Fluoro and Trifluoromethyl Derivatives of POCOP-Type Pincer Complexes of Nickel:Preparation and Reactivities in S_N_2 Fluorination and Direct Benzylation of Unactivated Arenes

摘要

This report describes the synthesis, characterization, and reactivities of the pincer-type complexes κ~P,κ~C,κ~P- (R-POCOP)NiX (R-POCOP = 2,6-(i-Pr_2PO)_2,4-R-C_6H_3; R = H, X = F (2a), CF3 (3a); R = OMe, X = F (2b), X = CF3 (3b)). These complexes were prepared by treating their Ni-Br analogues with excess AgF at rt (2a and 2b) or excess AgF and Me_3SiCF_3 (3a and 3b) at 45 °C. The Ni-F derivative 2a was shown to be an intermediate for the formation of the Ni-CF_3 derivative 3a. Complexes 2a, 3a, and 3b were fully characterized by multinuclear NMR spectroscopy, cyclic voltammetry, and X-ray diffraction analysis. Variable temperature, multinuclear NMR analysis of 2a helped elucidate interaction of the fluoride ligand with hydrofluoric acid present in the reaction medium. The bromo complexes 1a or 1b catalyzed fluorination of benzyl halides at 90 °C in the presence of excess AgF, whereas the Ni-CF_3 complexes 3 failed to promote trifluoromethylation of benzyl bromide, leading instead to a direct benzylation of the aromatic reaction solvent (benzene, toluene, xylene, or bromobenzene). Possible mechanisms for this benzylation reaction have been probed and discussed.