Regioselective Hydroamination of Acrylonitrile Catalyzed by Cationic Pincer Complexes of Nickel(II)

Abstract

The cationic pincer-type complexes [{(i-Pr2PCH2CH2)2CH}Ni(NCCH3)][BPh4] (1), [{2,6-(i-Pr2PCH2)2-C6H3}Ni(NCCH3)][BPh4] (2), [{2,6-(i-Pr2PO)2-C6H3}Ni(NCCH3)][BPh4] (3), and [{2,6-(i-Pr2PO)2-3,5-Cl2-C6H}Ni(NCCH3)][BPh4] (6) have been prepared and fully characterized by NMR spectroscopy and X-ray crystallography. Cyclic voltammetry measurements of the Ni−Br precursors of 2, 3, and 6 indicated that substituting the CH2 moiety in the ligand skeleton by O, or some of the aromatic protons by Cl, renders the metal center less susceptible to oxidation. Evaluating the catalytic activities of 1−3, 6, and the t-Bu analogue of 1 for addition of aniline to acrylonitrile showed 3 to be the most competent catalyst precursor. Isolation of [{(t-Bu2PCH2CH2)2CH}Ni(NCCH2CH2NHPh)][BPh4] (7) from the reaction of [{(t-Bu2PCH2CH2)2CH}Ni(NCCH═CH2)][BPh4] with aniline suggests that these cationic precursors act as Lewis acids that bind the nitrile moiety of acrylonitrile, thereby activating the olefinic moiety toward nucleophilic attack by aniline.