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

Abstract

The reactivity of mono-substituted HCCR (R=Ph, a; CH 2OH, b; CH 2CH 2CH 2CH 3, c) and di-substituted RCCR (R=CH 2CH 3, d; CO 2CH 3, e; Ph, f) acetylenes was studied in supercritical carbon dioxide (scCO 2) 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(η 4-C 4H 2[Ph] 2CO) ( 6a, mixture of isomers). The possible product formed by the incorporation of CO 2 to alkynes, i.e. diphenylpyrone ( 7a) was not observed. The reaction of the cobaltacyclopentadiene complex CpCo(1.4-σ-C 4[Ph] 4)(PPh) 3 ( 8f), in scCO 2, was performed. No insertion of CO 2 into the CoC σ-bond to form tetraphenylpyrone ( 7f) by reductive elimination was observed, instead the cobaltcyclobutadiene complex CpCo(η 4-C 4[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(η 4-C 4[Ph] 4) ( 9f) and CpCo(η 4-C 4[Ph] 4CO) ( 6f) in agreement with the results observed in conventional organic solvents.