Bonds oxidative addition

One other point to note in regard to this study (141) is that any evidence of oxidative addition, particularly with the chloro-olefins, was absent. The similarity of the spectra, coupled with the nonobservation of any bands in the visible region, as well as the observation of vc-c in the region commonly associated with 7r-complexation of an olefin (141, 142), all argue in favor of normal ir-coordination, rather than oxidative insertion of the metal atom into, for example, a C-Cl bond. Oxidative, addition reactions of metal atoms will be discussed subsequently. 

Let us consider the general trends of the reactivity of C-C, C-S, and C-Q (Q = Cl, Br, I) bonds towards oxidative addition and reductive elimination (Scheme 7-25). In many cases, either C-C bond-forming reductive elimination from a metal center (a) or the oxidative addition of a C-Q bond to a low-valent metal center is a thermodynamically favorable process (c). On the other hand, the thermodynamics of the C-S bond oxidative addition and reductive elimination (b) lies in between these two cases. In other words, one could more easily control the reaction course by the modulation of metal, ligand, and reactant Further progress for better understanding of S-X bond activation will be achieved by thorough stoichiometric investigations and computational studies. 

Ru, Os, and Ir carbene complexes have been prepared from reactions of anionic or low-valent metal complexes with some organic salts or neutral compounds with highly ionic bonds. Oxidative addition of halothiazole and -oxazole species to IrCl(CO)(PMe2Ph)2 affords Ir(III) complexes which on protonation yield cationic carbenes (69), e.g.,... 

This special feature arises from the combination of the transition metal behavior such as the coordination of a carbon-carbon multiple bond, oxidative addition, reductive elimination, P-hydride elimination, addition reactions and the behavior of classical c-carbanion towards electrophiles. 

Key words ONIOM, hydrogenation, enantioselectivity, asymmetric catalysis, DFT, reaction mechanism, chiral phosphine, ab initio, valence bond, oxidative addition, migratory insertion, reductive elimination. 

Many related complexes of iridium and rhodium undergo the oxidative addition reaction of alkanes and arenes [1]. Alkane C-H bond oxidative addition and the reverse reaction is supposed to proceed via the intermediacy of c-alkane metal complexes [4], which might involve several bonding modes, as shown in Figure 19.5 (for an arene the favoured bonding mode is r 2 via the K-electrons). 

Palladium(0)-catalyzed transformations generally involve three steps oxidative addition, insertion or transmetallation (really a special type of insertion), and reductive elimination. Together they comprise a pathway for the formation of new carbon-carbon bonds. Oxidative addition takes place when a coordinatively unsaturated Pd(0) species cleaves a covalent bond to give a new complex in which die palladium is oxidized to Pd(II). Typically dissociation of two phosphine ligands to a 14-electron complex is file first step followed by oxidative addition to give a 16-electron Pd(II) complex. 

The 1,2,4,3-triazaphospholes are colorless or pale yellow distillable liquids or crystalline solids. They are not oxidized by air and are reluctant to react with sulfur. Three isomeric heterocyclic systems of 277-1,2,4,3-triazaphospholes 15, 177-1,2,4,3-triazaphospholes 16, and 477-1,2,4,3-triazaphospholes 17 are known and they differ considerably in their behavior <1996CHEC-II(4)771>. The synthesis of 1,2,4,3-triazaphospholes and reactivity of different isomers of 1,2,4,3-triazaphospholes in the reactions at a ring nitrogen, in the addition to the P=N bond, oxidative addition to the ring phosphorus, cycloaddition reactions, and the formation of transition metal complexes are systematically covered in CHEC-II(1996) <1996CHEC-II(4)771>. The 1,3,4,2-thiadiazaphospholium ions 18 are only briefly mentioned in CHEC-II(1996) and no new results on their chemistry have been published in the last decade. 

Dissociation of the carbonyl ligand was initially postulated as the role of the photon (Route A, Scheme 3) [4], Mechanistic investigations by Goldman et al. indicated, however, that C-H bond oxidative addition to the photo-excited sixteen-elec-... 

