Reactivity oxidative addition reactions

Organometallic complexes of copper, silver, and gold are ideal precursors for carbene complexes along with some C- and N-coordinated species. Their reactivity pattern, in particular in oxidative addition reactions, was the most comprehensively studied. 

Another general process involves the reaction of Pd(0) species with halides or sulfonates by oxidative addition, generating reactive intermediates having the organic group attached to Pd(II) by a ct bond. The oxidative addition reaction is very useful for aryl and alkenyl halides, but the products from saturated alkyl halides often decompose by (3-elimination. The a-bonded species formed by oxidative addition can react with alkenes and other unsaturated compounds to form new carbon-carbon bonds. The... 

Formally, the metal oxidation number x increases to x+2, while the coordination number n of ML, increases to n+2. If such oxidative addition reactions are intended to be the first step in a sequence of transformations, which eventually will lead to a functionalization reaction of C-X, then the oxidative addition product 2 should still be capable of coordinating further substrate molecules in order to initiate their insertion, subsequent reductive elimination, or the like [1], This is why 14 electron intermediates MLu (1) are of particular interest. In this case species 2 are 16 electron complexes themselves, and as such may still be reactive enough to bind another reaction partner. 

Several basic approaches are possible and each has its own particular advantages. For some metals, all approaches lead to metal powders of identical reactivity. However, for some metals one method will lead to far superior reactivity. High reactivity, for the most part, refers to oxidative addition reactions. 

However, when the reductions were carried out with lithium and a catalytic amount of naphthalene as an electron carrier, far different results were obtained(36-39, 43-48). Using this approach a highly reactive form of finely divided nickel resulted. It should be pointed out that with the electron carrier approach the reductions can be conveniently monitored, for when the reductions are complete the solutions turn green from the buildup of lithium naphthalide. It was determined that 2.2 to 2.3 equivalents of lithium were required to reach complete reduction of Ni(+2) salts. It is also significant to point out that ESCA studies on the nickel powders produced from reductions using 2.0 equivalents of potassium showed considerable amounts of Ni(+2) on the metal surface. In contrast, little Ni(+2) was observed on the surface of the nickel powders generated by reductions using 2.3 equivalents of lithium. While it is only speculation, our interpretation of these results is that the absorption of the Ni(+2) ions on the nickel surface in effect raised the work function of the nickel and rendered it ineffective towards oxidative addition reactions. An alternative explanation is that the Ni(+2) ions were simply adsorbed on the active sites of the nickel surface. 

Klabunde has reported limited reactivity toward oxidative addition reactions of carbon halogen bonds with nickel slurries prepared by the metal vaporization technique(65). 

The iron slurries show exceptional reactivity toward oxidative addition reactions with carbon halogen bonds. In fact, the reaction with C.FcI is so exothermic that the slurry has to be cooled to 0 °C before the addition of C F L The reaction of iron with C F Br is also quite exothermic, hence, even for this addition, the iron slurry is cooled to about 0 ° C. The organoiron compound formed in the above reactions, solvated Fe(C.F )2, reacts with CO at room temperature and ambient pressure to yiela Fe(C,F3)2(CO)2(DMEL. 

The Ge(TMTAA) complex and the well known Sn(TMTAA) complex undergo facile oxidative addition reactions and reverse ylide formation with Mel and C6F5I because of the reactive M(II) (M = Sn, Ge) lone pair of electrons. In case of the oxidation with Mel it was assumed that, in solution, an ionic-covalent equilibrium exists (equation 48)95. 

Good stirring is important for the preparation of highly reactive calcium. A Schlenk tube is better than a flask for the reactor. Excess calcium salt was present during the oxidative addition reaction with 1 -bromoadamantane. 

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). 

Caulton and coworkers found that fluoride ligands in certain Ir complexes promote oxidative addition reactions [44]. This group s results showed that the fluoride complex lr(H)2F(P Bu2Ph)2 rapidly activated C—H bonds under dehydrogenation conditions. The reactive intermediate in these reactions may be a fluoro-bridged analogue of compounds 4-12, namely [lr(p-F)(P Bu2Ph)2]2. This would explain the improved reactivity in the Ir-catalyzed OHA reaction in the presence of cocatalytic naked fluoride . 

Arylated 5-chloro-2-methylpyridazin-3(277)-ones could be accessed regioselectively by exploiting the difference in reactivity of the C-Cl and the C-I bond of 186 in an oxidative addition reaction (Equation 36) <2005JHC427>. 

These oxidative—addition reactions have been treated extensively by Su et al. (29-31), using the VBSCD model. In all cases, a good correlation was obtained between the computed barriers of the reaction and the respective AEst quantities (which enter into the expression of G), including the relative reactivity of carbenoids, and of PtL2 versus PdL2 (29-31). Another treatment led to the same reactivity patterns for C—F bond activation reactions by Rh(PR3)2X and Ir(PR3)2X d8 complexes, which are isolobal to carbenoids (30). A similar extended correlation was found recently for C—Cl activation by d10-PdL2 (32), and is dealt with in Exercise 6.9. 

The reactivity of the aryl halide decreases in the halide order I > Br, with chlorides failing to react. The reactivity of substituted aryl halides increases upon going from electron-donating substituents through unsubstituted aryl halides to electron-withdrawing substituents. Both reactivity patterns of Arl > ArBr and activation of aryl halides with electron-withdrawing substituents follow the general reactivity of aryl halides in Pd(0) oxidative addition reactions [3],... 

Early attempts at producing dialkyltin compounds yielded polymers. More recently, Neumann has found several synthetic routes to reactive R2Sn intermediates which can be trapped by oxidative-addition reactions (J). In the absence of trapping agents the divalent tin compound polymerizes. Lappert and co-workers have shown that the bulky bistri-methylsilylmethyl ligand stabilizes the divalent tin species toward polymerization. This stable divalent tin species thus provides an excellent starting material for investigating a wide variety of oxidative-addition reactions, as shown in Fig. 10 (78). 

These 17-electron systems are excellent nucleophiles and undergo a variety of displacement, dimerization and oxidative addition reactions [Co(PMe3)4] is especially reactive (Scheme 40)435... 

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