Cobalt-Mediated η5-Pentadienyl/Alkyne [5 + 2] Cycloaddition Reactions: Substitution Effects, Bicyclic Synthesis, and Photochemical η4-Cycloheptadiene Demetalation

Abstract

The preparation of seven-membered carbocycles via traditional organic synthesis is difficult, yet essential, due to the prevalence of these moieties in bioactive compounds. As we report, the Co-mediated pentadienyl/alkyne [5 + 2] cycloaddition reaction generates kinetically stable η2,η3-cycloheptadienyl complexes in high yield at room temperature, which isomerize to the thermodynamically preferred η5-cycloheptadienyl complexes upon heating at 60–70 °C. Here we describe an extended investigation of this reaction manifold, exploring substituent effects and extending the reaction to tandem cycloaddition/nucleophilic cyclizations, generating fused bicyclic compounds. We also describe a new high-yielding photolytic method for the decomplexation of organic cycloheptadienes from Co(I) complexes. Both C5Me5 (Cp*) and C5H5 (Cp) half-sandwich complexes are active in [5 + 2] cycloaddition with alkynes, with Cp* generally providing higher yields of cycloheptadienyl complexes. Cp cycloheptadienyl complexes, however, are resistant to thermal η2,η3 → η5 isomerization. The reaction remains limited to open pentadienyl complexes incorporating substituents in the terminal (1 and 5) positions, except for the unsubstituted CpCo(η5-cycloheptadienyl)+ complex, which is modestly reactive. Incorporation of tethered latent nucleophiles allows cyclization onto the intermediate cycloheptadienyl cations, producing bicyclo[5.3.0]decadiene and bicyclo[5.4.0]undecadiene systems with complete diastereocontrol. A selection of intermediate complexes have been crystallographically characterized. Addition of tethered malonate nucleophiles occurs reversibly with equilibration to a thermodynamic elimination product, while enolate nucleophiles cyclize reliably under kinetic control. The resulting bicyclic products are decomplexed in high (>90%) yield by UV photolysis in the presence of allyl bromide to provide the organic bicyclic diene with complete retention of ring fusion geometry and without double-bond isomerization.