Oxidative addition versus reductive elimination

These are in principle reversible, hut because the position of equilibrium obeys the overall thermodynamics, in practice the reactions often tend to go in the oxidative or reduchve direchon only, depending on the case. Which is seen depends on the relative stabilities of the oxidahon states or, more quanhtahvely, on the A-B versus M-A and M-B bond strengths. Alkyl hydride complexes commonly eliminate alkane, but only rarely do alkanes oxidahvely add to a metal. Third row elements, which tend to have stronger metal-ligand bonds tend to give oxidahve addihon more easily. Occasionally, an equilibrium is established in which both the forward and back reachons are observed. It is typical for the two hydrogens to end up cis to one another in the product in Eq. 2.10 [6]. 

Oxididahve addihon is usually favored by strongly donor ligands because these stabilize the higher oxidation state. 

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In addition to the indirect evidence of O-complexes as intermediates in metal-mediated reductive elimination/oxidative addition transformations from isotopic scrambling, KIEs for C—H reductive elimination reactions lend credence to the intermediacy of coordinated alkane intermediates.20 Assuming coordinated alkane intermediates, the equilibria and rates of reductive elimination and oxidative addition can be described with four rate constants that correspond to reversible C—H reductive coupling/oxidative addition and dissociation/association of the hydrocarbon substrate (Scheme 11.18). Several groups have reported that the rates of C—H reductive elimination of alkanes of perprotio versus perdeuterio variants yield inverse KIEs (i.e., kyjki) < l).20... 

The Mizoroki-Heck reaction is a subtle and complex reaction which involves a great variety of intermediate palladium complexes. The four main steps proposed by Heck (oxidative addition, alkene insertion, )3-hydride elimination and reductive elimination) have been confirmed. However, they involved a considerable number of different Pd(0) and Pd(Il) intermediates whose structure and reactivity depend on the experimental conditions, namely the catalytic precursor (Pd(0) complexes, Pd(OAc)2, palladacycles), the Ugand (mono- or bis-phosphines, carbenes, bulky monophosphines), the additives (hahdes, acetates), the aryl derivatives (ArX, ArOTf), the alkenes (electron-rich versus electron-deficient ones), which may also be ligands for Pd(0) complexes, and at least the base, which can play a... 

Several studies have been conducted aiming ai the separation of steric and electronic effects [71-73]. For a single step process such as an oxidative addition or one-electron change in electrochemical processes this may be useful, but for multi-step reactions as we are dealing with in catalysis, this technique will encounter many problems. There will be different effects on the distinct steps and linear free-energy relationships will be an exception rather than the rule. When for instance both an oxidative addition step and a reductive elimination step are involved volcano curves must be expected for reactivity versus a ligand property, as in a series of metal oxides when... 

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