Carbon-metal bonds reductive elimination

Ca.ta.lysis, Iridium compounds do not have industrial appHcations as catalysts. However, these compounds have been studied to model fundamental catalytic steps (174), such as substrate binding of unsaturated molecules and dioxygen oxidative addition of hydrogen, alkyl haHdes, and the carbon—hydrogen bond reductive elimination and important metal-centered transformations such as carbonylation, -elimination, CO reduction, and... 

Key words hydrogenation of carbon dioxide, insertion of carbon dioxide into the metal-hydride bond, reductive elimination of formic acid, C-bond metathesis... 

Hosomi, A., Miura, K. Palladium-catalyzed carbon-metal bond formation via reductive elimination. Handbook of Organopalladium Chemistry for Organic Synthesis 2002, 1,1107-1119. 

Synthetic applications that consist of metal catalyzed addition of X-X to carbon-carbon unsaturated substrates, generally alkynes, have been described for X = X = SR, SeR, TeR and X = SeR, X = PR2 as well as X = SR, X = BR2, SiR3 [201], These transformations generally involve oxidative addition of the X-X reactant to the metal center followed by insertion of the alkyne, into the M-X bond. Reductive elimination leads to the final disubstituted alkene, which shows cis stereochemistry. 

III.3.4 Palladium-Catalyzed Carbon-Metal Bond Formation via Reductive Elimination... 

This section mainly deals with Pd-catalyzed reactions of aryl, alkenyl, benzyl, alkyl, and acyl electrophiles with metal nucleophiles M = Si, Ge, Sn, B, and transition metals) leading to carbon-metal bond formation (Scheme 1). Allylic and related metallations are described in Sect. V.2.3.3, The reaction mechanism is generally believed to involve, firstly, oxidative addition of R—X to a Pd(0) species second, transmetallation of the resulting Pd(II) species with M—M and finally, reductive elimination forming R—M and the active Pd(0) species. However, another catalytic cycle via oxidative addition of M—M to Pd(0) species is also proposed. ... 

Like the rates of reductive eliminations to form C-H and C-C bonds, the rates of reductive eliminations to form carbon-heteroatom bonds depend on the coordination number of the metal. The reductive elimination to form C-N bonds from Pd(0) has been shown to occur faster from three-coordinate complexes than from four-coordinate com-plexes." - The reaction of the triphenylphosphme complex in Equation 8.63 to form triarylamine and Pd(0) was conducted with varying concentrations of added ligand. The rate of the reaction was slower when conducted with higher concentrations of added PPhj. A detailed study of the dependence of the rate of reaction on the concentration of added PPhj revealed two pathways for reductive elimination of amine—one from a four-coordinate cis complex and one from a three-coordinate complex formed by dissociation of phosphine. Although the relative rates of these two pathways depend on the concentration of added ligand, reaction through the three-coordinate intermediate was the major pathway at the concentrations of free ligand that would be present in most reactions. 

Carbon-heteroatom reductive elimination from dinuclear transition metal complexes, as was proposed by us [96,109] as the product-forming step in Pd-catalyzed C-H acetoxylation and chlorination reactions, is rare. The two formulations of the high-valent, dinuclear Pd intermediate in arylation proposed by Sanford (60 and 61) highlight that reductive elimination from dinuclear Pd structures could, in principle, proceed with either redox chemistry at both metals (bimetallic reductive elimination reductive elimination from 60) or with redox chemistry at a single metal (monometallic redox chemistry reductive elimination from 61). While structures 60 and 61 do not differ in composition, they do differ in their respective potentials for metal-metal redox cooperation to be involved in C-C bond-forming reductive elimination. 

Baldwin has suggested that a C—H activation involving the formation of Fe—C bonds may be important in the biosynthesis of penicillin. In one model, the enzyme first forms the four-membered ring of penicillin. Then an iron 0x0 species abstracts an H atom from the substrate to leave a carbon-centered radical that in turn binds to the metal. A reductive elimination of a thiolate with the alkyl leads to the formation of the penicillin ring ... 

