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

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.

本研究成功合成了四种阳离子钳形配合物：[{(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) 以及[{2,6-(i-Pr₂PO)₂-3,5-Cl₂-C₆H}Ni(NCCH₃)][BPh₄] (6)，并通过核磁共振波谱法与单晶X射线衍射完成了全面表征。对配合物2、3、6的镍-溴前驱体开展循环伏安测试后发现：将配体骨架中的CH₂基团替换为氧原子，或是将芳环上的部分质子替换为氯原子，可使金属中心更难被氧化。针对配合物1~3、6以及1的叔丁基类似物，开展苯胺与丙烯腈加成反应的催化活性评价，结果显示配合物3为性能最优的催化前驱体。从[{(t-Bu₂PCH₂CH₂)₂CH}Ni(NCCH=CH₂)][BPh₄]与苯胺的反应体系中分离得到了[{(t-Bu₂PCH₂CH₂)₂CH}Ni(NCCH₂CH₂NHPh)][BPh₄] (7)，该结果表明这类阳离子前驱体可作为路易斯酸，结合丙烯腈的腈基官能团，从而活化烯烃官能团以接受苯胺的亲核进攻。