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

ABSTRACT

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.