Coupling and Dearomatization of Pyridines at a Transient η² -Cyclopropene/Bicyclobutane Zirconocene Complex

This paper reports on stereospecific coupling reactions between an η² -cyclopropene ligand and pyridine derivatives, which are preferred to alternative C-H bond activation reactions. The dicyclopropylzirconocene complex [Cp₂ Zr(c-C₃ H₅ )₂ ] (1) eliminates cyclopropane to generate the η² -cyclopropene/bicyclobutane intermediate [Cp₂ Zr(η² -c-C₃ H₄ )] (A). A does not activate any pyridine C-H bonds, but rather pyridine inserts into a Zr-C bond of A, yielding an azazirconacycle with a dearomatized pyridyl group [Cp₂ Zr{κ² -N,C⁸ -(2-c-C₃ H₄ )-C₅ H₅ N}] (2). Kinetic data, isotopelabelling experiments, and DFT calculations indicate that the rate-determining step of this stereospecific reaction is cyclopropane elimination, and that the stability of the intermediate [Cp₂ Zr(η² -c-C₃ H₄ )(NC₅ H₅ )] (A-py) governs the selectivity of the reaction. Complex 2 tautomerizes to [Cp₂ Zr{κ² -N,C⁸ -(2-C₃ H₅ )-C₅ H₄ N}] (6) through a base-catalyzed proton migration accompanied by cyclopropyl opening and restoration of conjugation within the zirconacycle.