Alkyne metathesis mechanism

There are of course borderline cases when the reacting hydrocarbon is acidic (as in the case of 1-alkynes) a direct attack of the proton at the carbanion can be envisaged. It has been proposed that acyl metal complexes of the late transition metals may also react with dihydrogen according to a o-bond metathesis mechanism. However, for the late elements an alternative exists in the form of an oxidative addition reaction. This alternative does not exist for d° complexes such as Sc(III), Ti(IV), Ta(V), W(VI) etc. and in such cases o-bond metathesis is the most plausible mechanism. 

This mechanism was later confirmed experimentally in 1981 by Schrock and others, who reported the first example of alkyne metathesis by tungsten(vi)-alkylidyne complex. They have prepared tungsten alkylidyne complex 120 (Equation (21)) and found that it reacts with diphenylacetylende to give tungsten alkylidyne complex 121 and another alkyne 122 (lequiv.) (Equation (22)). Furthermore, complex 121 works as a catalyst for the alkyne metathesis reaction. 

The polymerization of substituted alkynes is postulated to proceed either by the metathesis mechanism or by an insertion mechanism (18). Numerous alkyne derivates have been shown to polymerize in the presence of group V, VI, and VIII transition metal catalysts. 

Until recently, intermolecular enyne metathesis received scant attention. Competing CM homodimerisation of the alkene, alkyne metathesis and polymerisation were issues of concern which hampered the development of the enyne CM reaction. The first report of a selective ruthenium-catalysed enyne CM reaction came from our laboratories [106]. Reaction of various terminal alkynes 61 with terminal olefins 62 gave 1,3-substituted diene products 63 in good-to-excellent yields (Scheme 18). It is interesting that in these and all enyne CM reactions subsequently reported, terminal alkynes are more reactive than internal analogues, and 1,2-substituted diene products are never formed thus, in terms of reactivity and selectivity enyne CM is the antithesis of enyne RCM. The mechanism of enyne CM is not well understood. It would appear that initial attack is at the alkyne however, one report has demonstrated initial attack at the alkene (substrate-dependent) is also possible, see Ref. [107]. 

Recall the importance [reaction (o)] of this step for alkyne polymers formed by a metathesis mechanism. 

The mechanism of alkyne metathesis also consists of a series of [2 + 2] and [2 + 2] retro-cycloadditions. The key intermediate, a metallacyclobutadiene, appears to be antiaromatic, but apparently the metal uses a d orbital to form the W=C 77 bond so that the ring is aromatic. 

Metathesis of Alkynes. - Alkyne metathesis has been studied far less than has olefin metathesis. Mortreux and co-workers °° demonstrated that the metathesis of alkynes occurs at ambient temperatures with photochemical activation on a Mo(CO)6-3ClPhOH catalyst. The reaction proceeds via a two-step mechanism involving activation of the Mo(CO)6 by the alkyne and subsequent catalysis by the phenol. Heterogenization of the transition metal and... 

We first encountered alkyne metathesis in Chapter 10 in connection with reactions of metal-carbyne complexes. The mechanism of alkyne metathesis, first proposed by Katz,64 is analogous to that for alkenes, and it is shown in Scheme 11.9. 

By analogy to olefin metathesis, alkyne metathesis occurs between a complex containing a metal-carbon triple bond - a metal carbyne (or alkylidyne) - and an alkyne substrate [66]. As illustrated in Fig. 4.21, this mechanism parallels the Chauvin mechanism for olefin metathesis after alkyne coordination to the metal center, [2 - - 2] cycloaddition between the metal carbyne and the alkyne yields a metallacyclobutadiene, which rearranges and fragments productively to afford a new carbyne and a new alkyne (Fig. 4.21) [54]. 

The general equation and mechanism for alkyne metathesis is depicted in Scheme 31. Alkyne metathesis is considerably less well developed in comparison to alkene metathesis. Garbyne complexes or carbyne complex precursors are among the most effective alkyne metathesis catalysts representative catalysts are depicted in Scheme 32. Tungsten carbyne complex 276 is one of the earliest alkyne metathesis catalysts, and has frequently been employed to initiate... 

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. 

SCHEME 6.9 Mechanism of the formation of dialkyl-PPEs utilizing alkyne metathesis with a Schrock complex. 

Metallabenzenes have been invoked as possible intermediates in several other reaction types. Schrock, " for example, proposed tungstenabenzenes as possible intermediates in certain alkyne metathesis reactions that proceed by associative mechanisms. Shown in Scheme 32 is a proposed sequence for the metathesis of 3-heptyne to 3-hejQTie and 4-octyne using a tungstenacyclobutadiene complex as catalyst. The postulated metallabenzenes are formed by alkyne insertion into the metal carbon bonds of the metallacyclobutadienes. Of course, it is also possible to envisage a catalytic cycle based on Dewar metallabenzene intermediates. 

Scheme 6.18 (a) Alkyne metathesis reaction and (b) original proposed mechanism. 

Katz and coworkers [73] proposed the currently accepted mechanism for the alkyne metathesis reaction (Scheme 6.18b). In their proposal, the incoming alkyne reacts with a metal alkylidyne complex (56), generating a metallacyclobutadiene intermediate (57) that generates the expected products and a new alkylidyne species (58) through cycloreversion. The reaction of a second alkyne with the... 

Katz has proposed a carbenic Chauvin-type metathesis mechanism for the polymerization of alkynes. This mechanism involves two alkynes coordinated to in the catalytically active species. The ETC catalyzed ligand exchange process is adequate to rapidly introduce two or three alkyne ligands on in such a way that the Katz mechanism is then operating as shown on the following scheme. 

---