Cobalt-mediated pentadienyl/alkyne [5+2] cycloaddition reactions

A new method for the preparation of seven-membered carbocycles via cobalt-mediated [5+2] cycloaddition methodology is presented. We have demonstrated that Cp*Co(eta 5-pentadienyl)+ systems undergo cycloaddition reactions with alkynes in a diastereocontrolled and high-yielding process. When acetylene is employed as the cycloaddition partner, unprecedented Cp*Co(eta 2,eta3-cycloheptadienyl)+ complexes were isolated as the cycloaddition product under kinetic control. These allyl/olefin species were further transformed to the thermodynamic Cp*Co(eta5-cycloheptadienyl)+ complexes. Also described are two methods for the preparation of high-valent Co(III) eta5-pentadienyl complexes, a compound class that has been under-reported in the literature. This work fills this void and provides a valuable view of the structural properties of eta5-pentadienyl complexes as a function of the substitution pattern.;The mechanism and structure/reactivity relationships for the [5+2] cycloaddition process were studied via density functional theory calculations. These investigations revealed the existence of several convergent reaction pathways on the potential energy surface, and provided a new rationale for the eta2,eta 3→eta5 isomerization, thereby explaining the low activation barrier for the isomerization of 2-butyne cycloadducts. Of interest is the elucidation of a radical-type pathway, calculated to be of high energy for the Cp* ligand system, yet energetically competitive in the Cp complex reaction manifold. Further, computations on the Cp system demonstrate a potentially viable pathway on the triplet energy surface, suggesting spin-forbidden transitions may play a role in the mechanism. These observations provide an explanation for the differing cycloaddition efficiencies in these two pentadienyl systems. Calculations also suggest that reaction chemoselectivity is determined during the rate-limiting alkyne complexation step; the energetics of this process being dominated by steric interactions between the pentadienyl substituents and the ancillary ligand.;Finally, initial synthetic development of a new six-electron anionic ligand template is described. Both synthetic and theoretical investigations provide insight into the ligand design and function and suggest viable new avenues of study.;The incorporation of tethered pronucleophiles onto the pentadienyl ligand allowed the preparation of fused bicyclic structures of relevance to natural product synthesis. Both conjugated and unconjugated cycloheptadiene species were prepared, made possible via the differing cycloheptadienyl complex hapticity. The oxidative decomplexation of the organic products is also described. Initial steps towards a divergent pronucleophile-bearing pentadienyl synthesis were also undertaken.