Cobalt-Mediated η5-Pentadienyl/Alkyne [5 + 2] Cycloaddition. Synthesis and Characterization of Unbridged η2,η3-Coordinated Cycloheptadienyl Complexes

Abstract

A general new metal-mediated [5 + 2] cycloaddition reaction of η5-pentadienyl cobalt complexes and alkynes is reported, the first such cycloaddition reaction to provide controlled incorporation of 1 equiv of alkyne. The reaction of terminally substituted pentadienyl complexes proceeds thermally under exceptionally mild conditions, affording cycloheptadienyl complexes cleanly and in high yield. As a consequence of the unusually low kinetic barrier to the cycloaddition, reactions with acetylene at low temperature produce the cycloheptadienyl ring system exclusively as the η2,η3-isomer, an unprecedented coordination mode for unbridged seven-membered ring complexes. Isomerization of the kinetic product to the fully conjugated η5-cycloheptadienyl isomer is observed quantitatively upon heating. Disubstituted alkynes, as represented by 2-butyne, proceed through the η2,η3-intermediate to the thermodynamic η5-complex, but the rate of initial alkyne incorporation is attenuated to the extent that isomerization is competitive with the cycloaddition, preventing isolation of the η2,η3-intermediates. The reaction with terminal alkynes is relatively insensitive to modest steric or electronic influences, providing mixtures of regioisomeric products. Consistent with these results, a dissociative mechanism is proposed, initiated by the unusually facile η5→η3 isomerization of the pentadienyl ligand. The pentadienyl complexes themselves arise from protonolysis of an organic 1,4-pentadien-3-ol in the presence of the labile Co(I) complex, (η5-pentamethylcyclopentadienyl)cobalt(ethylene)2, suggesting that the reaction can be considered a novel “interrupted Nazarov” cyclization, amenable to further development in an explicitly organic context.