An elementary step to cleave C-C bonds is a reverse process of a C-C bond forming process. Oxidative addition of a C-C bond to a low-valent transition metal complex is a reverse process of reductive elimination, which occurs with a high-valent diorganometal, forming a C-C bond. P-Carbon elimination is a reverse process of insertion of an unsaturated bond into a carbon-metal bond, that is, carbometallation, or 1,2-addition of an organometal across a double bond. Such fundamental reactions are described along with typical examples. Besides this chapter, there are some excellent reviews on C-C bond cleavage available [1]. 

In Grignard reactions, Mg(0) metal reacts with organic halides of. sp carbons (alkyl halides) more easily than halides of sp carbons (aryl and alkenyl halides). On the other hand. Pd(0) complexes react more easily with halides of carbons. In other words, alkenyl and aryl halides undergo facile oxidative additions to Pd(0) to form complexes 1 which have a Pd—C tr-bond as an initial step. Then mainly two transformations of these intermediate complexes are possible insertion and transmetallation. Unsaturated compounds such as alkenes. conjugated dienes, alkynes, and CO insert into the Pd—C bond. The final step of the reactions is reductive elimination or elimination of /J-hydro-gen. At the same time, the Pd(0) catalytic species is regenerated to start a new catalytic cycle. The transmetallation takes place with organometallic compounds of Li, Mg, Zn, B, Al, Sn, Si, Hg, etc., and the reaction terminates by reductive elimination. 

Both Ni and Pd reactions are proposed to proceed via the general catalytic pathway shown in Scheme 8.1. Following the oxidative addition of a carbon-halogen bond to a coordinatively unsaturated zero valent metal centre (invariably formed in situ), displacement of the halide ligand by alkoxide and subsequent P-hydride elimination affords a Ni(II)/Pd(ll) aryl-hydride complex, which reductively eliminates the dehalogenated product and regenerates M(0)(NHC). ... 

Alternative paths for decomposition of the metal carboxylate can lead to ketones, acid anhydrides, esters, acid fluorides (1,11,22,68,77,78), and various coupling products (21,77,78), and aspects of these reactions have been reviewed (1,11). Competition from these routes is often substantial when thermal decomposition is carried out in the absence of a solvent (Section III,D), and their formation is attributable to homolytic pathways (11,21,77,78). Other alternative paths are reductive elimination rather than metal-carbon bond formation [Eq. (36)] (Section III,B) and formation of metal-oxygen rather than metal-carbon bonded compounds [e.g., Eqs. (107) (119) and (108) (120). Reactions (36) and (108) are reversible, and C02 activation (116) is involved in the reverse reactions (48,120). 

Each step includes elementary acts that require different properties of the metal, for example, sufficiently low ionization potential to favor oxidative addition, sufficiently weak metal-carbon bonds, tendency to form square-planar complexes and to reach pentacoordination to allow insertion, a sufficiently high electron affinity to allow reductive elimination, and so on. Some properties are conflicting and a compromise has to be reached. 

Sigma-bond metathesis at hypovalent metal centers Thermodynamically, reaction of H2 with a metal-carbon bond to produce new C—H and M—H bonds is a favorable process. If the metal has a lone pair available, a viable reaction pathway is initial oxidative addition of H2 to form a metal alkyl dihydride, followed by stepwise reductive elimination (the microscopic reverse of oxidative addition) of alkane. On the other hand, hypovalent complexes lack the... 

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. 

The proposed reaction mechanism is as follows (Scheme 16.83). Zinc metal reduces Ni(II) species to Ni(0). A nickelacyclopentadiene may be produced via coordination of two molecules of propiolates and regioselective head-to-head oxidative cyclometallation. Coordination and subsequent insertion of an allene into the Ni(II)-carbon bond give rise to a nickelacycloheptadiene intermediate. Finally, a benzene derivative is produced via reductive elimination followed by isomerization. 

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