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

The complex Pt{2,6-(R2PCH2)(2)C6H3}(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)(2)C6H3}{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)(2)C6H3}{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)(2)-C6H3}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)(2)C6H3}{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.