Alkyne metathesis

Alkyne metathesis is an organic reaction involving the redistribution of alkyne chemical bonds.^[1] This reaction is closely related to olefin metathesis. Alkyne metathesis was first observed in 1974 ^[2] by A. Mortreux as an alkyne scrambling phenomenon in which an asymmetric alkyne forms an equilibrium with its two symmetrical counterparts.

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Overview

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 capable of catalytic turnover.

The Schrock catalyst system Tris(t-butoxy)(2,2-dimethylpropylidyne)(VI)tungsten is based on tungsten ^[3]. This catalyst is not reactive towards alkenes in olefin metathesis. On the other hand Fischer carbenes have no value in alkyne metathesis.

The Schrock catalyst is commercially available and is prepared by amidation of tetrachloro tungsten with lithium dimethylamide to a di-tungsten complex followed by replacing the amide groups with tert-butoxy groups with tert-butanol.

This organometallic alkyne then undergoes a metathesis reaction with neoheptyne to the final product. In 2001 A. Fürstner developed a new molybdenum catalyst replacing alkoxide with aniline ligands ^[4].

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.

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

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) ^{[5] [6]}:

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