is not formed):

Nitrile-Alkyne Cross-Metathesis

External links

• Alkyne Metathesis in Organic Synthesis