Acetylene metathesis reaction

More recently, Schrock has reported the formation of coordinatively unsaturated Ta and W carbyne complexes (124). Like unsaturated carbene complexes, these carbyne compounds are now established as being active intermediates in a number of catalytic reactions. The discovery of acetylene metathesis reactions catalyzed by carbyne complexes (3), for example, has generated considerable interest in this class of compound. 

While dealing with the role of the alkoxide ligand in alkene and acetylene metathesis reactions, Schrock (155) emphasized that a combination of a (as... 

This reaction is formally analogous to organic acetylene metathesis. 

Acetylenic monomers also appeared to undergo polymerisation with conventional olefin metathesis catalysts. This relates to monosubstituted highly branched alkylacetylenes and arylacetylenes as well as disubstituted acetylenes (internal alkynes) [16-18], It has been demonstrated that acetylene itself may also be polymerised using olefin metathesis catalysts [19,20]. The polymerisation of alkynes [scheme (2)] involves a metathesis reaction [scheme (5) of Chapter 2] analogously to that of cycloolefins [21] ... 

Perfluoroalkynylcopper reagents can be prepared by metathesis reactions with the zinc or cadmium reagent (e.g., formation of 6) or by treatment of terminal (perfluoroalkyl)acetylenes with copper(I) chloride/ammonium hydroxide. Perfluoroalkynylcopper reagents react with... 

Addition of pyridine and acetylene results in generation and trapping of the complex shown in equation (87), which has a reduced bond order. In this complex, the acetylene bridges the two metals. This type of chemistry was initially developed by Schrock and coworkers for alkyne metathesis (see Alkyne Metathesis) reactions but has been subsequently developed by a number of other researchers. 

The reaction of the rhenium alkylidyne complex 277 with diisopropyl-acetylene and with diethylacetylene [Eq. (196)] demonstrates the sensitivity of metathesis reactions toward steric factors (57). With diisopropylace-tylene an alkylidyne complex is obtained whereas the reaction with diethylacetylene gives a metallacyclobutadiene. In the metathesis reactions the alkyne with the bulkiest groups cleaves most easily from intermediate metallacyclobutadiene complexes. The rhenacyclobutadienes with the smallest substituents thus become sinks and slow down the effective rate of metathesis. The alkylidyne alkylidene rhenium complex 278 is an active olefin metathesis catalyst (52). Reaction with hexene transforms the neo-pentylidene group into a propylidene group as shown in Eq. (197). 

The reaction of 17 with 18 is essentially irreversible, but the reaction of 17 with acyclic olefins is reversible and leads to the expected metathesis reactions, for example, the cis/trans isomerization of HDC=CMePh (Lee, J.B. 1982). Many isotopic labelling and kinetic experiments have been carried out in an attempt to discover whether a titanium-carbene-olefin complex plays a significant kinetic role in these reactions. The general conclusion is that this is unlikely and it is thought that complete dissociation to Ti(=CH2)Cp2 must occur before reaction takes place with an olefin or acetylene (Gilliom 1986a Anslyn 1987 Hawkins 1988, see ref 4 therein). If such a complex does have a finite existence, it probably corresponds only to a very shallow minimum in the energy profile for the reaction. Stereochemical evidence for this conclusion comes from a study of the isomerization reaction (17). 

Table 4.1 Some types of observed [2 + 2] reactions involving metal atoms attached to multiple bonds (other than those involved in olefin or acetylene metathesis) cf. eqn. (1)...

The simple one-step metathesis, reaction (7), between a metal carbyne complex and a symmetrical disubstituted acetylene was first observed by Wengrovius (1981). 

Some examples of single-step metathesis reactions of acetylenes have already been mentioned. This type of reaction can also occur with binuclear complexes eqns. (10-12) (Schrock 1982 Chisholm 1984a,b, 1989 Strutz 1984 Listemann 1985 Latham 1986 Koutsantonis 1991). Reaction (10) proceeds via a fi-C2H2 complex. Reaction (12), in the absence of quinuclidine (quin), gives only [Mo]=CPh. 

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