Metallacyclobutane complexes reactions with alkynes

Whereas Fischer-type chromium carbenes react with alkenes, dienes, and alkynes to afford cyclopropanes, vinylcyclopropanes, and aromatic compounds, the iron Fischer-type carbene (47, e.g. R = Ph) reacts with alkenes and dienes to afford primarily coupled products (58) and (59) (Scheme 21). The mechanism proposed involves a [2 -F 2] cycloaddition of the alkene the carbene to form a metallacyclobutane see Metallacycle) (60). This intermediate undergoes jS-hydride elimination followed by reductive elimination to generate the coupled products. Carbenes (47) also react with alkynes under CO pressure (ca. 3.7 atm) to afford 6-ethoxy-o -pyrone complexes (61). The unstable metallacyclobutene (62) is produced by the reaction of (47) with 2-butyne in the absence of CO. Complex (62) decomposes to the pyrone complex (61). It has been suggested that the intermediate (62) is transformed into the vinylketene complex... 

In the same way, the reactions of metal carbene complexes with alkynes give metallacyclobutenes, which leads to applications in organic synthesis. Tebbe s methylene complex is in equilibrium with the metallacyclobutane by... 

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. 

Attempts to exploit the reaction of the dianion with alkyl halides to produce a c/.v-dialkyl complex by using 1,2- or 1,3-dihaloalkanes did not indeed give this result. The reaction of Ru(Por) " with 1,2-dibromoethane was sucessful, but the resulting metallacyclopropane product is better formulated as a /r-complex of ethene, and will be discussed below in the section on alkenc and alkyne complexes. The corresponding reaction of the diiinion with 1,3-dichloropropane gave no evidence for a metallacyclobutane. but instead free cyclopropane was detected by GC analysis and the porphyrin product was Ru(TTP)(THF)2. ... 

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