Addition oxydante | EPFL Graph Search

L'addition oxydante et l'élimination réductrice sont deux réactions importantes de la chimie organométallique. L'addition oxydante est un processus qui augmente le degré d'oxydation et le nombre de coordination du centre métallique du complexe qui réagit. Cette réaction est généralement une étape d'un cycle catalytique ; il en est de même de étape inverse : l'élimination réductrice. Il est impossible d'avoir une de ces deux réactions dans un cycle sans que l'autre ne soit présente, car il faut restaurer le degré d'oxydation ainsi que le nombre de coordination du centre métallique. Rôle dans la chimie des métaux de transitions Pour les métaux de transitions, les réactions d'oxydation changent le nombre d'électrons , généralement par une perte d'électrons : dn devient dn-2. Les additions oxydantes sont favorisées pour des métaux qui satisfont une des deux conditions : (1) être basique et (2) être facilement oxydable. Les métaux avec un degré d'oxydati

New Synthetic Applications of Water Acetate Pd/TPPTS Catalyst Generated in situ. Evidence for true Pd(0) Valent Species Intermediate

Christian Amatore

Studies on the sp-sp intermolecular coupling reactions with the palladium water-soluble catalyst prepared in situ from palladium(II) acetate and sulfonated triphenylphosphine P(C6H4-m-SO3Na)(3) (TPPTS) in a homogeneous acetonitrile-water system, without Cu(I) promotor, afford diynes with moderate yields (45-65%). Under the same conditions, the sp(2)-sp coupling of 2-iodophenols or 2-iodoanilines and terminal alkynes followed by intramolecular cyclization gives indolic and furanic products in good yields (60-99%). Under these aqueous conditions, an efficient and short synthesis of eutypine illustrated the synthetic potentiality of the coupling. Furthermore, through a series of kinetic and P-31 NMR experiments, we have demonstrated that a mixture of Pd(OAc)(2) and TPPTS affords spontaneously a palladium(0) complex, through formation of bivalent complex Pd(OAc)(2)-(TPPTS)(2). A detailed mechanism of the reaction has been investigated thoroughly and the pertinent rate constants measured. The resulting palladium(0) complex reacts with phenyl iodide via an oxidative addition. This complex is considerably less reactive than the corresponding complex generated from PPh(3), probably due to steric effects.

1995

Reactivity and fluxionality of late transition metal cluster compounds

Zoltan Beni

The first part of this work deals with oxidative addition reactions of the triangulo platinum cluster [Pt3(μ-CO)3(PCy3)3]. Reactions with the electrophilic molecules ICH2CN and X2 (X = I and Br) and with the weakly polar reagent Ph3SnH have been studied. All three reactants convert the starting cluster firstly to novel 44-electron clusters probably via a SN2 mechanism. In the case of ICH2CN the adduct [Pt3(μ-CO)3(PCy3)3(I)(CH2CN)] is the first example where a Pt(II)L3 moiety is bonded to two Pt(0)L units via direct metal-metal bonds. A similar adduct was observed in the oxidative addition of Ph3SnH to the starting cluster, e.g. [Pt3(SnPh3)(μ-CO)3(PCy3)3H] which is the first reported neutral hydrido Pt-Sn cluster. With elementary halogens oxidative addition is followed by nucleophilic substitution of a CO ligand giving adducts [Pt3X(μ-CO)2(μ-X)(PCy3)3] (X = I, Br). Excess of reactants led to fragmentation of these adducts giving Pt(I) dimers and Pt(II) monomers. The observed trends in the nuclear spin-spin coupling constants in Pt(I) dimers have been reproduced by computation at the DFT level. The second part reports novel synthetic routes for pentanuclear platinum cluster compounds starting from trinuclear ones. The first complete NMR characterization of these Pt5 phosphino-carbonyl clusters was achieved as well. The final part of our work deals with CO scrambling processes in tetranuclear iridium and rhodium carbonyl clusters as studied by variable pressure NMR. The two major processes are the 'merry-go-round' and the 'change of basal face' which may be distinguished by the sign of the activation volume, negative for the former, positive for the latter process. On the basis of a theoretical study at the DFT level, transition state structures of the 'change of basal face' were proposed in the case of Ir2Rh2(CO)12 and Ir4(μ-SO2)(CO)11. These transition state structures have larger molecular volumes compared to those of ground state geometries in accordance with the experimentally determined, positive values of activation volume for this fluxional process.

