Acetylene metathesis

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. 

On the basis of the fact that tungsten(VI) alkylidene complexes will metathesize olefins one might predict that acetylenes should be metathesized by tungsten(VI) alkylidyne complexes (29). Acetylene metathesis is not unknown, but the catalysts are inefficient and poorly understood (30, 31). 

CH3OC//2CH2OCH3), S 1.28 (s, 9H, CC(CH3)3). The complex Cl3(dme)W= CC(CH3)3 is an important intermediate for preparing olefin and acetylene metathesis catalysts.3,4 It is also a useful starting material for preparing certain tungstenacyclobutadiene, /3-cyclopropenyl, and 5-cyclopentadienyl... 

This reaction is formally analogous to organic acetylene metathesis. 

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). 

Leigh, G. J., Rahman, M. T., Walton, D. R. M. Carbon-carbon triple bond fission in the homogeneous catalysis of acetylene metathesis. J. Chem. Soc., Chem. Commun. 1982, 541-542. 

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

Out of the two groups (fluoro and chloro) of title compounds so far studied, the chemistry of fluoroalkoxides (24, 160, 303-305) and fluoro-/3-diketonates (146, 306-309) has recently attracted considerable attention, mainly due to their enhanced volatility compared with the unfluorinated analogues. Another interesting feature of fluoroalkoxide derivatives in their role as an effective alkene and acetylene metathesis (160). 

Figure 10-8 Acetylene metathesis polymerization by an alkylidene complex.

Figure 10-10 Polymerization of 1,6-diynes using a molybdenum alkylidene catalyst [Rp is (CFjljCHjC] [67]. The 1,6-diyne monomer is drawn in two different exaggerated conformations to illustrate that head-tail polymerization leads to six-membered rings, and tail-tail polymerization leads to five-membered rings. See Fig. 10-8 for a more mechanistic diagram of acetylene metathesis.

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)...

Scheme 10.1 Propagation of acetylene metathesis by metal carbyne and metallacyclo-...

Scheme 10.4 Propagation of acetylene metathesis by a metal carbene chain carrier, facilitated by proton transfer.

Compounds of type 7 proved to be remarkably active catalysts for the metathesis of internal olefins. [44,68,69] The activity of such species for the metathesis of ordinary internal olefins (e.g., c 5 -2-pentene) appeared to maximize for the OCMe(CF3)2 species. New alkylidene complexes such as W(NAr)(CHPh)[OCMe(CF3)2]2 could be isolated, and in some cases trigonal bipyramidal (TBP) tungstacyclobutane intermediates were stable enough to be observed and isolated. On the basis of this work it was proposed that the rate of reaction of alkylidene complexes with olefins correlated directly with the electron-withdrawing ability of the alkoxide, as found in acetylene metathesis systems described earlier. In many circumstances trigonal bipyramidal or square pyramidal tungstacyclobutane intermediates could be observed. [44] In any system in which ethylene could be formed, unsubstituted metallacycles could... 

Zhang, X.-P. and Bazan, G.C. (1994) Regiospecific head-to-tail ring-opening acetylene metathesis polymerization of tetrasilacycloocta-3,7-diynes. Macromolecules, 27, 4627-4628. 

The electronic structure of [MoCl3(T 3-cyclopiopenyl)] complexes was studied using nonlocal density functional theory as part of a study of acetylene metathesis catalysed by high-oxidation state molybdenum complexes3. ... 

Schrock and his co-workers have prepared several tungsten(vi) neopentylidyne complexes, e.g. (174), which react with diphenylacetylene to give the product of acetylene metathesis (175). More importantly, the complexes catalytically metathesize many acetylenes with remarkable ease. ... 

---