Alkyne Metathesis in Organic Synthesis

While alkyne metathesis is not going to displace alkene methathesis as a synthetic method, it is a complementary approach that can offer advantages. 

In each of the alkyne metatheses outlined here, the byproduct is the volatile 2-butyne, The alkyne metathesis can only be carried out on internal alkynes, since the metathesis catalysts cyclotrimerize terminal alkynes such as 11 to benzene derivatives. In this context, it may prove useful that readily-available terminal alkynes such as 14 are easily isomerized specifically to methyl alkynes such as 15 (Tetrahedron Lett. 1990, 31, 5843). 

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The use of alkyne metathesis in organic synthesis is not as extensive as alkene metathesis. Nevertheless, there are some interesting applications that have been reported over the past decade. We will briefly consider a couple of these here. 

Thiel, O. R. Athene and alkyne metathesis in organic synthesis. In Transition Metals for Organic Synthesis (2ndEdn.), 2004,1, pp 321-333. (Review). 

Although much emphasis has been placed on the importance of alkene metathesis in organic synthesis, alkyne metathesis is also significant. Metathesis reactions of alkynes with alkencs and metathesis reactions of alkynes with alkynes have both been carried out efficiently. 

Lindel T (2003) Alkyne metathesis in natural product synthesis. In Schmalz HG, Wirth T (eds) Organic synthesis highlights, vol V. Wiley-VCH, Weinheim, p 27... 

Lindel, T. Alkyne metathesis in natural product synthesis. Organic Synthesis Highlights V 2003, 27-35. 

In Section 24.12, we introduced alkene (olefin) metathesis, i.e. metal-catalysed reactions in which C=C bonds are redistributed. The importance of alkene and alkyne metathesis was recognized by the award of the 2005 Nobel Prize in Chemistry to Yves Chauvin, Robert H. Grubbs and Richard R. Schrock for the development of the metathesis method in organic synthesis . Examples of alkene metathesis are shown in Figure 27.3. The Chauvin mechanism for metal-catalysed alkene metathesis involves a metal alkyli-dene species and a series of [2 + 2]-cycloadditions and cycloreversions (Figure 27.4). Scheme 27.6 shows the mechanism for alkyne metathesis which involves a high oxidation state metal alkylidyne complex, L M=CR. 

The discussed reactions of carbene and carbyne complexes show that they have essential significance as catalysts or unstable transient intermediate compounds in such catalytic processes as metathesis of olefins and other unsaturated compounds, Fischer-Tropsch synthesis, syntheses of cyclopropanes from diazoalkanes and olefins, and polymerization of olefins and alkynes as well as in organic synthesis. Except for alkynes [reaction (5.132) ] some compounds containing double bonds react with carbon monoxide and carbene ligands to form bonds with those groups. Examples of such compounds are enamines, ynamines, and Schiff bases. The JV-vinylpyrrolidone (enamine), methoxyphenylcarbene, and excess of CO (higher pressure) react to furnish enaminoketone. 

The organometallic chemistry of alkynes is very rich, including oxidative addition and coupling, insertion and metathesis and leads to a variety of applications in organic synthesis such as functionalization, C-H and C-C bond cleavage and formation, dimerization, oligomerization and polymerization of alkynes (see Parts IV and V)... 

M. and Kitamura, T. (2005) Ene-Yne and alkyne metathesis, in Comprehensive Organometallic Chemistry III, Transition Metal Organometailics in Organic Synthesis (ed. T. Hiyama), Elsevier, pp. 271-310. 

Alkyne metathesis catalyzed by metal carbynes combined with subsequent stereoselective reduction provides an option to overcome the problem of insufficient E/Z-selectivity in olefin metathesis. For a leading reference, see A. Fiirsmer, in (R.H. Gmbbs, ed.), Handbook of Metathesis, Vd. 2 Application in Organic Synthesis, Wiley-VCH, Weinheim 2003, 432. 

Metal allenylidene complexes (M=C=C=CR2) are organometallic species having a double bond betv een a metal and a carbon, such as metal carbenes (M=CR2), metal vinylidenes (M=C=CR2), and other metal cumulenylidenes like M=C=C= C=CR2 [1]. These metal-carbon double bonds are reactive enough to be employed for many organic transformations, both catalytically and stoichiometrically [1, 2]. Especially, the metathesis of alkenes via metal carbenes may be one ofthe most useful reactions in the field of recent organic synthesis [3], vhile metal vinylidenes are also revealed to be the important species in many organic syntheses such as alkyne polymerization and cycloaromatization [4, 5]. 

This contribution describes the synthesis and properties of organic alkyne-bridged materials that have been made by alkyne metathesis or by Acyclic Diyne Metathesis (AD I MET). This review covers mostly poly(aryleneethynylene)s made at the author s laboratory in South Carolina. A comprehensive review covering poly(aryleneethynylene)s listing important contributions in adjacent areas has appeared [1, 2]. This chapter discusses the synthesis and properties of dialkyl-poly(paraphenyleneethynylene)s (dialkyl-PPEs) in relation to the concept of ADIMET with simple catalyst systems. 

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