Alkyne metathesis

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

## Contents

* 1 Overview
* 2 Ring closing alkyne metathesis
* 3 Nitrile-Alkyne Cross-Metathesis
* 4 External links
* 5 References

## Overview

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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.

timeAlkyne metathesis mechanism through a metallacyclobutadiene intermediate

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.

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 tetrachloro tungsten with lithium dimethylamide to a di-tungsten complex followed by replacing the amide groups with tert-butoxy groups with tert-butanol.

Synthesis of Schrock catalyst starting from tetrachloro tungsten

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

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

## Ring closing alkyne metathesis

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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

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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]:

Nitrile-Alkyne Cross-Metathesis

## External links

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* Supplier Schrock catalyst
* Alkyne Metathesis in Organic Synthesis

## References

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1. ↑ Alkyne metathesis Alois Fürstner and Paul W. Davies, Chemical Communications, 2005, (18), 2307-2320. doi:10.1039/b419143a
2. ↑ Mo[N(t-Bu)(Ar)]3 Complexes As Catalyst Precursors: In Situ Activation and Application to Metathesis Reactions of Alkynes and Diynes Furstner, A. Mathes, C. Lehmann, C. W. J. Am. Chem. Soc.; (Communication); 1999; 121(40); 9453-9454. doi:10.1021/ja991340r 10.1021/ja991340r
3. ↑ Tungsten(VI) neopentylidyne complexes R. R. Schrock, D. N. Clark, J. Sancho, J. H. Wengrovius, S. M. Rocklage, S. F. Pedersen; Organometallics; 1982; 1(12); 1645-1651. doi:10.1021/om00072a
4. ↑ Metathesis of alkynes by a molybdenum hexacarbonyl–resorcinol catalyst Journal of the Chemical Society, Chemical Communications, 1974, (19), 786 - 787 doi:10.1039/C39740000786
5. ↑ Catalytic Nitrile-Alkyne Cross-MetathesisAndrea M. Geyer, Robyn L. Gdula, Eric S. Wiedner, and Marc J. A. Johnson J. Am. Chem. Soc.; 2007; 129(13) pp 3800 - 3801; (Communication) doi:10.1021/ja0693439
6. ↑ Nitrile-Alkyne Cross-Metathesis Steve Ritter March 26 2007 Chemical & Engineering News Link