Alkylidyne complexes, reactions with alkyne

Molybdenum and tungsten carbyne (alkylidyne) complexes frequently undergo 2+2 cycloaddition reactions with alkynes to give the corresponding metallacyclobutadiene... 

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). 

Metal carbenes are also initiators for the metathetical polymerization of alkynes by, e. g., catalysts such as M0CI5 and WClg. Reaction of the metal carbene with an alkyne gives a metallacyclobutene intermediate, followed by opening of this intermediate metallacycle into a new metal carbene that in its turn can interact with another alkyne molecule, and so on eq. (13). Metathesis of acetylenes proceeds through reactions between an alkylidyne complex and an alkyne via a metallacyclobutadiene intermediate (eq. (14)). 

This mechanism was later confirmed experimentally in 1981 by Schrock and others, who reported the first example of alkyne metathesis by tungsten(vi)-alkylidyne complex. They have prepared tungsten alkylidyne complex 120 (Equation (21)) and found that it reacts with diphenylacetylende to give tungsten alkylidyne complex 121 and another alkyne 122 (lequiv.) (Equation (22)). Furthermore, complex 121 works as a catalyst for the alkyne metathesis reaction. 

The organic reactivity of the alkylidyne complexes has been extensively studied especially with regard to alkene and alkyne metathesis reactions.353,356... 

Several reactions of low-valent alkylidyne complexes with alkynes were reported. Treatment of the benzylidene complex 241 with base in the presence of alkynes gives the alkylidene alkyne complexes 242 [Eq. (183)]... 

Geoffroy and co-workers investigated the protonation of cyclopenta-dienyl dicarbonyltungsten alkylidyne complexes with HBF4 in the presence of alkynes (195,196). The outcome of the reactions depends on the nature of both the alkylidyne ligand and the alkyne. Protonation of complex 254 in the presence of diphenylacetylene affords the vinylcarbene complex 255... 

A route to alkylidynes containing metals with high oxidation states involves a metathesis exchange reaction that we have referred to briefly in earlier sections of Chapter 10. Scheme 10.10 shows the triply-bonded ditungsten complex 79 reacting with either alkynes or nitriles to give the corresponding metal carbyne (path a)90 or metal-carbyne plus nitride complex (path b).91 Scheme 10.10 also shows results of molecular orbital calculations at the DFT level.92 In the case... 

---