EPFL2004

Mechanistic Studies Related to Nickel-Catalyzed Alkyl-Alkyl Cross Coupling Reactions

Jan Breitenfeld

Cross-coupling reactions of non-activated alkyl halides are potentially useful chemical transformations. At the same time, however, they are challenging due to a series of unproductive side reactions. Recently, significant progress has been made to overcome these difficulties. With judicious choice of metal, ligands, and reaction conditions, the normally problematic β- H elimination can be suppressed. As a consequence, a number of examples of the successful coupling of non-activated alkyl halides have been demonstrated. The mechanistic understanding of such reactions is, in most cases, still primitive. In the majority of cases, the active catalysts are generated in situ and remain unidentified and unknown. This dissertation is devoted to mechanistic studies of alkyl-alkyl cross-coupling catalyzed by a well-defined nickel pincer complex [(MeNN2)Ni-Cl] (1; Nickamine). This complex showed excellent activity for the cross-coupling of unactivated alkyl halides. Moreover, several nickel alkyl complexes bearing β-hydrogen atoms have been isolated. These features offered a unique platform to carry out mechanistic studies to gain a better understanding of nickel-catalyzed crosscoupling reactions. Chapter one provides examples of palladium-, iron-, cobalt-, and nickel-catalyzed alkyl-alkyl cross-coupling reactions. In each case mechanistic details are discussed. At the end of the chapter cross-coupling reactions using Nickamine are presented. In chapter two the propensity of isolated nickel alkyl complexes to undergo -hydride elimination is explored. Isomerization and olefin exchange experiments show that -hydride elimination is kinetically viable but thermodynamically unfavorable in [(MeNN2)Ni-alkyl] complexes. The intermediacy of an [(MeNN2)Ni-H] (6) species was corroborated by trapping experiments. The alkyl complex [(MeNN2)Ni-propyl] catalyzes olefin isomerization reactions. In chapter three the aforementioned nickel(II) hydride complex 2 was synthesized by reaction of [(MeNN2)Ni-OMe] (10) with Ph2SiH2, and was characterized by NMR and IR spectroscopy, as well as X-ray crystallography. Complex 6 was unstable in solution, and decomposed via two reaction pathways. The first pathway was through intramolecular N-H reductive elimination to III give MeNN2H and Ni particles. The second pathway was intermolecular, giving H2, Ni particles, and a five-coordinate Ni(II) complex [(MeNN2)2Ni] (12) as the products. Compound 6 reacted with ethylene and acetone, forming [(MeNN2)Ni-Et] (2) and [(MeNN2)Ni-OiPr] (13), respectively. Complex 6 also reacted with alkyl halides, yielding nickel(II) halide complexes and alkanes. The reduction of alkyl halides was rendered catalytic, using 1 as catalyst, NaOiPr or NaOMe as base, and Ph2SiH2 or Me(EtO)2SiH as the hydride source. The catalysis appeared to operate via a radical mechanism. In chapter four a detailed mechanistic study of alkyl-alkyl Kumada coupling catalyzed by the preformed nickel(II) pincer complex 1 is reported. The coupling proceeds through a radical process, involving two nickel centers for the oxidative addition of alkyl halide. The catalysis is 2nd order in Grignard reagent, 1st order in catalyst, and 0th order in alkyl halide. A transient species, (MeNN2)Ni-Alkyl2, is identified as the key intermediate responsible for the activation of alkyl halide, the formation of which is the turnover-determining step of the catalysis. Finally, a catalytic cycle for Nickamine-catalyzed cross-coupling reactions was established. This dissertation elaborates the properties of nickel-alkyl and nickel-hydride complexes of the Nickamine system. It elucidates many mechanistic features of the alkyl-alkyl Kumada coupling reactions catalyzed by Nickamine, and establishes, for the first time, a bimetallic oxidative addition mechanism for nickel-catalyzed coupling reactions. The work significantly enhances the current understanding of cross-coupling reactions of alkyl halides.

EPFL2013