Neutral and cationic chiral NCN pincer nickel(II) complexes with 1,3-bis(2'-imidazolinyl)benzenes: Synthesis and characterization

摘要

Chiral 1,3-bis(2'-imidazolinyl)benzenes 1a-e easily undergo direct nickelation at the C2 position of the central benzene ring via the C-H bond activation in the reaction with anhydrous NiCl_2 giving neutral NCN pincer nickel(ii) complexes 2a-e in 40-87% yields. Treatment of the nickel pincers 2a or 2c with AgBF_4 in CH_3CN-CH_2Cl _2 afforded the cationic nickel pincers 3a or 3c in good yields. All the complexes were characterized by elemental analysis, ~1H, ~(13)C NMR, and IR spectra. Molecular structures of the neutral complexes 2a, 2b and 2c as well as the cationic complex 3c have been determined by X-ray single-crystal diffraction. The cationic nickel pincers 3 are found to be effective catalysts for the Michael addition of ethyl 2-cyanopropionate to methyl vinyl ketone in the presence of i-Pr_2NEt base with a catalyst loading of 5 mol% even at -78 °C, producing the adduct in >99% yield after 24 h albeit with no ee.