Application of cationic acetonitrile Nickel pincer complexes as catalysts in Michael-type hydroamination and hydroalkoxylation of nitriles

This report presents the results of a catalyst screening and mechanistic study on the aza-Michael addition reaction of acrylonitrile and its substituted derivatives catalyzed by a family of cationic POCOP-type pincer nickel complexes featuring different substitution patterns on the central aromatic ring of the type [(R-POCOPR′)Ni(NCMe)][OSO2CF3] (R-POCOPR′ = 2,6-(R′2PO)2(RnC6H3–n); R′ = i-Pr: R = H (1), p-Me (2), p-OMe (3), p-CO2Me (4), p-Br (5), m,m-t-Bu2 (6)) that were previously published by the Zargarian group in 2015.1 Whereas the impact of POCOP-substituents on the catalytic activities of the pre-catalysts is not clear-cut, our results show that the type of amine, alcohol, and nitrile substituent used has significant influence on the outcome of the catalytic reactions. Thus, the addition of aniline to acrylonitrile proceeds with mostly quantitative yields in only 2 h at 50 °C with all catalysts screened (turnover numbers, TON, of 100 or 200 with 1 mol% or less of catalyst). The catalysis is quite rapid at 50 °C (TON ~83 in 5 min), but much slower reactions are observed at r.t. (TON~ 20 in 4 h). Lower catalytic activities were also noted with anilines bearing a halide at the ortho position of the Ph ring: Yields = 71 % for F, ~64 % for Cl and Br, and 36 % for I. The corresponding activity with 2,5-Me2-aniline was 80 %. It should be noted that significant amounts of double-addition products RN(CH2CH2CN)2 are also generated from the addition to acrylonitrile by octylamine (yield = 34 %) as well as aniline and its substituted derivatives ArNH2 (Yield =3-16 %). In comparison to acrylonitrile, all other nitrile substrates proved less reactive. Thus, addition various amines to crotonitrile and methacrylonitrile proceeded with the following yields: aniline (21 % and 28 %), cyclohexylamine (39 % and 36 %), octylamine (34 % and 56 %), and ethanol amine (14 % and 18 %). Very small amounts of double-addition side-products are observed with crotonitrile and methacrylonitrile, but are still present. The least reactivity was observed with cinnamonitrile: among the amines tested, morpholine was the only one that showed some reactivity (22 % yield). Finally, addition of phenol and its substituted derivatives to acrylonitrile proceeded with much lower yields compared to amines: PhOH (16 %), 3,4,5-Me3-PhOH (20 %), 3-OMe-PhOH (14 %), and 3-CF3-PhOH (12 %). Addition of these alcohols to crotonitrile gave even lower activities (2-6 % yield), while methacrylonitrile and cinnamonitrile were completely inert. Several kinetic studies and ligand exchange tests have provided interesting insights on the mechanism of this aza-Michael addition (hydroamination) in our system. Monitoring the addition of aniline to acrylonitrile by 1H NMR under pseudo-first order conditions showed results consistent with a second order reaction, but there are some indications that more complex behavior takes place at different stages of the catalysis, after ca. 90% of the nitrile substrate has been consumed. Product inhibition is also believed to play a role in the kinetics as inferred from the competitive binding of the nitrile moiety in the addition product.

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