Alkyne metathesis

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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.[1] 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 unsymmetric alkyne equilibrates with its two symmetrical derivative.[2]

History[edit]

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.[3] 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.[4]

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

Ring closing alkyne metathesis[edit]

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[edit]

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[edit]

References[edit]

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