Reductive elimination three-coordinate intermediates

Symmetry requirements for concerted reductive elimination of dialkyls have been considered (314), and for trialkylgold(III) species reductive elimination from a trigonal, three-coordinate intermediate was found to be symmetry forbidden. Solvent participation or the involvement of T-shaped species, however, was suggested as possible. Charge transfer to the high-oxidation state gold(III) center and reductive elimination from such a charge transfer state was proposed as an alternative reaction pathway. 

The mechanisms of the reductive eliminations in Scheme 5 were studied [49,83], and potential pathways for these reactions are shown in Scheme 6. The reductive eliminations from the monomeric diarylamido aryl complex 20 illustrate two important points in the elimination reactions. First, these reactions were first order, demonstrating that the actual C-N bond formation occurred from a monomeric complex. Second, the observed rate constant for the elimination reaction contained two terms (Eq. (49)). One of these terms was inverse first order in PPh3 concentration, and the other was zero order in PPh3. These results were consistent with two competing mechanisms, Path B and Path C in Scheme 6, occurring simultaneously. One of these mechanisms involves initial, reversible phosphine dissociation followed by C-N bond formation in the resulting 14-electron, three-coordinate intermediate. The second mechanism involves reductive elimination from a 16-electron four-coordinate intermediate, presumably after trans-to-cis isomerization. 

The three-coordinate intermediate in Equation 19 would then allow for greater mobility of the groups from which reductive elimination could proceed.(19) Rearrangement followed by a very rapid reductive elimination the cis isomer is also a possibility. However, our unsuccessful attempts to synthesize cis-aryl-methyl-nickel(ll) complexes still leave open the question of reductive elimination from such stereoisomers. The results we obtained in studies with the bidentate diphosphine ligand (dppe), though qualitatively in this direction, unfortunately lack definitiveness as yet. 

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. 

The mechanism of conjugate addition reactions probably involves an initial complex between the cuprate and enone.51 The key intermediate for formation of the new carbon-carbon bond is an adduct formed between the enone and the organocopper reagent. The adduct is formulated as a Cu(III) species, which then undergoes reductive elimination. The lithium ion also plays a key role, presumably by Lewis acid coordination at the carbonyl oxygen.52 Solvent molecules also affect the reactivity of the complex.53 The mechanism can be outlined as occurring in three steps. 

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. 

Thermolysis of these complexes at 9°C produced ethylene, cyclobutane, and butenes. The ratio of the gaseous products was found to be a function of the coordination number of the complex, or intermediate. Thus three coordinate complexes favoured butene formation, while four coordinate complexes favoured reductive elimination to form cyclobutane, and five coordinate complexes produced ethylene as shown in Scheme 25.83... 

The reductive elimination step (iv) is a three-centre mechanism, which creates the carbon-carbon bond, regenerates the catalyst and needs the R1 and R3 groups to be cis on the palladium. This may be the case when cis bidentate ligands are used188. On the other hand, a trans to cis isomerization may precede the reductive elimination, which operates through T-shaped Pd(II)189,190 or Pd(IV)191-193 intermediates. Finally, recent studies argued that a T-shaped three-coordinate species c -[PdR1R2L] may be formed directly by an associative transmetallation step. 

To effectively model the asymmetric hydrogenation reaction, we must look at the mechanism carefully. The first step involves the displacement of solvent and the coordination of the enamide to produce the two diastereomers (Fig. 3) (17-20). It appears as though the enamide-coordinated diastereomers are in rapid equilibrium with each other through the solvento species (Fig. 4). This square planar rhodium(I) cation is then attacked by dihydrogen to form an octahedral rhodium(III) complex (Fig. 4). Hydrogen then inserts into the Rh-C bonds, and the product is reductively eliminated (Fig. 4). From a molecular mechanics standpoint we have three entities to model the square planar rhodium(I) solvento species and the two intermediates (square pyramidal dihydrogen complex and the octahedral dihydride). 

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