Reductive elimination oxidative addition compared

The coordination chemistry of NO is often compared to that of CO but, whereas carbonyls are frequently prepared by reactions involving CO at high pressures and temperatures, this route is less viable for nitrosyls because of the thermodynamic instability of NO and its propensity to disproportionate or decompose under such conditions (p. 446). Nitrosyl complexes can sometimes be made by transformations involving pre-existing NO complexes, e.g. by ligand replacement, oxidative addition, reductive elimination or condensation reactions (reductive, thermal or photolytic). Typical examples are ... 

The high formal oxidation states of metals in some of these adducts is noteworthy, e.g., Fe(IV) (entries 17 and 18), Ru(IV) (entries 21 and 22), and Pt(IV) (entries 55 and 56). Such adducts are important because they provide definite examples of species often postulated as intermediates in oxidative addition-reductive elimination processes (compare Section II,G,1) and in homogeneous catalysis (134,220a, 410a). In the case of germanium, a tris(germyl) adduct of Pt(IV) has been described (57), but no more than two silyl groups per metal atom are known to result from oxidative addition. 

Similar kinetic parameters have been reported by Stille and co-workers for the reaction of ds-PdMe2(PPh2Me)2 with Mel to form tra 5 -PdlMe(PPh2Me)2 and ethane (Scheme 5), except that this reaction is 6000-7000 times slower than the reaction of PdMe2(bpy). In the case of c/5 -PdMe2(PPh2Me)2, oxidative addition is slow compared with reductive elimination of ethane and hence the Pd(lV) intermediate is not detected spectroscopically. The detection of a cation intermediate for the reaction of PdMe2(bpy)... 

This reaction typifies the two possibilities of reaction routes for M-catalyzed addition of an S-X (or Se-X) bond to alkyne (a) oxidative addition of the S-X bond to M(0) to form 94, (b) insertion of alkyne into either the M-S or M-X bond to provide 95 or 96 (c) C-X or C-S bond-forming reductive elimination to give 97 (Scheme 7-21). Comparable reaction sequences are also discussed when the Chalk-Harrod mechanism is compared with the modified Chalk-Harrod mechanism in hydrosily-lations [1,3]. The palladium-catalyzed thioboratiori, that is, addition of an S-B bond to an alkyne was reported by Miyaura and Suzuki et al. to furnish the cis-adducts 98 with the sulfur bound to the internal carbon and the boron center to the terminal carbon (Eq. 7.61) [62]. 

Complexes 6 undergo the second migratory insertion in this scheme to form the acyl complexes 7. Complexes 7 can react either with CO to give the saturated acyl intermediates 8, which have been observed spectroscopically, or with H2 to give the aldehyde product and the unsaturated intermediates 3. The reaction with H2 involves presumably oxidative addition and reductive elimination, but for rhodium no trivalent intermediates have been observed. For iridium the trivalent intermediate acyl dihydrides have been observed [29], The Rh-acyl intermediates 8 have also been observed [26] and due to the influence of the more bulky acyl group, as compared to the hydride atom in 2e and 2a, isomer 8ae is the most abundant species. 

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). 

Addition of hydrosilane to alkenes, dienes and alkynes is called hydrosilylation, or hydrosilation, and is a commercially important process for the production of many organosilicon compounds. As related reactions, silylformylation of alkynes is treated in Section 7.1.2, and the reduction of carbonyl compounds to alcohols by hydrosilylation is treated in Section 10.2. Compared with other hydrometallations discussed so far, hydrosilylation is sluggish and proceeds satisfactorily only in the presence of catalysts [214], Chloroplatinic acid is the most active catalyst and the hydrosilylation of alkenes catalysed by E PtCU is operated commercially [215]. Colloidal Pt is said to be an active catalytic species. Even the internal alkenes 558 can be hydrosilylated in the presence of a Pt catalyst with concomitant isomerization of the double bond from an internal to a terminal position to give terminal silylalkanes 559. The oxidative addition of hydrosilane to form R Si—Pt—H 560 is the first step of the hydrosilylation, and insertion of alkenes to the Pt—H bond gives 561, and the alkylsilane 562 is obtained by reductive elimination. 

