Closed-shell and open-shell ferracyclobutadienes-syntheses, characterization and reactivity studies

Metallacyclobutadienes (MCBs) are an important class of organometallic compounds, especially known for their role as reactive intermediates in alkyne metathesis. The first heterogeneous alkyne metathesis catalyst consisting of WO3 on silica (6.8% w/w) was reported by Banks and Bailey in 1968. This disclosure was followed by the work of Mortreux and Blanchard who found that the alkyne metathesis can be also effected by a homogenous mixture of Mo(CO)6 and resorcinol in an inert solvent at 160°C. Although variants of this catalyst have been successfully used in the synthesis of natural products, semiconducting polymers and macrocyclic compounds, these catalysts do not give any clues about the nature of the catalytically active species. In 1975, one year after Mortreux`s seminal discovery, the mechanism of alkyne metathesis was proposed by Katz and McGinnis that follows the logic of Chauvin´s mechanistic proposal for olefin metathesis.[15] According to this proposal, a reactive species having a metal-carbon triple bond (M=C-R2) as the operative unit combines with a alkyne (R1-C=C-R1) in a formal [2+2] cycloaddition to give a metallacyclobutadiene. Cycloreversion of this intermediate from the other resonance extreme releases the metathesized alkyne and regenerates a competent catalyst entity (M=C-R1), which then undergoes the same steps to complete the turnover.