DFT and Experimental Studies on the PtX 2 /X – -Catalyzed Olefin Hydroamination: Effect of Halogen, Amine Basicity, and Olefin on Activity, Regioselectivity, and Catalyst Deactivation
Résumé
A DFT/B3LYP study with inclusion of solvent and temperature effects has probed the olefin activation mechanism for the intermolecular hydroamination of ethylene and 1-hexene by aniline derivatives catalyzed by the PtX2/X– system on the basis of a variety of experimental results, including new experiments on catalyst deactivation. For ethylene and aniline, the calculated ΔG‡cycle between the resting state [PtX3(C2H4)]−, 1X, and the TOF-determining transition state of the C–H reductive elimination from [PtX3(H)(CH2CH2NHPh)]−, TS2X, is slightly smaller for X = Br than for Cl or I. The ΔG‡cycle decreases as the aniline basicity decreases. For the slightly less efficient hydroamination of 1-hexene, ΔG‡cycle is greater than that for the hydroamination of ethylene, with a preference for the Markovnikov addition, in agreement with experiment and with essentially equivalent ΔG‡cycle values for the Br and I systems. In general, the results of the calculations are in agreement with the experimental observation. A clear-cut comparison of trends is hampered by the small energy differences and by the possibility, proven in certain cases, that the reaction parameters under investigation affect the catalyst degradation rate in addition to its intrinsic activity. Extrapolation of the computational study to the fluoride system suggests that this should be even more active. However, experimental studies show that this is not the case. The reason for this anomaly has been traced to the basicity of the fluoride ion, which triggers more rapid catalyst decomposition. A bonding analysis of 1X indicates a significant push–pull π interaction between the C2H4 and the trans-F ligand.
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