Oxidative addition reactions catalysts

Lithiation at C2 can also be the starting point for 2-arylatioii or vinylation. The lithiated indoles can be converted to stannanes or zinc reagents which can undergo Pd-catalysed coupling with aryl, vinyl, benzyl and allyl halides or sulfonates. The mechanism of the coupling reaction involves formation of a disubstituted palladium intermediate by a combination of ligand exchange and oxidative addition. Phosphine catalysts and salts are often important reaction components. 

RhCl(PPh3)3 is a very active homogenous hydrogenation catalyst, because of its readiness to engage in oxidative addition reactions with molecules like H2, forming Rh—H bonds of moderate strength that can subsequently be broken to allow hydride transfer to the alkene substrate. A further factor is the lability of the bulky triphenylphosphines that creates coordinative unsaturation necessary to bind the substrate molecules [44]. 

The effectiveness of an organic chloride in activating the catalyst appears to be related to the lability of the C—Cl bonds and probably also to their coordinating ability with the Rh. For example, CH30—CH2C1 and CH2=CH—CH2C1 were found to be extremely effective activators presumably, the presence of the ether or the allylic donor sites adds to the ease of the oxidative addition reaction ... 

In this chapter, theoretical studies on various transition metal catalyzed boration reactions have been summarized. The hydroboration of olefins catalyzed by the Wilkinson catalyst was studied most. The oxidative addition of borane to the Rh metal center is commonly believed to be the first step followed by the coordination of olefin. The extensive calculations on the experimentally proposed associative and dissociative reaction pathways do not yield a definitive conclusion on which pathway is preferred. Clearly, the reaction mechanism is a complicated one. It is believed that the properties of the substrate and the nature of ligands in the catalyst together with temperature and solvent affect the reaction pathways significantly. Early transition metal catalyzed hydroboration is believed to involve a G-bond metathesis process because of the difficulty in having an oxidative addition reaction due to less available metal d electrons. 

Cluster or bimetallic reactions have also been proposed in addition to monometallic oxidative addition reactions. The reactions do not basically change. Reactions involving breaking of C-H bonds have been proposed. For palladium catalysed decomposition of triarylphosphines this is not the case [32], Likewise, Rh, Co, and Ru hydroformylation catalysts give aryl derivatives not involving C-H activation [33], Several rhodium complexes catalyse the exchange of aryl substituents at triarylphosphines [34] ... 

The fact that complex 38 does not react further - that is, it does not oxidatively add the N—H bond - is due to the comparatively low electron density present on the Ir center. However, in the presence of more electron-rich phosphines an adduct similar to 38 may be observed in situ by NMR (see Section 6.5.3 see also below), but then readily activates N—H or C—H bonds. Amine coordination to an electron-rich Ir(I) center further augments its electron density and thus its propensity to oxidative addition reactions. Not only accessible N—H bonds are therefore readily activated but also C—H bonds [32] (cf. cyclo-metallations in Equation 6.14 and Scheme 6.10 below). This latter activation is a possible side reaction and mode of catalyst deactivation in OHA reactions that follow the CMM mechanism. Phosphine-free cationic Ir(I)-amine complexes were also shown to be quite reactive towards C—H bonds [30aj. The stable Ir-ammonia complex 39, which was isolated and structurally characterized by Hartwig and coworkers (Figure 6.7) [33], is accessible either by thermally induced reductive elimination of the corresponding Ir(III)-amido-hydrido precursor or by an acid-base reaction between the 14-electron Ir(I) intermediate 53 and ammonia (see Scheme 6.9). 

The reduced donor ability of the phosphinite complexes such as 5e and 5f has an impact beyond the catalyst activation stipulated above. Apparently, the decreased tendency to undergo oxidative addition reactions also disfavors catalyst deactivation via oxidative olefin addition. Accordingly, (vinyl) (hydride) complexes such as 3 are less relevant. Simultaneously, product oxidative addition is restricted and, as... 

The presumed catalytic cycle for this coupling is the following Once formed from 23, the highly coordinatively unsaturated 14-electron palladium(O) complex 24 participates in an oxidative addition reaction with the aryl or vinyl halide to give the 16-electron palladium(II) complex 25. A copper(I)-catalyzed alkynylation of 25 then furnishes an aryl- or vinylalkynyl palladium(II) complex 27. Finally, a terminating reductive elimination step reveals the coupling prduct 9 and regenerates the active palladium(O) catalyst 24. 

Their advantage over other types of dendrimers is their straightforward synthesis and, most importantly, their chemical and thermal stabilities. Two distinct steps characterize their synthesis a) an alkenylation reaction of a chlorosilane compound with an alkenyl Grignard reagent, and b) a Pt-cata-lyzed hydrosilylation reaction of a peripheral alkenyl moiety with an appropriate hydrosilane species. Scheme 2 shows the synthesis of catalysts Go-1 and Gi-1 via this methodology. In this case, the carbosilane synthesis was followed by the introduction of diamino-bromo-aryl groupings as the precursor for the arylnickel catalysts at the dendrimer periphery. The nickel centers of the so-called NCN-pincer nickel complexes were introduced by multiple oxidative addition reactions with Ni(PPh3)4. 

In support of this suggestion, when [Rh2(CO)2( r-SP AN-POP) (p-Cl)2] was used as a catalyst precursor, the catalytic reaction rate was four times higher than when a 1 1 (molar) diphosphine Rh ratio was used. In a subsequent computational investigation, the oxidative addition reactions of Mel with di-rhodium complexes, [Rh(CO)(PR3)( r-Cl)]2 (R = H, Me) and that with mononuclear [Rh(CO)(PH3)2Cl] and [Rh(CO)2I2] were compared on the basis of DFT calculations [96]. Calculated activation parameters for nucleophilic attack by rhodium on Mel showed good agreement with experimental results. 

The proposed iminium intermediate 2 in the oxidative alkylation reaction implied that other C-H based nucleophiles could undergo oxidative addition reactions to the C-H bond of tertiary amines. Li and co-workers demonstrated the cross-coupling of indoles 18 with tertiary amines 6 using simple copper salts as catalyst (Scheme 11) [27]. 

Some saturated complexes are easily converted into unsaturated states without an extra energy source. For example, four-coordinate Pt and Pd phosphine complexes, [M(PPh3)4, M = Pt or Pd] have d10 configurations and are regarded as saturated. However, it was proved by Malatesta and CarieHo that the coordinated phosphines dissociate in solution, forming dicoordinate complexes which readily undergo oxidative addition reactions, as will be shown later 6 r>. These complexes are said to be potentially unsaturated, and are thus useful as catalysts in several reactions. 

The 16-electron halide complexes ciy-[Rh(CO)2l2] and tran -[Ir(CO)Cl(PPh3)2] (Vaska s compound) undergo many oxidative addition reactions and have important catalytic applications (see Chapter 26). The product of reaction 23.61 is a catalyst precursor for alkene hydrogenation. Vaska s compound readily takes up O2 to give the peroxo complex 23.44. 

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