Transition metals reductive elimination reactions

As mentioned earlier, reductive elimination reactions are commonly observed processes that involve M-Si bond cleavage. Usually the transition-metal reductive elimination product is trapped by an added reagent such as a silane (equation 63)204, a germane (equation 64)205, a phosphine (equation 65)167 or hydrogen (equation 66, dppe = Ph2PCH2CH2PPh2)206. The latter reaction with hydrogen probably proceeds via initial oxidative addition of H2 to form a Pt(IV) intermediate. In the case of chiral complex ds-(SX-)-[(l-Naph)PhMeSi]PtH(PPh3)2, elimination of the silane upon addi-... 

Schanke CA, LP Wackett (1992) Environmental reductive elimination reactions of polychlorinated ethanes mimicked by transition-metal coenzymes. Environ Sci Technol 26 830-833. 

Besides dissociation of ligands, photoexcitation of transition metal complexes can facilitate (1) - oxidative addition to metal atoms of C-C, C-H, H-H, C-Hal, H-Si, C-0 and C-P moieties (2) - reductive elimination reactions, forming C-C, C-H, H-H, C-Hal, Hal-Hal and H-Hal moieties (3) - various rearrangements of atoms and chemical bonds in the coordination sphere of metal atoms, such as migratory insertion to C=C bonds, carbonyl and carbenes, ot- and P-elimination, a- and P-cleavage of C-C bonds, coupling of various moieties and bonds, isomerizations, etc. (see [11, 12] and refs, therein). 

Reductive elimination on transition metal complexes seems to be enhanced by coordination of electron-withdrawing 71-acids such as cyanobenzene and cyanoethylene. For example, the reductive elimination reaction of NiR2(bpy) (R=alkyl or aryl group bpy =2,2 -bipyridyl) is enhanced by electron-withdrawing olefinic and aromatic compounds [12-16] (Scheme 1). 

The reductive elimination to form C-C and C-H bonds [45] is a crucial step in the cross-coupling processes, as well as many other transition metal-catalyzed reactions. Reductive elimination reactions comprise an early chapter in any organometallic text. Many examples of these reactions have been studied, and a great deal is known about the mechanisms of these processes. Similarly, the cleavage of C-H bonds by oxidative addition, including the C-H bond in methane, is now known [46]. Again, questions remain about how these reactions occur, but a variety of mechanistic studies have revealed key features of these reactions. Even some remarkably mild C-C cleavage reactions have now been observed with soluble transition metal complexes [47,48]. 

In one group of reductive-elimination reactions, an HSiR3 molecule can be displaced from a silyl-substituted transition-metal hydride ML (H)(SiR3) by more efficient 7r-bonding ligands such as CO, PR3, C2H4, acetylenes or N2 which favor a lower oxidation state of M ... 

The transition metal-catalyzed coupling reaction that forms and cleaves the bonds of two organic molecules occurs by a sequence of oxidative addition-transmetalation (alkylation)-reductive elimination (Fig. 1) [1,7d, 32d-f,41]. 

A versatile application of Co(allyl)3 has been found in its reaction with butadiene, which effects catalytic dimerization under very mild conditions. Individual reaction steps are reproduced in Scheme 19. These reactions lead to the isolated and structurally characterized complex (13) as the true catalyst. Scheme 19 is in fact a collection of all the classical steps involved in homogeneous transition metal bond-forming reactions. The first step consists of replacement of two allyl groups by butadiene, which is reductively eliminated as diallyl, similar to what has been found with cod and with other two-or four-electron ligands. 

Oxidative-addition and reductive elimination reactions of transition metal complexes are crucial to many homogeneously catalyzed reactions and are important for bond formation. Reactant molecules such as H2 and Oj undergo oxidative addition to many transition metal centers. Oxidative addition of carbon-hydrogen bonds has been an active area of research. Reductive elimination is the process whereby products are eliminated from transition metal centers. 

Transition metal-based hydrogenation reactions most often operate on a catalytic cycle that involves oxidative addition, olefin insertion, and reductive elimination. The mechanistic basis for organolanthanide hydrogenation is quite different, and involves olefin insertion and a-bond metathesis (Fig. 3). 

A second major route to metal-metal complexes, related to the salt-elimination method described above, is elimination of neutral molecules with concurrent formation of metal-metal bonded complexes. Transition metal hydrides readily undergo these dinuclear reductive elimination reactions. The oxidative addition/reductive elimination see Oxidative Addition and Reductive Elimination) reaction of molecular hydrogen is a key reaction in this area (equation 47). 

Research has also focused on understanding the mechanism of the transition metal-catalyzed ROP reactions for [l]ferrocenophanes. A logical first step in the polymerization is insertion of the transition metal into the strained Cp-carbon-bridging element bond in the ferrocenophane. Polymers 93 formed in the presence of hydrosilanes are believed to result from competitive oxidative addition between the Si-H bond of the hydrosilane and the strained Gp-Si bond of the ferrocenophane at the catalytic center followed by reductive elimination. Detailed work has indicated that colloidal metal is the likely catalyst in the ROP reactions. 

We extended the study of C-C reductive elimination reactions to other members of late transition metals in order to find possible alternatives to Pd/Pt complexes for catalytic coupling reactions. The calculations were performed only for the corresponding phosphine complexes, for which experimental precedents for the reaction were reported. In the case of Rh and Ir derivatives, an extra o-bonded ligand has to be added to maintain correct oxidation state, because Rh and Ir compounds are rarely known [48]. The chloride has been chosen for this purpose. Thus the model compounds studied are 19 [Rh (CH=CH2)2(PH3)3Cl], 20 [Ir (CH=CH2)2(PH3)3Cl],21 [Ru (CH=CH2)2(PH3)3],and22 [Os (CH=CH2)2-... 

Oxidative addition and reductive elimination reactions play key roles in C—H activation reactions, where a strong C—bond is cleaved by a transition-metal complex. These are important reactions because they permit unfunctionalized hydrocarbons to be transformed into complex molecules. Bergman reported the following classic C—H reductive elimination/oxidative addition sequence. ... 

Of course, no one wants to make ethane that way (if at all) but many other pairs of ligands can be coupled by reductive elimination. Reductive elimination is one of the most important methods for the removal of a transition metal from a reaction sequence, leaving a neutral organic product. We will see many examples as the chapter develops but here is an indole synthesis that depends on a reductive elimination at palladium as a last step. In the starting material, palladium has two a bonds sharing electrons with C, and is Pd(II). In the reaction the two C substituents bond together to form the indole ring and a Pd(0) species is eliminated. 

In a reductive elimination reaction a dialkyl transition metal complex, symbolized by 19.43, decomposes into an alkane and a coordinatively unsaluratcd complex. The reaction is synthetically useful under catalytic and stoichiometric conditions ... 

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