CpCo(CO)2-catalysed cyclotrimerisation of alkynes in supercritical carbon dioxide

The reactivity of mono-substituted HCCR (R=Ph, a; CH2OH, b; CH2CH2CH2CH3, c) and di-substituted RCCR (R=CH2CH3, d; CO2CH3, e; Ph, f) acetylenes was studied in supercritical carbon dioxide (scCO2) using the easily available complex CpCo(CO)2 as catalyst. The reaction of phenylacetylene produced a mixture of the isomeric cyclotrimers 1,3,5- (2a) and 1,2,4-triphenylbenzene (2a′), in a 1:5 ratio, and traces of cobaltcyclopentadienone complexes CpCo(η4-C4H2[Ph]2CO) (6a, mixture of isomers). The possible product formed by the incorporation of CO2 to alkynes, i.e. diphenylpyrone (7a) was not observed. The reaction of the cobaltacyclopentadiene complex CpCo(1.4-σ-C4[Ph]4)(PPh)3 (8f), in scCO2, was performed. No insertion of CO2 into the CoC σ-bond to form tetraphenylpyrone (7f) by reductive elimination was observed, instead the cobaltcyclobutadiene complex CpCo(η4-C4[Ph]4) (9f) was formed. In the reactions with other alkynes, lower yields were obtained in general, except in the cyclotrimerisation of the highly activated alkyne, propargyl alcohol (b). Reaction of the non-activated alkynes, 1-hexyne (c) and 3-hexyne (d), produced complex mixtures of cobalt complexes in low yield in which the alkyne was coordinated to cobalt. Finally, the highly hindered diphenylacetylene (f) gave a mixture of the known complexes CpCo(η4-C4[Ph]4) (9f) and CpCo(η4-C4[Ph]4CO) (6f) in agreement with the results observed in conventional organic solvents.