Complexation-Initiated Intramolecular [4+2] Cycloaddition: Construction of Bridged-Type Cycloadducts

Dehydrotropylium-Co2(CO)6 ion: generation, reactivity and evaluation of cation stability

The dehydrotropylium-Co(2)(CO)(6) ion was generated by the action of HBF(4) or BF(3)⋅OEt(2) on the corresponding cycloheptadienynol complex, which in turn has been prepared in four steps from a known diacetoxycycloheptenyne complex. The reaction of the cycloheptadienynol complex via the dehydrotropylium-Co(2)(CO)(6) ion with several nucleophiles results in substitution reactions with reactive nucleophiles (N>1) under normal conditions, and a radical dimerisation reaction in the presence of less reactive nucleophiles. Competitive reactions of the cycloheptadienynol complex with an acyclic trienynol complex show no preference for generation of the dehydrotropylium-Co(2)(CO)(6) ion over an acyclic cation. DFT studies on the dehydrotropylium-Co(2)(CO)(6) ion, specifically evaluation of its harmonic oscillator model of aromaticity (HOMA) value (+0.95), its homodesmotic-reaction-based stabilisation energy (≈2.8 kcal mol(-1)) and its NICS(1) value (-2.9), taken together with the experimental studies suggest that the dehydrotropylium-Co(2)(CO)(6) ion is weakly aromatic.