Protonolysis of a toluidinoalkyl platinum(ii) complex derived from the insertion of the CC bond into the Pt–NHR (amido) bond: the role of amine in Pt-catalyzed hydroamination of acrylonitrile

Abstract

The complex Pt{2,6-(R₂PCH₂)₂C₆H₃}(OTf) [R = Ph (1a), Cy (1b)] catalyzes the hydroamination of acrylonitrile with p-toluidine to produce CH₂(CN)CH₂NH(Tol-p), exclusively. In the catalyzed reactions, platinum intermediates were detected by NMR spectroscopy. The p-tolylamido platinum complex Pt{2,6-(Ph₂PCH₂)₂C₆H₃}{NH(Tol-p)} (4), containing the pincer ligand, was synthesized from the reaction of 1a and NaNH(Tol-p). Complex 4 reacted with acrylonitrile to yield the regiospecific insertion product Pt{2,6-(Ph₂PCH₂)₂C₆H₃}{CH(CN)CH₂NH(Tol-p)} (5), quantitatively. Reaction of 5 with HX (X = Cl, OTf) generated free acrylonitrile, p-toluidine and Pt{2,6-(Ph₂PCH₂)₂C₆H₃}X. Reacting 5 with a proton source having a non-coordinating counter anion, [NH₃(Tol-p)]BPh₄, also produced free acrylonitrile along with a cationic amine complex [Pt{2,6-(Ph₂PCH₂)₂C₆H₃}{NH₂(Tol-p)}]⁺. On the other hand, reaction of 5 with [NH₃(Tol-p)]BPh₄ in the presence of excess p-toluidine (ca. 30 equiv.) generated the hydroaminated product CH₂(CN)CH₂NH(Tol-p), predominantly. Treatment of 5 with [NH₂Me₂]BPh₄ in the absence of amine bases also released the hydroaminated product. These results apparently reveal that the amine substrate plays a critical role in driving the catalytic cycle.