Metal complexes heterolytic oxidations

Eq. (8) requires determination of the two-electron oxidation potential of L M by electrochemical methods. When combined with the two-electron reduction of protons in Eq. (9), the sum provides Eq. (10), the AGh- values of which can be compared for a series of metal hydrides. Another way to determine the AGh-entails the thermochemical cycle is shown in Scheme 7.3. This method requires measurement of the K of Eq. (11) for a metal complex capable of heterolytic cleavage of H2, using a base (B), where the pK., of BH+ must be known in the solvent in which the other measurements are conducted. In several cases, Du-Bois et al. were able to demonstrate that the two methods gave the same results. The thermodynamic hydricity data (AGh- in CH3CN) for a series of metal hydrides are listed in Table 7.4. Transition metal hydrides exhibit a remarkably large range of thermodynamic hydricity, spanning some 30 kcal mol-1. 

Although hydroxyl radical is commonly assumed to be the most toxic of the oxygen radicals (with little direct evidence), other direct reactions are more likely to be important for understanding the cytotoxicity of peroxynitrite. A second oxidative pathway involves the heterolytic cleavage of peroxynitrite to form a nitronium-like species (N02 ), which is catalyzed hy transition metals (Beckman et al., 1992). Low molecular weight metal complexes as well as metals bound in superoxide dismutase and other proteins catalyze the nitration of a wide range of phenolics, including tyrosine residues in most proteins (Beckman et al., 1992). 

There are also several situations where the metal can act as both a homolytic and heterolytic catalyst. For example, vanadium complexes catalyze the epoxidation of allylic alcohols by alkyl hydroperoxides stereoselectively,57 and they involve vanadium(V) alkyl peroxides as reactive intermediates. However, vanadium(V)-alkyl peroxide complexes such as (dipic)VO(OOR)L, having no available coordination site for the complexation of alkenes to occur, react homolyti-cally.46 On the other hand, Group VIII dioxygen complexes generally oxidize alkenes homolytically under forced conditions, while some rhodium-dioxygen complexes oxidize terminal alkenes to methyl ketones at room temperature. 

A major question is whether heterolytic cleavage of the H3C-H bond occurs as depicted in Scheme 10 (maintaining the Ptn state) or whether oxidative addition to a PtIV methyl hydride complex takes place. In such systems transfer of protons would be expected to be very facile because of the extremely high mobility of H +, and even a short-lived, very weak a complex could be a key intermediate. The C-H bond is likely to be polarized towards Cs H5+ on such highly electrophilic cationic metal complexes, where H+ can very rapidly split off and transfer to either a cis ligand or the anion as soon as the... 

Historically the homolytic type of catalysis has been known and studied for a long time. The heterolytic catalysts represent a relatively recent innovation but, nevertheless, include important developments such as the Wacker process for the oxidation of olefins. Regardless of the mechanism involved, the most important characteristics of metal catalysts for effecting oxidation are the accessibility of several oxidation states as well as the accommodation of various coordination numbers, both of which are properties of transition metal complexes. 

Homolytic autoxidations of hydrocarbons often give complex mixtures of products-the autoxidation of olefins is a prime example. There is a great incentive, therefore, to search for catalysts that can promote the selective oxidation of olefins by essentially nonradical mechanisms. For example, there is no method available for carrying out the selective epoxidation or oxidative cleavage of olefins (see Section III.C) by molecular oxygen. In order to be successful, any heterolytic pathway for the metal-catalyzed oxidation of a substrate must, of course, be considerably faster than the ubiquitous homolytic processes for autoxidation. Thus, the metal catalysts discussed in the following sections, in addition to being able to promote heterolytic oxidations, are also able to catalyze homolytic processes. 

We have seen in the first section how the concepts of electron and ligand transfer via 1-electron changes provides a basis for the understanding of homolytic oxidation mechanisms. Similarly, the concepts of substrate activation by coordination380 to metal complexes and by oxidative addition381 386 provide a basis for discussing heterolytic mechanisms. Examples of the former are the activation of hydroperoxides (Section III.B.2) and olefins (Section III.D) to nucleophilic attack by coordination to metal centers. 

Reaction (414) is formally analogous to the oxidative addition of alkyl halides to noble metal complexes described earlier, and both homolytic and heterolytic processes can be envisaged. Heterolytic cleavage of C—H bonds represented in Eq. (415) is analogous to the interaction of the powerful oxidant Co3+ with alkanes in TFA in reaction (229). 

Ans. No. Other mechanisms (radical, heterolytic, etc.) may be available with transition metal complexes. With lanthanide complexes in the highly stable 3 + oxidation state, an OA/RE-based mechanism is not possible. 

Heterolytic oxidations generally involve the (metal-mediated) oxidation of a substrate by an active oxygen compound, e.g. H202 or R02H. Alternatively, stoichiometric oxidation of a substrate by a metal ion or complex is coupled with the reoxidation of the reduced metal species by the primary oxidant (e.g. 02 or H202). 

A database search yields more than 20 000 references that contain Z-E isomerization, cis-trans isomerization, or geometric isomerization as keywords, and the general tendency is an increase in the number of papers devoted to the kinetic aspects of cis-trans isomerization (CTI) in all fields. The main isomerization pathways have probably been discovered, though many remain the object of intense theoretical (see Chapter 7) and experimental research (see Chapters 4—6, 8-10, 13, and 14). In the present chapter, general CTI mechanisms will be divided into homolytic and heterolytic cleavage of the 7r-bond which allows isomerization, though some molecular motifs such as amides are able to switch from cis to trans via both processes. An overview of CTI in metal complex (mainly thermal, photochemical, and oxidative isomerizations) will be the purpose of Chapter 14 and will not be detailed here. 

The term homolytic splitting of Hj by metal complexes, in contrast to heterolytic cleavage (H2 H + H" ) to be discussed in the following section, is generally used synonymously with oxidative addition. Equation (a) illustrates an oxidative addition reaction of Hj to a univalent metal center ... 

Peroxides of transition metals are themselves active intermediates in heterolytic and homolytic liquid-phase catalytic oxidation reactions of alkenes, aromatic hydrocarbons and alkanes. Heterolytic oxidations are characterized by a requirement for a free coordination volume near the transition metal atom. Homolytic oxidations proceed via M-O bond cleavage in peroxo complexes. 

Hydrocarbons can also be activated through reactions that do not involve an oxidative addition to an unsaturated metal complex but rather proceed by a four-centered mechanism. This is usually called heterolytic activation of C-H bonds because these bonds are polarized in the transition state, by action of an electrophilic metal and a... 

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