Cobalt carbonyl-mediated carbocyclizations of enynes: generation of bicyclooctanones or monocyclic alkenes.

TLDR: Depending on the thermolytic conditions, dicobalthexacarbonyl-complexed enynes underwent cyclizations to provide different carbocyclic frameworks, which depicts another example of an interruption of the normal route in the Pauson-Khand reaction pathway.

Abstract: Depending on the thermolytic conditions, dicobalthexacarbonyl-complexed enynes underwent cyclizations to provide different carbocyclic frameworks. Bicyclopentanones were formed from enyne−Co2(CO)6 complexes, or from enynes that were treated with Co2(CO)8, or more effectively, with Co4(CO)12 in an alcoholic solvent under a H2 or N2 atmosphere. This transformation proceeded via a sequential cyclocarbonylation and 1,4-reduction and is the first account using the cobalt carbonyl cluster. Under these conditions a cobalt hydride was presumably generated, which mediated reduction of the enone to the saturated ketone. In contrast, thermolysis of dicobalthexacarbonyl-complexed enynes under a hydrogen atmosphere in toluene resulted in their reductive cyclization to form monocyclic alkenes in moderate yields, in addition to the bicyclopentenone product. In some cases, addition of a hydrosilane to the reaction induced a complete suppression of the bicyclopentenone formation. While the former results demonstrate a rea...