Pauson-Khand Reaction of Allenic Hydrocarbons: Synthesis of 4-Alkylidenecyclopentenones

The carbonyldicobalt-mediated alkyne/allene/CO cocyclization gives 4-alkylidenecyclopentenones as the major [2+2+1] cycloadducts. The regio- and stereoselectivities depend mainly on the substitution pattern of both the alkyne and the allenic moieties, which can be rationalized using the Magnus mechanism. However, contrary to this model, and in agreement with more recent mechanistic studies, our results provide evidence that both initial pseudo-equatorial and pseudo-axial coordination modes of the allenic hydrocarbons onto one of the cobalt atoms of the primary alkyne dicobalt complex are involved. DFT calculations supporting both these coordination modes are given.