Alkylpalladium complexes generated by oxidative addition of Pd(0) to alkyl halides with a /3 hydrogen can undergo /3-elimination to yield an alkene and a Pd-hydrido complex (as in the Heck reaction Scheme8.7). Nevertheless, this process is relatively slow compared with transmetalations and reductive eliminations, and simple alkyl halides or tosylates with /3 hydrogen can be cross-coupled with carbon nucleophiles under optimized conditions if the nucleophile is sufficiently reactive [9, 73-75] (Scheme8.6). 

The reaction sequence in the vinylation of aromatic halides and vinyl halides, i.e. the Heck reaction, is oxidative addition of the alkyl halide to a zerovalent palladium complex, then insertion of an alkene and completed by /3-hydride elimination and HX elimination. Initially though, C-H activation of a C-H alkene bond had also been taken into consideration. Although the Heck reaction reduces the formation of salt by-products by half compared with cross-coupling reactions, salts are still formed in stoichiometric amounts. Further reduction of salt production by a proper choice of aryl precursors has been reported (Chapter III.2.1) [1]. In these examples aromatic carboxylic anhydrides were used instead of halides and the co-produced acid can be recycled and one molecule of carbon monoxide is sacrificed. Catalytic activation of aromatic C-H bonds and subsequent insertion of alkenes leads to new C-C bond formation without production of halide salt byproducts, as shown in Scheme 1. When the hydroarylation reaction is performed with alkynes one obtains arylalkenes, the products of the Heck reaction, which now are synthesized without the co-production of salts. No reoxidation of the metal is required, because palladium(II) is regenerated. 

The evidence is in accord with an addition-elimination mechanism (addition of ArPdX followed by elimination of HPdX) in most cases." In the conventionally accepted reaction mechanism," a four-coordinate aryl-Pd(II) intermediate is formed by oxidative addition of the aryl halide to a Pd(0) complex prior to olefin addition. This suggests that cleavage of the dimeric precursor complex, reduction of Pd , and ligand dissociation combine to give a viable catalytic species." If these processes occur on a time scale comparable to that of the catalytic reaction, nonsteady-state catalysis could occur while the active catalyst is forming, and an... 

The action of nickel is so much more powerful than that of alumina that the dehydrating action of the latter is practically eliminated when catalysts containing mixtures of reduced nickel and alumina are used. In fact, the alumina apparently only acts as a support for the active metal. However, comparative measurements have shown that the oxides of aluminium, iron, magnesium, and calcium may act as strong promoters for nickel catalysts. This effect has been explained as a mechanical effect, viz., the development of a large surface by which relatively more active metal is effectively exposed.10 When only small amounts of oxide are present the effect is predominantly that of support. The increased addition of oxide may increase the catalytic activity up to a certain point beyond which it only serves to dilute the catalyst and reduce its selectivity. Other explanations of the promoter action postulate the removal of catalyst poisons by the oxide, or regeneration of the active metallic catalyst by oxidations and reductions.20... 

The mechanism of this unique H-D exchange is of considerable importance. The usual mechanism proposed for H-D exchange in d-block transition metals involves a series of reductive-elimination, oxidative-addition cycles. The prerequisite in this type of process is the ability of the metal atom to shuttle between two readily available oxidation states. This type of mechanism could be invoked to explain the exchange reaction in the uranium hydride since uranium (VI) is a well-known oxidation state. The observation that the thorium hydride also undergoes exchange at a comparable rate shows that such a process is not viable as thorium (VI) is unknown. Scheme III outlines an... 

Depending on the solvent, the relative reactivities of 1 and 2 (as well as of [RhCl(CO)(SbPh3)2]) towards the oxidative addition and reductive elimination span ranges of up to two orders-of-magnitude (Table 3). The half-lives for the reaction at ambient temperature and [CH3I] = 1 M, range from a few minutes to hours, and are comparable with those observed for [RhCl(CO)(YPh3)2] (Y = Por As). o... 

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