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.

The Reactivity of Mercapto Groups against Boron Hydrides in Pincer Ligated Nickel Mercapto Complexes

“…Similar 1 H NMR resonances and H‐P coupling constants were previously reported for other transition metal mercapto complexes ,. Generally, a tert ‐butyl group is more electron‐donating than an isopropyl group and a PCP pincer ligand is more electron rich than the corresponding POCOP pincer ligand ,,. It is supposed that the chemical shifts of the SH protons are influenced by the electronic nature of the pincer backbone.…”

Charge Effects in PCP Pincer Complexes of NiII bearing Phosphinite and Imidazol(i)ophosphine Coordinating Jaws: From Synthesis to Catalysis through Bonding Analysis

“…In the search for a mechanism adapted for the Ni‐catalyzed hydroamination of nitriles under discussion, we selected as guiding concepts the following known reactivities of Ni II species with nitriles and amines: i) Nitriles coordinate very readily to the Ni II center in cationic species; ii) cationic adducts of the type [(pincer)Ni(NCMe)] + promote Michael‐type additions of amines to N ‐coordinated acrylonitrile derivatives 13b,39. Therefore, an outer‐sphere mechanism similar to that proposed by Forsberg et al.…”

Transition‐Metal‐Catalyzed Hydroamination Reactions

“…Stoichiometric reactions in this case suggest that a Lewis acid mechanism promoting the anti-Markovnikov hydroamination is active (Figure 15.7) [258].…”

“…Cationic Ni(II)-pincer complexes have also been exploited for the hydroamination of acrylonitrile with aniline [258]. Stoichiometric reactions in this case suggest that a Lewis acid mechanism promoting the anti-Markovnikov hydroamination is active (Figure 15.7) [258].…”