Metathesis of alkynes

A few publications have appeared concerning the metathesis of alkynes so far only heterogeneous systems with acyclic alkynes have been reported (31-33). From experiments with [l-14C]2-hexyne this reaction was found to be analogous to the metathesis of alkenes, because it turned out to be a transalkylidynation reaction (33) ... 

Moulijn et al. (33) studied the reactions of some linear alkynes over a W08-Si02 catalyst in a fixed-bed flow reactor. Besides metathesis, cyclotrimerization to benzene derivatives occurred. Thus, propyne yielded, in addition to metathesis products, a mixture of trimethylbenzenes. From this an indication of the mechanism of the metathesis of alkynes can be obtained. 

Since cyclotrimerization and metathesis of alkynes occur simultaneously, a common intermediate might be involved, which would mean that the metathesis of alkynes does not proceed via a cyclobutadiene complex. 

Alkylidyne complexes can be used as catalysts for the metathesis of alkynes. For a classic review see Schrock [18],... 

Homogeneous, single-component catalysts such as, e.g., W(=CCMe3)(OCMe3)3 or W(=CMe)(OCMe2CF3)3, cannot only be used for exchange metathesis of alkynes but also for ROMP of cycloalkynes, ADMET of a,to-diynes, and RCM of a,co-diynes [751]. 

Metathesis of alkynes is a much less general reaction, and only a few catalysts were found to be active. Terminal alkynes characteristically undergo cyclotrimeri-zation to yield benzene derivatives in the presence of most metathesis catalysts7,8... 

It is generally accepted that ring-opening polymerization of cycloalkenes proceeds via metallocarbenes. Although less extensively studied, metathesis of alkynes... 

Metathesis of alkynes is a much less general reaction and has not yet found the attention that alkene metathesis has received.158-160 Nonetheless, a few observations have shown that it has the potential of useful applications emerging as a new synthetic tool.214-216... 

By analogy with alkene metathesis, carbyne complexes might be expected to mediate the metathesis of alkynes, which indeed they do, but with some specific limitations. The basic mechanism parallels that for alkene metathesis, with the key intermediate being a metallacyclobutadiene which may break down in one of two possible directions (Figure 5.44). 

The metathesis of alkynes by metal alkylidyne complexes is now a well-established process. The reaction is useful for the catalytic metathesis of alkynes as well as for the synthesis of new metal alkylidyne complexes. The general electronic and steric factors favoring the metathesis reaction were discussed in recent reviews by Schrock (6,7). 

Mortreux, A., Blanchard, M. Metathesis of alkynes by a molybdenum hexacarbonyl-resorcinol catalyst. J. Chem. Soc., Chem. Commun. 1974, 786-787. 

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... 

The tris[Al-aryl-iV-(t-butyl)amino]molybdenum(0) complexes (3) are good catalyst precursors readily activated in situ for metathesis of alkynes and diynes. ... 

Technology for a number of applications of olefin metathesis has been developed (, fO At Phillips, potential processes for producing isoamylenes for polyisoprene synthesis and long-chain linear olefins from propylene have been through pilot plant development. In the area of specialty petrochemicals, potential industrial applications include the preparation of numerous olefins and diolefins. High selectivities can be achieved by selection of catalyst and process conditions. The development of new classes of catalysts allows the metathesis of certain functional olefins (, 14). The metathesis of alkynes is also feasible (15) ... 

M. (1980) Improving selectivity and G experiments during metathesis of alkynes on M0O3—Si02 catalysts. J. Mol. Catal, 8, 97-106. 

Metathesis of alkynes proceeds according to a mechanism (13.177) analogous to that already described for olefins. The formation of catalytically active metal-lacyclobutadiene complexes was confirmed by both kinetic and isotopic studies of... 

Carbyne (alkylidyne) complexes are implicated in the metathesis of alkynes (p. 377). 

In accordance with this alkylidene—metallacyclobutane mechanism, metathesis of alkynes has been performed using tungsten metallacarbyne complexes [36]. The analogy between the reactions of olefins and acetylenic hydrocarbons has led to the assumption that metallacyclobuta-dienes could be intermediates for this reaction. 

Cross-metathesis of Alkyne with Cycloalkene (ROM-CM-RCM Sequence) A cascade metathesis with cycloalkene compounds is an interesting reaction because the different ring-size cyclic diene is formed from the starting cycloalkene via multiple processes in... 

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