There are two completely different pathways for the cleavage of H-H bonds oxidative addition and heterolytic cleavage. Both pathways have been identified in catalytic hydrogenation and may also be applicable to other types of X-H a bond activation such as C-H cleavage. 

Bergman RG. A physical organic road to organometallic C-H bond oxidative addition reactions. J Organomet Chem 1990 400 273-282. 

Pt(0), NVE 16/Pt(II), NVE 16). Oxidative additions generally occur most readily for low valent complexes, and for metals in the order 5d > Ad 3d. In addition to those that formally cleave C-X, H-X, and X-X (X = halide) bonds, oxidative addition reactions are also known where the metal is inserted into C-0, C-H, and some strained or activated C-C bonds. Another reaction which is effectively an oxidative addition is the formation of metallacycles from a low valent metal and an olefin (Equation 7). 

A possible mechanistic scheme for carbocupration may involve initial complexa-tion of the RCu(I)MgBr2 species with the triple bond, oxidative addition (insertion) of RCu to the activated triple bond, transfer of the R to the vinylic carbon, reductive elimination of Cu(I), and finally metal-metal exchange to furnish the 2,2-disubstituted vinylcopper intermediate. 

Reactions of transition metal carbonyls with an In(I) center also take place by oxidative addition here InX inserts into the M-X bond. Oxidative addition to the low-valent halides of Ga, In, or T1 thus provides a useful route to compounds with group IIIB-metal bonds This approach is particularly useful when the salt elimination route cannot be employed because a suitable transition metal nucleophile is lacking. 

A much wider range of organopalladium compounds is available, however, by means of cyclopalladation reactions. Such reactions involve a Pd(II) salt and an N or P ligand capable of undergoing intramolecular C—H bond oxidative addition. HCl is eliminated as a by-product, and the reactions occur most readily when a five-membered chelate ring is formed " ... 

Oxidative addition can also occur at C(sp2)-X bonds (i.e., at vinyl and aryl halides). It always occurs with retention of configuration about the double bond. Oxidative addition at C(sp2)-X obviously cannot proceed by an Sn2 mechanism. The SRN1 mechanism is a possibility. Another possibility is that the Pd coordinates first to the 77 bond of, say, vinyl iodide, to form a 7r complex. The metal-lacyclopropane resonance structure of the 77 complex can be drawn. An electro-cyclic ring opening of the metallacyclopropane (Chapter 4) can then occur. One Pd-C bond breaks, and I- leaves to give a new cationic Pd-vinyl complex. When I associates with the Pd, the overall result is oxidative addition. 

Numerous compounds with Sn—M bonds, where M is a transition metal, have become available by various routes (e.g. insertion of SnCl2 into M—Cl bonds, oxidative addition, or simply by metathesis accompanied by alkali metal halide formation). Examples are given in Table 2.1.9. For the tin coordination number 4, one observes an increase in Sn nuclear shielding by a bond between tin and more heavy metals a similar trend is well known for and nuclear shielding in C—M and P—M bonds, respectively. 

Few reports have appeared addressing the stereochemistry of either C-H bond oxidative addition or reductive elimination. The most convincing paper was an intramolecular activation examined by Flood in which a chiral 8-ethylquinoline derivative underwent benzylic activation by PdCl42-. The reaction proceeds with the net retention of configuration at carbon (Eq. 10) [49]. 

Keywords C-0 bond cleavage, Activation of C-0 bonds, Oxidative addition, Transition metal complexes, Allylic compounds, Esters, Ethers, Anhydrides, Alcohols... 

The search for new reactivity and new reactions is an important target in homogeneous catalysis. A declared goal is the selective activation of C-H bonds under mild conditions. Although there are numerous examples of stoichiometric C-H bond oxidative additions to transition metal centers, successful examples regarding catalytic functionalization of C-H bonds have been made only during the last five years. Notable advances have been achieved by Moore and coworkers who described in 1992 the ortAo-acylation of pyridine with olefins and carbon monoxide. The cluster compound triruthenium dodecacarbonyl has been used as catalyst (Scheme 10). 

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