Platinum-Catalyzed Hydroamination of Ethylene: Study of the Catalyst Decomposition Mechanism

A study of the addition of nucleophilic reagents that are also strong Brønsted bases (Et₃N, pyridine, quinuclidine, MeO^–) to <i>n</i>Bu₄P⁺[PtBr₃(C₂H₄)]⁻ (<b>1</b>) and <i>trans</i>-[PtBr₂(NHEt₂)­(C₂H₄)] (<b>7</b>) has provided key information on the deactivation of the hydroamination PtBr₂/Br^– catalyst, leading to metallic platinum. The addition of NEt₃ to <b>1</b> in CD₂Cl₂ is reversible and temperature dependent; the quantitative formation of the zwitterionic complex <i>trans</i>-[Pt⁽⁻⁾Br₂(NEt₃)­(CH₂CH₂N⁽⁺⁾Et₃)] (<b>9</b>) is observed only at low temperature, whereas slow deposition of metallic platinum occurs at room temperature. The addition of NEt₃ to <b>7</b> in CD₂Cl₂ is also reversible and temperature dependent, yielding <i>trans</i>-[Pt⁽⁻⁾Br₂(NHEt₂)­(CH₂CH₂N⁽⁺⁾Et₃)] (<b>10</b>) quantitatively at low temperature. At room temperature, this reaction led to the deposition of metallic platinum and to the formation of a complex identified as <i>trans</i>-[PtBr₂(NHEt₂)­(CH₂CHNEt₂)] (<b>11</b>). The carbyl ligand in <b>11</b> is shown by an X-ray structural study to be between the π-bonded enamine and the σ-bonded iminiumalkyl configurations. The addition of MeONa to <b>7</b> results in the formation of the same products <b>11</b> and Pt⁰. On the basis of these results, a mechanism for the base-induced decomposition of Pt^II(C₂H₄) complexes that involves Wacker-type β-H elimination followed by intermolecular hydride transfer, ligand rearrangements, and final deprotonation is proposed. Addition of more nucleophilic N-based ligands (pyridine, quinuclidine) to <b>7</b> ultimately leads to C₂H₄ and Et₂NH substitution rather than to metal reduction, even though evidence for a kinetically controlled nucleophilic addition to the coordinated ethylene is given by the quinuclidine system. From the reaction with pyridine, the complex <i>cis</i>-PtBr₂(py)₂ was isolated and structurally characterized