The reductive elimination reaction

The elimination of the molecule R —R (R and R are X-type ligands such as H, alkyl, halogen, etc.) is a decomposition mode that is frequently found for organometallic complexes [L MRR ] (4-52). 

The removal of two X-type ligands leads to a decrease of two units in the metal s oxidation state the electronic configuration changes from d to 4 . The metal is therefore reduced, which explains the name reductive eliniination . The opposite reaction is called oxidative addition . 

Four electrons are intimately involved in the reorganization of the bonds. In the reactant, these are the electrons associated with the two M—R 

In the reactant, two (occupied) bonding MO that describe the bonds that are to be broken, and the two corresponding (empty) antibonding MO. 

In the products, the (occupied) bonding and (empty) antibonding MO associated with the new bond that is formed (R—R), and the two nonbonding MO that remain on the metal (with two electrons in total) after the departure of the two ligands. 

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Study of the reductive elimination reactions of /ac-PtMe3(dppe)I leads to estimated Pt-Me and Pt-I bond energies of 132 and 196 kJmol-1, respectively [200],... 

Reaction 2.22 a may be followed by various other reactions such as insertions, 13-eliminations or regular reductive eliminations (See Figure 2.24). The reductive elimination reaction is governed by the common rules given in the section on reductive elimination. The reaction shown has been observed for nickel complexes. 

The reductive elimination reactions of halogen from 6-11 illustrate several examples of ligand loss from trigonal bipyramidal species B to generate onium species A. Activation parameters in these processes were of 73 to 100 kJ mol and Eyring activation parameters of 70-97 kJ molfor and —4 to... 

A C-Cu bond is a stable covalent bond, and is difEcult to cleave by itself [93]. After charge transfer from cuprate(I) to substrate, however, cleavage of the resulting R-Cu bond becomes easy. The reductive elimination reaction regenerates RCu , which may take part in further catalytic cycles. Thus, in copper-... 

Density functional calculations, incorporating clusters with and without solvent coordination to lithium and/or copper, reveal that the 5 n2 transition state always features inversion and retention at the electrophilic and nucleophilic centres, respectively. This transition state (100) is such that the carbons of the three alkyl groups are in a different electronic and spatial environment thus, the formation of RR, rather than RR, is governed by the transition state (101) for the reductive elimination reaction of the Cu(II) intermediate. 

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 controlling factor of the reductive elimination on Pd(R)(C=CR )Lm (Eq. 5) may be different from that observed with the nickel complex. However, participation of a similar activation process by coordination of electron-with-drawing RX and R C=CH is conceivable. The Pd(R)(C=CR )Lm-type complex can be isolated, and it has been shown that isolated Pd(R)(C=CR )Lm undergoes the reductive elimination exhibited in Eq. 5 [8]. The reductive elimination seems to be enhanced by addition of Cul. Cul may interact with the Pd complex, and an acceleration effect of Lewis acids on the reductive elimination reaction of NiR2(bpy) has been shown [22]. The X-ray crystallographic structure of an isolated Pd(R)(C=CR )Lm (R=C6H4Me-p R =C6H5) has been determined [8]. 

Important features favoring the reductive elimination reaction have been discussed based on theoretical and experimental studies [128-130]. The reductive elimination of an azohum salt from a palladium NHC alkyl complex (Fig. 13) proceeds under direct formation of Pd° in an exothermic process with a low activation barrier [128]. The coligands at the palladium atom play an important role. It has been shown that the Caikyi-Pd-CNHC angle becomes more acute during the reductive elimination to allow for an optimal orbital overlap of the groups to be... 

Another possible reason that ethylene glycol is not produced by this system could be that the hydroxymethyl complex of (51) and (52) may undergo preferential reductive elimination to methanol, (52), rather than CO insertion, (51). However, CO insertion appears to take place in the formation of methyl formate, (53), where a similar insertion-reductive elimination branch appears to be involved. Insertion of CO should be much more favorable for the hydroxymethyl complex than for the methoxy complex (67, 83). Further, ruthenium carbonyl complexes are known to hydro-formylate olefins under conditions similar to those used in these CO hydrogenation reactions (183, 184). Based on the studies of equilibrium (46) previously described, a mononuclear catalyst and ruthenium hydride alkyl intermediate analogous to the hydroxymethyl complex of (51) seem probable. In such reactions, hydroformylation is achieved by CO insertion, and olefin hydrogenation is the result of competitive reductive elimination. The results reported for these reactions show that olefin hydroformylation predominates over hydrogenation, indicating that the CO insertion process of (51) should be quite competitive with the reductive elimination reaction of (52). 

Let us take, as an example, the reductive elimination reaction in order to illustrate the approach. The general description of this reaction is given by Eq. (3). 

The dimeric amido complexes underwent reductive elimination after cleavage to form two monomeric, 3-coordinate, 14-electron amido complexes. In the case of the anilido dimer 20, a half-order rate dependence in the palladium complex showed that the reductive elimination occurred after reversible cleavage of the dimer to form two monomers. In the case of the f-butylamido complex 21, rapid reductive elimination occurred after irreversible dimer cleavage. This conclusion was supported by reaction rates that were first order in palladium dimer and by the lack of crossover during the reductive elimination reactions containing two doubly-labeled dimers. 

These reactions involve a decrease in the oxidation number of the metal (in this case Ir to Ir ), and a decrease in coordination number for the metal. Reductive elimination reactions are very important in catalysis as the product-removing step. Frequently, the reductive elimination reactions are rapid, making detailed study difficult. 

Adsorption followed by a surface dissociation step is the analogue of the oxidative addition reaction we have met earlier. Associative recombination reactions become favoured when the adatom energies become destabilized, i.e. to the right of the periodic system, when the d-valence electrons have become highly occupied. The associative recombination reaction followed by desorption is the analogue of the reductive elimination reaction. 

The conversion of 21 to 22 with loss of H2 may be viewed as a reductive elimination and the equilibrium between 21 and 22 provides the reaction which leads to H2—D2 exchange. The reductive elimination reaction 21-do, 19-d2 results in... 

Similar considerations are also applicable for the reductive elimination reactions for the other platinum group metals. Our calculations on reductive eUmination reactions have shown an energetic preference of Rh compounds to Ir , Ru and Os compounds. The fact can be rationaUzed taking into account that the only exothermic promotion energy of -37.6 kcal/moR is expected for s d — s d (Rh —>Rh )- In contrast, s d —>sid (Ir —>Ir ) and s d —>d (Ru — Ru ) are endothermic by 25.1 and 9.2 kcal/mol respectively [13]. 

Figure 4.11. Schematic correlation diagram for the reductive elimination reaction [LnMR ] —> [LnM] + R—R. Only the four most strongly perturbed orbitals are shown.

Figure 4.12. Correlation diagram for the MO involved in the reductive elimination reaction d -[L2MR2] d °-[L2M] (bent) + R2 in the...

The reductive elimination reaction of the /3-hydroxy imidazolyl sulfone derivatives with sodium amalgam or samarium diiodide provided mainly the desired -alkenes in good yields. 

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