Platinum-Catalyzed Ethylene Hydroamination with Aniline: Synthesis, Characterization, and Studies of Intermediates
Résumé
Starting from either K2PtCl4 or K[PtCl3(C2H4)]·H2O (Zeise’s salt), complexes (nBu4P)2[PtBr4] (1), nBu4P[PtBr3(C2H4)] (2), nBu4P[PtBr3(PhNH2)] (3), trans-[PtBr2(C2H4)(PhNH2)] (4), cis-[PtBr2(C2H4)(PhNH2)] (5), and cis-[PtBr2(PhNH2)2] (6) have been obtained by efficient one-pot procedures. All have been fully characterized by microanalysis (C, H, N), multinuclear NMR spectrometry (1H, 13C, 195Pt), UV−visible spectroscopy, and single-crystal X-ray diffraction. Compound 1 slowly loses Br− in solution to yield (nBu4P)2[Pt2Br6] (1′), which has also been characterized crystallographically. The relative stability of the various compounds has been probed experimentally by NMR studies in several solvents and computationally by gas-phase geometry optimizations followed by C-PCM calculations of the solvation effects in dichloromethane and aniline. The calculations also included the bis(ethylene) complexes [PtBr2(C2H4)2] in the trans (two different conformations 7 and 7′) and cis (8) configurations. The solution experiments gave no evidence for a nucleophilic attack of aniline onto coordinated ethylene under mild conditions (T up to 68 °C), setting a lower limit of 29 kcal mol−1 for the activation barrier of this process. Therefore, the relative energies computed for the other compounds suggest that all ethylene-containing complexes (2, 4, 5, 7, and 8) are viable candidates for the nucleophilic addition step of the PtBr2-catalyzed ethylene hydroamination by aniline. Use of the isolated complex 2, 4, or 5 in combination with nBu4Br as precatalysts for the ethylene hydroamination by aniline yields similar catalytic activities.
Domaines
Chimie
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