Regioselectivity in the Pauson–Khand reaction of allenic hydrocarbons

The regioselectivity of the cobalt-mediated cocyclisation of alkynes with allenic hydrocarbons leading to 4- and 5-alkylidenecyclopentenones was shown to depend on the substitution pattern of both the acetylenic and allenic components. An initial pseudoaxial coordination of the allenic hydrocarbon may account for the formation of the 5-alkylidenecyclopentenone from acetylene.