Redox ligands elimination

Haim and Taube reacted [CoN3(NH3)s]2+ with nitrosyl chloride in water S which led to the formation of [Co(NH3)5(H20)]3+, dinitrogen, and nitrous oxide. [Co(N3)Cl(en)2]+ underwent a similar reaction in water56 57 with formation of [CoCl(en)2(H20)]2+. Following this early work, several methods were developed that similarly employed azide ligand elimination as the result of redox transformations in non-aqueous solvents. The complex [CoN3(NH3)s]2+ and nitrosonium salts were reacted in trimethylphosphate, tetramethylene sulfone,59 or organonitriles with the formation of the... 

Platinum(IV) is kinetically inert, but substitution reactions are observed. Deceptively simple substitution reactions such as that in equation (554) do not proceed by a simple SN1 or 5 2 process. In almost all cases the reaction mechanism involves redox steps. The platinum(II)-catalyzed substitution of platinum(IV) is the common kind of redox reaction which leads to formal nucleophilic substitution of platinum(IV) complexes. In such cases substitution results from an atom-transfer redox reaction between the platinum(IV) complex and a five-coordinate adduct of the platinum(II) compound (Scheme 22). The platinum(II) complex can be added to the solution, or it may be present as an impurity, possibly being formed by a reductive elimination step. These reactions show characteristic third-order kinetics, first order each in the platinum(IV) complex, the entering ligand Y, and the platinum(II) complex. The pathway is catalytic in PtnL4, but a consequence of such a mechanism is the transfer of platinum between the catalyst and the substrate. 10 This premise has been verified using a 195Pt tracer.2011... 

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. 

It is useful to consider the reactions of carbonyl metallates separately, since their reactivity is generally concerned with the nucleophilic metal centre and will be discussed below. Simple ligand substitution reactions have already been discussed above, as have redox processes that provide access to carbonyl metallates through reduction of the metal centre. These redox or ligand addition/elimination processes are in principle no different from those encountered for classical ligands. We will now consider reactions in which the carbonyl ligand itself enters directly into the reaction and emerges transformed. 

The redox reactions of carbon free radicals and copper(II) compounds have been portrayed as ligand-transfer and electron-transfer processes 178). The electron-transfer oxidation of alkyl radicals by copper(II) complexes, which are efficient radical interceptors, is considered to proceed via a metastable alkylcopper species which is consumed primarily by oxidative elimination [Eq. (129)] and oxidative solvolysis [Eq. (130)] 143b). The anionic counterion exerts a dominant effect in the selection... 

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). 

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