Metal carbyne complexes acetylenes

Rather surprisingly some metal carbyne complexes bring about the polymerization of acetylenes by a reaction of type (2) rather than their metathesis by a reaction... 

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)(OR)2 is active for metathesis of internal acetylenes when OR is OCMe(CF3)2, but not when OR is OCMe2CF3, OCMe3 or OC6H3-i-Pr2-2,65 6. The complex W(=CMe)(Cl)(PMe3)4 undergoes stoichiometric metathesis with PhC=CPh but the product PhC=CMe remains coordinated to the metal centre759. 

Since the Schrock carbyne is active in the transalkylidynation reaction, the possibility of ring-opening polymerisation of cycloalkynes by acetylene transalkylidynation catalysts (metal alkylidyne complexes) has been evaluated [151]. Unfortunately, cyclooctyne is one of the few relatively stable strained cyclic acetylenes, but it is not strained enough to react selectively with the catalyst and yield a polymer according to the scheme... 

The carbyne complexes [Mo(=CBu )(SAr)3] (SAr = TMT, TIPT) have been synthesized by adding 3 eq of Li[SAr] to [Mo(=CBu )Cl3(dme)] (dme = dimethoxyethane). The analogous W derivatives were made by a slightly modified route (32). The initial aim was to probe the acetylene metathesis catalytic properties of the complexes [M ( Bu KSArlg] (M = Mo, W SAr = TMT, TIPT). However, none of the complexes were active for metathesis, which was in contrast to the high activity of the analogous alkoxide compounds for metathesis. This was attributed to the stronger electron donation power of thiolate, which reduces the electrophilic nature of the metal center (32). 

The metallacyclobutadiene formation from acetylene and molybdenum carbyne complex CI3M0CH was studied [45] along a postulated least-motion pathway as shown in Fig. 12. The CI3M0CH was brought together with the acetylene molecule such that the three carbon atoms and the metal center remained coplanar throughout the course of the reaction. It has been found... 

The interaction of the carbyne ligand with the substituents on the acetylene is an important factor in determining whether reaction will occur or not. Thus reaction (13) occurs readily, whereas reaction (14) does not, while reactions (15) and (16), leading to complete metathesis of the substrate, proceed rather slowly at 25°C (McCullough 1985 Krouse 1987, 1989). As already discussed (Scheme 10.3), reaction (13) faces strong competition from a reaction leading to a metal carbene complex and subsequent polymerization of the monosubstituted acetylene. 

The preparation and study of metallacycles has been a subject of active investigation for organometallic chemists. We have just seen one example where metallacycle formation is a key step in a catalytic process and there are several others most notably, olefin metathesis. The metal acts as a geometrical and electronic template in these reactions. For unsaturated metallacycles there are interesting questions concerning delocalization [29]. Certain metal carbynes can react with acetylene to give metallacyclobutadienes as intermediates [30]. One such example of an insoluble molecule is the tungstenacyclobutadiene complex, 18.36 [31]. The compound is quite stable and not very reactive (in contrast to cyclobutadienes... 

Several groups have completed computational studies on the relative stabilities of osmium carbyne, carbene, and vinylidene species. DFT calculations on the relative thermodynamic stability of the possible products from the reaction of OsH3Cl(PTr3)2 with a vinyl ether CH2=CH(OR) showed that the carbyne was favored. Ab initio calculations indicate that the vinylidene complex [CpOs(=C=CHR)L]+ is more stable than the acetylide, CpOs(-C=CR)L, or acetylene, [CpOs() -HC=CR)L]+, complexes but it doesn t form from these complexes spontaneously. The unsaturated osmium center in [CpOsL]+ oxidatively adds terminal alkynes to give [CpOsH(-C=CR)L]+. Deprotonation of the metal followed by protonation of the acetylide ligand gives the vinylidene product. 

An important use of transition metal complexation has been in the stabilization of reactive molecules including cyclobutadiene, trimethylene-methane, o-xylylene, carbenes, carbynes, and others. In many cases, release of the ligand by chemical means has proven possible, making the complexed precursors valuable as shelf-stable sources of the reactive species for structural study or synthetic deployment. There also has been considerable interest in analogous complexes of benzyne, other strained cycloalkynes, and highly reactive acetylenes. 

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