A two-pronged synthetic approach was designed to cultivate the ionic-ionic reactions in dicobalthexacarbonyl-complexed acetylenic hexads and radical-ionic reactions in the respective tetrads. Molecular templates such as [Co2(CO)6-μ,η2-(MeO)C(α’)H2C≡C[C(α)(OMe)CH2(CH2)0,1CH2CH(γ)=CH(β)] contained – by design – primary electrophilic centers (α’) and competing cycloalkenyl groups (C5,C6) both positioned alpha to the metal core. With a six-membered ring, carbon-carbon bond-forming reactions with various reagents such as methallyltrimethylsilane or 1,3,5-trimethoxybenzene were accompanied by a double bond migration, placing incoming nucleophiles into a remote, γ-position of the cyclohexenyl ring. A carbon-carbon bond can also be introduced into a 1° C(α’) position, although the latter, by its reactivity, is by far inferior to its competition, a 3° C(α) atom. Regioselectivity studies will be presented for five- and six-membered substrates in which C(α) is a part of the unsaturated carbocycle, and also for those containing cyclohexenyl and cyclopentenyl groups directly connected to the α-carbon atoms, either 2° or 3°. A second, complimentary strategy involved the generation of the cobalt-stabilized cations in unsymmetrical acetylenic tetrads wherein α,α‘-(OMe)2 diesters were replaced with methoxy-substituted propargyl alcohols, i.e., [Co2(CO)6-μ,η2-(MeOC(α’)H2C≡CC(α)HPh(OH)]. An HBF4-induced elimination at the alpha position revealed the formation of the ether-soluble, colorless cobalt-complexed propargyl cations, a new type of cationic intermediates, supposedly of a dimeric nature. A head-to-tail coordination of the cobalt-complexed propargyl cations is allegedly enabled by the Lewis base located in α’-position, and further facilitated by a bent structure of the π-coordinated alkyne unit. A soluble propargyl cation regains its dark-red color in the open air yielding starting alcohols; it could also be precipitated from an ethereal solution by introducing less polar solvents, such as pentane. The isolated propargyl cations were reduced with cobaltocene, or zinc, forming radical α,α-dimer with pending α’,α’ termini. The latter – with BF3 assistance – reacted with nucleophiles such as 1,3,5-trimethoxybenzene, forming twin C-C bonds at the periphery of the molecular assembly. The first case of the radical-ionic reactions will be presented, along with the mechanism of the reaction and structural characterization of the novel reactive intermediates.

Cobalt-complexed acetylenic tetrads and hexads: Ionic-ionic and radical-ionic designs

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