Redox coupling reductive elimination

In several examples the reductive halide-hydrogen exchange has been studied on a preparative scale. For example, the indirect electroreduction of 2-chloropyridine in DMF using anthracene as mediator gives pyridine in 83-86 % yield 2 . For the dehalogenation of 1-chlorohexane (80% yield), naphthalene is applied as redox catalyst. Similarly, 6-chloro-hexene yields 1-hexene (60%) and methylcyclopentane (25%), which is the product of the radical cyclization . The indirect electrochemical reduction of p- and y-bromocarboxylic esters forms coupling and elimination products besides the dehalogenated products... 

Polymer-modified electrodes have shown considerable utility as redox catalysts. In many cases, modified electrode surfaces show an improved electrochemical behavior towards redox species in solution, thus allowing them to be oxidized or reduced at less extreme potentials. In this manner, overpotentials can be eliminated and more selective determination of target molecules can be achieved. In this discussion, a mediated reduction process will be considered, although similar considerations can be used to discuss mediated oxidation processes. This mediation process between a surface-bound redox couple A/B and a solution-based species Y can be describes by the following ... 

Aminations of five-membered heterocyclic halides, such as furans and thiophenes, are limited. These substrates are particularly electron-rich. As a result, oxidative addition of the heteroaryl halide and reductive elimination of the amine are slower than for simple aryl halides (see Sections 4.7.1 and 4.7.3). In addition, the amine products can be air-sensitive and require special conditions for their isolation. Nevertheless, Watanabe has reported examples of successful couplings between diarylamines and bromothiophenes [126]. Triaryl-amines are important for materials applications because of their redox properties, and these particular triarylamines should be especially susceptible to electrochemical oxidation. Chart 1 shows the products formed from the amination of bromothiophenes and the associated yields. As can be seen, 3-bromothiophene reacted in higher yields than 2-bromothiophene, but the yields were more variable with substituted bromothiophenes. In some cases, acceptable yields for double additions to dibromothiophenes were achieved. These reactions all employed a third-generation catalyst (vide infra), containing a combination of Pd(OAc)2 and P(tBu)3. The yields for reactions of these substrates were much higher in the presence of this catalyst than they were in the presence of arylphosphine ligands. 

The decomposition of the dimethyl palladium(II) carbene complex with excess methyl iodide is a stepwise process. Although the authors [282] propose oxidative addition of methyl iodide on the palladium centre forming an octahedral palladium(IV) complex, it seems much more likely, with respect to the rarity of palladium(IV) compounds [284,285], that the first step is reductive elimination of an imidazolium salt, a decomposition pathway found to be fairly common after the initial publication of McGuinness et al. [283], Oxidative addition of methyl iodide followed by reductive elimination of ethane would account for the accumulation of iodide ligands on the palladium centre and a Pd(0)/Pd(II) redox couple. However, in the last step, a six coordinate Pd(IV) centre still seems to be necessary (see Figure 3.91). 

Oxidative addition and the microscopic reverse, reductive elimination, involving formal Pt(0)/Pt(II) as well as Pt(II)/Pt(IV) redox couples, have been of long-standing interest.44-47 Using bisphosphine platinum systems, ab initio calculations provided insight into the thermodynamics and activation barriers for oxidative addition reactions as a function of the substrate being activated (Scheme 11.20). The calculated... 

Transition-metal-silyl complexes are also formed by the reactions of metal-alkyl complexes with silanes to form free alkane and a metal-silyl complex. Two examples are shown in Equations 4.114 and 4.115. ° The synthesis of silyl complexes by this method has been accomplished with both early and late transition metal complexes. The formation of metal-silyl complexes from late-metal-alkyl complexes resembles the hydrogenolysis of metal-alkyl complexes to form metal hydrides and an alkane. The mechanisms of these reactions are discussed in Chapter 6. In brief, these reactions with late transition metal complexes to form silyl complexes typically occur by a sequence of oxidative addition of the silane, followed by reductive elimination of alkane. An example of this is shown in the coupling of 1,2-bis-dimethylsilyl benzene with a dimethyl platinum(II) complex (Equation 4.114). Similar reactions occur with d° early metal complexes by a a-bond metathesis process that avoids these redox events. For example, the reaction of Cp ScPh with MesSiH, has been shown to proceed through this pathway (Equation 4.115). ... 

Therefore, the enzyme peroxidase acts as an electrocatalyst (bio-electrocatalyst) eliminating the over-voltage for hydrogen peroxide reduction at the electrode. As result a significant increase of the electrode potential (AE) - anodic shift towards the equilibrium potential of the redox couple H2O2/H2O occurs. The rate of this increase (AE/At) is proportional to the hydrogen peroxide production rate. 

A tridentate ligand based on 1,8-naphthyridine (NP), which bears a ferroceneyl amide pendant arm, was synthesized and used to support dipalladium(I) and diruthenium(I) compounds [69]. The synthesis of the dipalladium(I) compound 104 started with a Pd(II) precursor, but the detail of the redox reaction was not discussed in the original article (Scheme 10.54). Complex 104 is diamagnetic and the Pd(I)-Pd(I) bond is short at 2.3952(8) A (Entry 14, Table 10.7). Compound 104 was proven to be an active catalyst for Suzuki and Heck coupling, and a bimetallic-synergy mechanism was proposed for the pivotal oxidative addition/reductive elimination steps. 

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