Metathesis of acetylenes by well-defined metal carbyne initiators

2 Metathesis of acetylenes by well-defined metal carbyne initiators 

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

The ability of metal carbyne complexes of the type Mt(=CCMc3)(OR)3 to metathesize internal acetylenes by a chain mechanism depends on a delicate 

The rhenium carbyne complex Re(=CCMe3)(=NAr)[OCMe(CF3)2]2 is active for metathesis of internal acetylenes, but analogous complexes, where OR is OCMe2(Cp3), OCMe3, or OC6H3-/-Pr2-2,6, are not active the rhenium centre is insufficiently electrophilic to enable the acetylenic substrate to overcome the barrier to reaction (Schrock 1988d Weinstock 1991). 

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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