Cobalt-stabilized propargyl cations

The scope of the intramolecular Pauson-Khand has been rapidly expanded as both more complex and heteroatom-containing substrates have been employed. Cycloadditions involving a cycloalkene reaction partner afford the direct construction of tricyclic systems in a single step. Triquinacene derivatives are efficiently obtained from 3-(3-butynyl)cyclopentenes [Eq. (55)]. An unusual characteristic of this system is the epimerization that occurs at the pro-pargylic position subsequent to cobalt complexation but prior to cyclization. The steric demands on the reaction are evidently so large that one stereoisomer is unable to cyclize and, instead, isomerizes through a cobalt-stabilized propargylic cation [123]. 

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)]. 

An additional advantage of the intramolecular protocol stems from the opportunity to prepare easily the required polyfunctional precursors via cobalt carbonyl stabilized propargyl cations. The approach based on the tandem utilization of Co-mediated alkylation and Pauson-Khand annulation was developed in Schreiber s studies to elaborate short pathways for the synthesis of polycyclic compounds. An example of the efficiency of this protocol is the two-step transformation of the acyclic precursor 409 into the tricyclic derivative 410. The cobalt-complexed acetal 409 was first transformed into the cyclooctyne derivative 411 via intramolecular reaction of the in situ generated propargyl cation 409a with the allylsilane moiety. Cyclooctyne 411 underwent smooth cycloaddition in the presence of carbon monoxide to give the target compound 410 with excellent stereoselectivity. 

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. ... 

The configuration of the acetylene group was inverted through an acetylene-cobalt complex prepared by treatment of the acetylene with Co2(CO)g (O Scheme 31) [52]. Upon treatment of the complex with acid, epimerization occurred via a propargylic cation intermediate stabilized by the cobalt complex to afford the thermodynamically more stable -C-glycoside as the... 

A general mechanism is illustrated below to demonstrate the stabilization of propargylic cation from the cobalt complex, in which a general alcohol is used as a representative nucleophile. 

An underused property of cobalt-coordinated alkynes is the stabilization of propargyllic cations. The Nicholas reaction is a propargylic substitution reaction facilitated by the ability of the adjacent cobalt complex to stabilize the propargylic cation, 67 to 68. Both carbon and heteroatom nucleophiles have been used to effect this transformation. " This transformation has been been used as a strategy to introduce the alkene component for an intramolecular PKR. Shea has probed the use of an... 

---