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 w as studied in supercritical carbon dioxide (scCO(2)) using the easily avail able complex CpCo(CO)(2) as catalyst. The reaction of phenylacetylene produ ced a mixture of the isomeric cyclotrimers 1,3,5- (2a) and 1.2,4-triphenylb enzene (2a '). in a 1:5 ratio, and traces of cobaltcyclopentadienone comple xes CPCO(eta (4)-C4H2[Ph](2)CO) (6a, mixture of isomers). The possible prod uct formed by the incorporation of CO, to alkynes, i.e. diphenylpyrone (7a) was not observed. The reaction of the cobaltacyclopentadiene complex CpCo( 1.4-sigma -C-4[Ph](4))(PPh)(3) (8f), in scCO(2), was performed. No insertio n of CO2 into the Co-C a-bond to form tetraphenylpyrone (7f) by reductive e limination was observed, instead the cobaItcyclobutadiene Complex CpCo(eta (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 h ighly activated alkyne, propargyl alcohol (b). Reaction of the non-activate d alkynes, 1-hexyne (c) and 3-hexyne (d), produced complex mixtures of coba lt complexes in low yield in which the alkyne was coordinated to cobalt. Fi nally, the highly hindered diphenylacetylene (f) gave a mixture of the know n Complexes CpCo(eta (4)-C-4[Ph](4)) (9f) and CpCo(eta (4)- C-4[Ph](4)CO) ( 6f) in agreement with the results observed in conventional organic solvents . (C) 2001 Elsevier Science B.V, All rights reserved.