Regioselectivity in the Pauson–Khand reaction of allenic hydrocarbons
作者:Frédéric Antras、Mohammed Ahmar、Bernard Cazes
DOI:10.1016/s0040-4039(01)01749-x
日期:2001.11
The regioselectivity of the cobalt-mediated cocyclisation of alkynes with allenichydrocarbons 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 allenichydrocarbon may account for the formation of the 5-alkylidenecyclopentenone from acetylene.
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