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

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(NCCH )][OSO CF ] bearing an electron-donating or electron-withdrawing substituent R on the central aromatic ring of the POCOP ligand (R-POCOP = κ ,κ ,κ -2,6-(i-Pr PO) -4-R-C H ; 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 > OSO CF ), 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(NCCH[double bond, length as m-dash]CHR')][OSO CF ] (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(NH )][OSO CF ] (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.