Regioselective C–F Bond Activation/C–C Bond Formation between Fluoropyridines and Cyclopropyl Groups at Zirconium

This paper addresses the problem
of the strong and inert C–F
bond activation of various fluoropyridines by zirconocene derivatives.
Dicyclopropylzirconocene, [Cp2Zr­(c-C3H5)2], thermally eliminates cyclopropane,
giving the transient intermediate zirconabicyclobutane [Cp2Zr­(η2-c-C3H4)] that cleaves a C–F bond of pentafluoropyridine selectively
at position 2, forming new Zr–F and C–C bonds stereoselectively
to give [Cp2ZrF­{c-cis-CHCH2CH­(2-NC5F4)}]. DFT computations
indicate the selectivity results from the initial formation of an
azazirconacycle intermediate that undergoes ring opening and C–F
bond cleavage. Transmetalation with a variety of cyclopropyl donors
yields [Cp2Zr­(c-C3H5)­{c-cis-CHCH2CH­(2-NC5F4)}] with the selectivity depending on the nature
of the donor. A synthetic cycle is realized when [Cp2Zr­(c-C3H5)­{c-cis-CHCH2CH­(2-NC5F4)}]
reacts with pentafluoropyridine, yielding 2-(c-C3H5)­NC5F4 and [Cp2ZrF­{c-cis-CHCH2CH­(2-NC5F4)}] with C–F bond activation. Attempts
at converting this reaction sequence to a catalytic version failed
due to either decomposition of the active species or multiple C–F
bond substitutions by the transmetalating agent.