Alkyne metathesis
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[[Image:Alkyne montreux.png|200px|right|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 bond]]s.<ref>''Alkyne metathesis'' Alois Fürstner and Paul W. Davies, [[Chemical Communications]], '''2005''', (18), 2307-2320. {{doi|10.1039/b419143a}}</ref> This reaction is closely related to [[olefin metathesis]]. Alkyne metathesis was first observed in 1974 <ref>''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}}</ref> by A. Mortreux as an alkyne scrambling phenomenon in which an asymmetric alkyne forms an equilibrium with its two symmetrical counterparts.
==Overview==
The Mortreux system consists of the [[molybdenum]] catalyst [[molybdenum hexacarbonyl]] Mo(CO)<sub>6</sub> and [[resorcinol]] cocatalyst. In 1975 T.J. Katz proposed a metal carbyne and a '''metallacyclobutadiene''' as an intermediate and in 1981 [[Richard R. Schrock|R.R. Schrock]] characterized several metallacyclobutadiene complexes that were capable of catalytic turnover.
:[[Image:Alkyne schrock.png|500px|timeAlkyne metathesis mechanism through a metallacyclobutadiene intermediate]]
The Schrock catalyst system '''Tris(t-butoxy)(2,2-dimethylpropylidyne)(VI)tungsten ''' is based on [[tungsten]] <ref>''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}}</ref>. This catalyst is not reactive towards alkenes in olefin metathesis. On the other hand [[Fischer carbene]]s have no value in alkyne metathesis.
:[[Image:Alkynemetathesis3.png|400px|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|''tert''-butanol]].
:[[Image:Alkyne schrock catalyst.png|600px|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 [[alkoxide]] with [[aniline]] [[ligand]]s <ref>''Metathesis of alkynes by a molybdenum hexacarbonyl–resorcinol catalyst'' Journal of the Chemical Society, [[Chemical Communications]], '''1974''', (19), 786 - 787 {{doi|10.1039/C39740000786}}</ref>.
:[[Image:Alkyne cat furstner.png|200px|A. Fürstner developed a new molybdenum catalyst replacing [[alkoxy]] with [[aryl]] [[ligand]]s]]
== Ring closing alkyne metathesis==
Alkyne metathesis is extensively used in [[ring-closing reaction|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 [[alkyne|2-butyne]].
:[[Image:Alkyne kivetone.png|400px|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'''.
:[[Image:Alkyne turriane.png|400px|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 N<sub>2</sub> is not formed) <ref>''Catalytic Nitrile-Alkyne Cross-Metathesis''Andrea 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}}</ref> <ref>''Nitrile-Alkyne Cross-Metathesis'' Steve Ritter March 26 '''2007''' [[Chemical & Engineering News]] [http://pubs.acs.org/cen/news/85/i13/8513notw7.html Link]</ref>:
:[[Image:Nitrile-Alkyne Cross-Metathesis.png|400px|Nitrile-Alkyne Cross-Metathesis]]
== External links ==
* [http://www.strem.com/code/technical_notes/74-1800tech.pdf Supplier Schrock catalyst]
* [http://www.organic-chemistry.org/Highlights/2005/28March.shtm Alkyne Metathesis in Organic Synthesis]
== References ==
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[[Category:Carbon-carbon bond forming reactions]]