Synthesis of lithium N-pentafluorophenyl. N′-trimethylsilyl 2-pyridylamidinate and its cyclization to lithium tetrafluoro-2-(2-pyridyl) benzimidazolate via a Me3SiF elimination. Coordination chemistry, reactivity, and mechanism

Abstract

When N-trimethylsilylpentafluoro aniline reacts with BuLi in the presence of THF or TMEDA, the corresponding THF (2) or TMEDA (4) lithium anilides are obtained. Complex 4 is monomeric in the solid state, with a distorted trigonal pyramidal coordination geometry at the lithium center. The strong C-F→Li interaction in this complex is also accompanied by the elongation of the C-F bond. Mild heating of complex 4 with 2-cyanopyridine results in rapid evolution of Me3SiF to quantitatively yield the tetrafluoro-2-(2-pyridyl)benzimidazolate lithium complex (6). From the reaction mixture of complex 4 the asymmetric N-trimethylsilyl, Af-pentafluorophenyl-2-pyridylamidinate lithium complex (5) was isolated as a dormant intermediate. Complex 5 possesses several unusual bonding features with the most notably being a very short Si-N bond length and a rare κ1-Z-syn amidinate bonding mode. A mechanism that includes silicon assisted C-F bond activation is proposed for the cyclization reaction based on the structural parameters of complexes 5 and 6, and on 1H, 19F, and 13C NMR studies. © 2009 American Chemical Society.