Platinum-Catalyzed Hydroamination of Ethylene: Study of the Catalyst Decomposition Mechanism
Résumé
A study of the addition of nucleophilic reagents that are also strong Brønsted bases (Et3N, pyridine, quinuclidine, MeO−) to nBu4P+[PtBr3(C2H4)]− (1) and trans-[PtBr2(NHEt2)(C2H4)] (7) has provided key information on the deactivation of the hydroamination PtBr2/Br− catalyst, leading to metallic platinum. The addition of NEt3 to 1 in CD2Cl2 is reversible and temperature dependent; the quantitative formation of the zwitterionic complex trans-[Pt(−)Br2(NEt3)-(CH2CH2N(+)Et3)] (9) is observed only at low temperature, whereas slow deposition of metallic platinum occurs at room temperature. The addition of NEt3 to 7 in CD2Cl2 is also reversible and temperature
dependent, yielding trans-[Pt(−)Br2(NHEt2)(CH2CH2N(+)Et3)] (10) 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 trans-[PtBr2(NHEt2 (CH2CHNEt2)] (11). The carbyl ligand in 11 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 7 results in the formation of the same products 11 and Pt0. On the basis of these results, a mechanism for the base-induced decomposition of PtII(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 7 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 cis-PtBr2(py)2 was isolated and structurally characterized.
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