Hexacarbonyldicobalt-stabilized propargyl cation

Hexacarbonyldicobalt-stabilized propargyl cation is captured by a nucleophile. Subsequent oxidative demetallation then gives propargylated product. 

The chemistry and synthetic utility of cobalt complexed propargyl cations have been demonstrated by Nicholas in an impressive series of papers, and the area was reviewed in 1987.72 More recently, reviews of cluster-stabilized cations73 and propargylium complexes74 have appeared. Two general routes for the synthesis of dicobalt-propargylium complexes have been developed. The most commonly used method is the treatment of an alkynic ether or alkynic alcohol-hexacarbonyldicobalt complex with a Lewis or Bronsted acid [Eq. (7)]. 

Cyclization-reairangement. Intramolecular capture of a propargyl cation, which is stabilized by hexacarbonyldicobalt complexation at the triple bond by the double bond of an ally lie alcohol, is followed by rearrangement to give a cyclic ketone. The tricyclic portion of ingenol is thereby created. The present organoaluminum catalyst appears to be the most efficient among those screened. 

Cobalt-Complexed Propargyl Cations and Their Reactions. The stability and synthetic utility of hexacarbonyldicobalt complexed cations was first reported in 1971. They are readily formed by protonation of propargyl alcohol complexes or addition of electrophiles (protons, or alkyl or acyl cations) to vinylacetylene complexes. They can be isolated in pure form, usually by precipitation as hexafluorophosphates or tetrafluoroborates, but are more commonly generated in situ and used directly. Most of the early work is due to Nicholas, who has comprehensively reviewed the work up to 1986. A detailed description of the procedure for generating the l-methyl-2-propynyl complex and for its reaction with trimethylsilyloxycyclohexene (eq 51) has been given. ... 

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