Synthesis of Lithium N-pentafluorophenyl, N'-trimethylsilyl 2-Pyridylamidinate and Its Cyclization to Lithium Tetrafluoro-2-(2-pyridyl)benzimidazolate via a Me_3SiF Elimination. Coordination Chemistry, Reactivity, and Mechanism

摘要

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_úLi 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 Me_3SiF to quantitatively yield the tetrafluoro-2-(2-pyridyl)benzimidazolate lithium complex (6). From the reaction mixture of complex 4 the asymmetric N-trimethylsilyl, N'-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 κ~l-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.