Alkylidene complexes, acetylene metathesis

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). 

Molybdenum imido alkylidene complexes have been prepared that contain bulky carboxylate ligands such as triphenylacetate [35]. Such species are isola-ble, perhaps in part because the carboxylate is bound to the metal in an r 2 fashion and the steric bulk prevents a carboxylate from bridging between metals. If carboxylates are counted as chelating three electron donors, and the linear imido ligand forms a pseudo triple bond to the metal, then bis(r 2-carboxylate) species are formally 18 electron complexes. They are poor catalysts for the metathesis of ordinary olefins, because the metal is electronically saturated unless one of the carboxylates slips to an ri1 coordination mode. However, they do react with terminal acetylenes of the propargylic type (see below). 

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

Alkyne metathesis is catalysed by alkylidene complexes of tungsten and molybdenum, but not by all alkylidene complexes of these metals. Complexes which do not catalyse metathesis catalyse polymerization of acetylenes to give polypropynes. 

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... 

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. 

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). 

Alkyhdene derivatives of titanium and of phosphorus catalyse methylene exchange between olefins. Although exchange of CH2 groups is not useful for synthesis, these systems provide insight into the mechanisms of alkylidene exchange, a basic step in conventional metathesis. Titanacyclobutenes have been isolated from reactions of acetylenes with methylene-titanium complexes but titanacyclobutanes, the assumed intermediate for the case of olefins, have not been isolated. Bis(cyclopentadiene)titanacyclohexane decomposes to produce ethylene as the major product apparently via a-C-C bond cleavage. ... 

Subsequently, other Ti-cyclobutane species were shown to be the reaction products of the Tebbe complex with olefins the analogous reaction with acetylenes gives metallacyclobutenes." Utilization of Ti-metaUacycles as initiators in metathesis provides the first example of a living metathesis polymerization system. Clear evidence of the intervention of metaUacarbenes and metallacyclobutanes in olefin metathesis was later furnished by Kress et al. through minute nuclear magnetic resonance (NMR) studies on norbomene polymerization with tungsten alkylidenes. 

Alkyne polymerization can be initiated by metal-alkylidenes or metal-alkylidyne complexes. The mechanism involves metallacyclobutene or metallacyclobutadiene key intermediates, in the same way as alkene metathesis and ROM involve metal-lacyclobutane intermediates Katz mechanism, bottom of this page, an extension of the hauvin mechanism for alkyne metathesis. For instance, Schrock s catalyst W - u O - u 3, shown in section. as catalyst of disynunetrical alkyne metathesis, also initiates the polymerization of acetylene and terminal alkynes rather than metathesizing them. Indeed, the molecules of terminal alkynes successively insert into the W- bond of the metallacyclobutadiene intermediate... 

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