Palladium(II) and Palladium(0) Complexes of Pyridylthioether-Based Metallodendrimers. Synthesis, Characterization, and Mechanistic Study of the Influence of Wedge Size on Allyl Amination

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• 作者：Luciano Canovese ; Gavino Chessa ; Claudio Santo ; Fabiano Visentin ; and Paolo Uguagliati
• 刊名：Organometallics
• 出版年：2002
• 出版时间：October 14, 2002
• 年：2002
• 卷：21
• 期：21
• 页码：4342 - 4349
• 全文大小：125K
• 年卷期：v.21,no.21(October 14, 2002)
• ISSN：1520-6041

文摘

Pd(II) allyl and Pd(0) fumaronitrile complexes bearing pyridyl-dithioether-based dendrimers as ancillary ligands have been synthesized and fully characterized by means of NMRspectrometry, elemental analysis, and MALDI-TOF technique. The fluxional behavior of thespecies was investigated by studying the ¹H NMR spectra recorded in CD₂Cl₂ at variabletemperature and interpreted on the basis of a windshield-wiper motion of the dendritic wingswhich alternatively coordinate at the metal core. The reactivity of piperidine on the Pd(II)allyl substrates to give the allylamine derivative and the corresponding Pd(0) fumaronitrilespecies was also studied. The second-order rate constants k₂ and the equilibrium constantsK_E relating to piperidine attack on the allyl fragment and to the concomitant displacementof the dendrimer ligand by piperidine, respectively, were determined and discussed takinginto account the increasing dendritic size. Surprisingly, no macroscopic effects are observedon going from the model molecule to the second-generation dendritic substrates, and onlywith the third generation dendritic wedge complex are remarkable variations in the rateand equilibrium constants observed. We therefore advance the hypothesis that a suddenrearrangement at this stage occurs in solution. Such a rearrangement would induce anincrease of steric hindrance at the allyl fragment and a concomitant distortion of the ligandin the environment of the metal core, thereby justifying the decrease of k₂ and the increaseof K_E values.