On the mechanism of Ni(ii)-promoted Michael-type hydroamination of acrylonitrile and its substituted derivatives

Abstract

Michael-type hydroamination of acrylonitrile and its substituted derivatives promoted by Ni(II) complexes is believed to proceed via an outer-sphere nucleophilic attack on the cationic adduct of the nitrile-coordinated substrate. As a test for the validity of this mechanistic postulate, we have sought to establish a correlation between the electrophilic character of the Ni(II) center and the degree to which it can activate the substrate toward nucleophilic attack by amines. This has been done by screening the catalytic activities of the cationic acetonitrile adducts [(R-POCOP)Ni(NCCH₃)][OSO₂CF₃] bearing an electron-donating or electron-withdrawing substituent R on the central aromatic ring of the POCOP ligand (R-POCOP = κ^P,κ^C,κ^P-2,6-(i-Pr₂PO)₂-4-R-C₆H₂; R = OMe (3), COOMe (4)). The catalytic activities for the addition of primary amines to crotonitrile, methacrylonitrile, and cinnamonitrile were found to depend on the precursor and the amine used, as well as on the reaction time. These studies were complemented by ligand exchange studies that established the relative binding order among the main components of a typical hydroamination mixture (RCN > amine > OSO₂CF₃), thus supporting the assertion that cationic nitrile adducts constitute the resting state in the catalytic manifold. We have also prepared and characterized the cationic acrylonitrile and cinnamonitrile adducts [(R-POCOP)Ni(NCCHCHR′)][OSO₂CF₃] (R′ = H: R = COOMe (7) or OMe (8); R′ = Ph: R = COOMe (9) or OMe (10)) as models of the postulated catalytic intermediates in the addition of amines to these substrates. To allow structural comparisons to the nitrile adducts, we have prepared and characterized the ammonia adducts [(R-POCOP)Ni(NH₃)][OSO₂CF₃] (R = H, 11, and COOMe, 12). The results of structural, spectroscopic, and reactivity studies carried out on these compounds and their implications for the mechanism of Michael-type hydroamination reactions promoted by the title system have been discussed.