Simple, convenient methods for the preparation of alkyne-Co<SUB>2</SUB>(CO)<SUB>6</SUB> complexes from CoBr<SUB>2 </SUB>for application in Pauson-Khand cyclopentenone synthesis

Alkyne-dicobalthexacarbonyl complexes are readily prepared by the reduction of cobalt bromide by Zn in the presence of alkynes in THF while bubbling carbon monoxide under ambient conditions. The complexes prepared in this way react with olefins to give the corresponding Pauson-Khand cyclopentenones in moderate to good yields. The alkyne-dicobalthexacarbonyl complexes can be also prepared in situ by the reduction of cobalt bromide with sodium naphthalene under CO atmosphere followed by CuBr treatment in the presence of alkynes. The complexes prepared in this way also give cyclopentenones on heating with norbornene, but the former method utilizing zinc gives better results