Conclusive evidence against a cyclobutadiene intermediate in the cyclotrimerization of diarylacetylene using the diarylacetylene hexacarbonyl dicobalt complex as catalyst

To decide whether the cyclotrimerization of diarylacetylene with diarylacetylene hexacarbonyl dicobalt as catalyst, proceeds via a cyclobutadiene intermediate, two experiments were performed, both relying on the experimentally verified hypothesis that a 3-methoxyphenyl ring can be considered as an ideally chemically labelled phenyl ring : (1) the co-cyclotrimerization of diphenylacetylene with bis(3-methoxyphenyl)acetylene, and (2) the cyclotrimerization of 1-(3-methoxyphenyl)-2-phenylacetylene. Analysis of the resulting meta methoxy substituted hexaphenylbenzenes using our chemical shift model, is for both experiments conclusive against the involvement of a cyclobutadiene intermediate.