Oxidative addition reaction pathway

Periana et al. have reported a mercury system that catalyzes the partial oxidation of methane to methanol.81 Hg(II) is typically considered to be a soft electrophile and is known to initiate electrophilic substitution of protons from aromatic substrates. The catalytic reaction employs mercuric triflate in sulfuric acid, and a key step in the catalytic cycle is Hg(II)-mediated methane C—H activation. For methane C—H activation by Hg(II), an oxidative addition reaction pathway via the formation of Hg(IV) is unlikely. Thus, an electrophilic substitution pathway has been proposed, although differentiation between proton transfer to an uncoordinated anion versus intramolecular proton transfer to a coordinated anion (i.e., o-bond metathesis) has not been established. Hg(II)-based methane C H activation was confirmed by the observation of H/D exchange between CH4 and D2S04 (Equation 11.9). 

SCHEME 2.16 Additional reaction pathway for the generation of the quinone methide in the gas phase oxidation of 2-methylphenyl radical, investigated by the hybrid functional MPW1K (reproduced from Ref. [23] with permission from American Chemical Society). 

In this chapter, theoretical studies on various transition metal catalyzed boration reactions have been summarized. The hydroboration of olefins catalyzed by the Wilkinson catalyst was studied most. The oxidative addition of borane to the Rh metal center is commonly believed to be the first step followed by the coordination of olefin. The extensive calculations on the experimentally proposed associative and dissociative reaction pathways do not yield a definitive conclusion on which pathway is preferred. Clearly, the reaction mechanism is a complicated one. It is believed that the properties of the substrate and the nature of ligands in the catalyst together with temperature and solvent affect the reaction pathways significantly. Early transition metal catalyzed hydroboration is believed to involve a G-bond metathesis process because of the difficulty in having an oxidative addition reaction due to less available metal d electrons. 

As already mentioned, the reverse reactions of Fig. 2.6 are reductive elimination reactions. By the principle of microscopic reversibility, the existence of an oxidative addition reaction means that reductive elimination, if it were to take place, would follow the reverse pathway. The reductive elimination of an alkane from a metal-bonded alkyl and hydride ligand in most cases poses a mechanistic problem. This is because clean oxidative addition of an alkane onto a metal center to give a hydrido metal alkyl, such as a reaction like Reaction 2.5, is rare. 

Alkyl halides that do not readily undergo nucleophilic attack may oxidatively add to a metal by radical mechanisms. Oxidative addition reactions that occur by radical mechanisms show loss of stereochemistry, nomeproducible rates, inhibition by radical inhibitors, and acceleration by O2 or light. Reactions of lr(Cl)(CO)(PMe3)2 with methyl and benzyl halides showed no indication of radical behavior, but other saturated alkyl halides, vinyl, and aryl halides showed characteristics consistent with a radical-chain pathway. 

Such oxidative-addition reactions play a widespread role in homogeneous catalytic processes and constitute an important pathway of activation and dissociation of H—H, C—C and C—H bonds. Correspondingly, the reverse reductive-elimination reactions represent an important process for the formation of such bonds, and thus frequently constitute the product-forming steps in catalytic cycles . ... 

In addition to these atom transfer reactions, electron transfer reactions can occur. Reduction of 17 e MLn to 18 e M L anions is a common reaction. Oxidation of 19 e MLn(A-B) adducts to 18 e ML (A-B)+ cations is also frequently observed. Thus, an additional reaction pathway of the intermediate L M (A-B)ML is disproportionation to an ionic compound L M (A-B)]+ M L . The detailed mechanisms of these reactions are more complex than what is shown in Scheme 10.1. Many of the reactions are reversible and, in addition, they can couple to other reactions. 

The second direct reaction pathway, one-electron reduction of a target by nitric oxide, could occur only if the target was itself a strong oxidant, since nitric oxide does not readily give up its unpaired electron. Oxidation of nitric oxide would result in the formation of NO, which would rapidly nitrosate nucleophiles such as amines, sulfhydryls, or aromatics. In fact, the best one-electron oxidants would be radicals such as -NOi or hydroxyl radical or even ONOO itself. In such cases the net effect would be nitric oxide addition reactions (nitrosations), regardless of whether the mechanism is considered to be transfer of an electron from nitric oxide followed by attack of NO or simple radical-radical combination. Thus, under most conditions, one-electron reduction of a target by nitric oxide becomes a simple addition reaction. 

General principles of the parahydrogen effect Oxidative addition reactions studied via parahydrogen Observations on polyhydride complexes Ligand Exchange Pathways... 

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