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