Theoretical Study of the Mechanism of Oxidative Addition of Allyl-Ammonium and -Iminium Salts to Low-Valent Metal Complexes. Rationalization of Selective C-N and N-H Bond Activation

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• 作者：Maricel Torrent ; Djamaladdin G. Musaev ; and Keiji Morokuma
• 刊名：Organometallics
• 出版年：2000
• 出版时间：October 16, 2000
• 年：2000
• 卷：19
• 期：21
• 页码：4402 - 4415
• 全文大小：209K
• 年卷期：v.19,no.21(October 16, 2000)
• ISSN：1520-6041

文摘

A theoretical study has been carried out for the mechanism of the reactions of allylammonium and -iminium substrates with Ni(0) complexes using the B3LYP density functionalmethod. The main findings are as follows: (1) The actual active catalyst in the oxidativeaddition of ammonium and iminium salts is the bisphosphine-nickel complex. (2) Forallylammonium salts the reaction is found to proceed via an associative mechanism (ratherthan a dissociative path) which involves (i) coordination of the allylammonium cation to theactive metal catalyst, (ii) oxidative addition of the C-N bond to the Ni(0) complex, (iii)coordination of NH₃ to yield a pentacoordinated intermediate, and (iv) loss of a phosphineligand. The analogous reaction involving allyliminium salts does not follow the samemechanistic pattern; once the allyliminium cation is coordinated to the metal, the reactionprefers to proceed in a dissociative fashion. This preference is likely to be enhanced insolution. (3) The main difference between allylammonium and -iminium reaction mechanismslies in the relative barriers corresponding to the insertion of the catalyst into the C-N bondleading to the pentacoordinated intermediate. For allylammonium cations, such a step iscalculated to be less energy demanding than phosphine loss, whereas for allyliminium, thesituation is reversed. (4) Finally the observed preference of allyliminium substrates toundergo C-N cleavage rather than N-H cleavage has been also investigated. The reasonthat allyl salt does not yield hydrido complexes is the kinetic impairment found for the N-Hbond cleavage as compared to C-N. Intramolecular N-H activation is not allowed becauseof the high barriers corresponding to the three-centered transition states involved in thereaction. The structural rearrangements required for the coordinated iminium to form an²-NH complex make the process prohibitive.