Oxidation-addition mechanism

From the foregoing, however, it should not be concluded that the approach of Mango and Schachtschneider is appropriate for the understanding of the metathesis reaction. The main difficulty is the supposition that the metathesis is a concerted reaction. If the reaction is not concerted, it makes no sense, of course, to correlate directly the orbitals of the reactants with those of the products. Recently, non-concertedness has been proved probable for several similar reactions, which were formerly believed to be concerted. For instance, Cassar et al. (84) demonstrated that the Rh catalyzed valence isomerization of cubane to sj/w-tricyclooctadiene proceeds stepwise. They concluded that a metallocyclic intermediate is formed via an oxidative addition mechanism ... 

These copper-catalyzed reactions are generally applicable to aryl halides with either EWG or ERG substituents. The order of reactivity is I > Br> Cl > 0S02R, which is consistent with an oxidative addition mechanism. 

Treatment of a-tocopherol (1) with elemental bromine provided quantitative yields of 5a-bromo-a-tocopherol (46). The reaction was assumed to proceed according to a radical mechanism, but later a nonradical oxidation-addition mechanism was proven (Fig. 6.33). Bromine oxidized a-tocopherol (1) to the intermediate ortho-qainone methide (3), which in turn added the HBr produced in the oxidation step.60 If the HBr was removed by flushing with nitrogen, the spiro dimer (9) became the main product, and if it was purged by HC1 gas, mainly 5a-chloro-a-tocopherol was produced. 

FIGURE 6.33 Synthesis of 5a-bromo-a-tocopherol (46) from a-tocopherol (1) according to an oxidation-addition mechanism involving the o-QM intermediate 3. 

Recently, Fu and coworkers have shown that secondary alkyl halides do not react under palladium catalysis since the oxidative addition is too slow. They have demonstrated that this lack of reactivity is mainly due to steric effects. Under iron catalysis, the coupling reaction is clearly less sensitive to such steric influences since cyclic and acyclic secondary alkyl bromides were used successfully. Such a difference could be explained by the mechanism proposed by Cahiez and coworkers (Figure 2). Contrary to Pd°, which reacts with alkyl halides according to a concerted oxidative addition mechanism, the iron-catalyzed reaction could involve a two-step monoelectronic transfer. 

Equation (1) depicts an early example of an intermolecular addition of an alkane C-H bond to a low valent transition metal complex [12], Mechanistic investigations provided strong evidence that these reactions occur via concerted oxidative addition wherein the metal activates the C-H bond directly by formation of the dative bond, followed by formation of an alkylmetal hydride as the product (Boxl). Considering the overall low reactivity of alkanes, transition metals were able to make the C-H bonds more reactive or activate them via a new process. Many in the modern organometallic community equated C-H bond activation with the concerted oxidative addition mechanism [10b,c]. 

Another instructive scenario may be found when considering the metalation of arenes. There are two distinct mechanisms for the metalation of aromatic C-H bonds - electrophilic substitution and concerted oxidative addition (Box2). The classical arene mercuration, known for more than a century, serves to illustrate the electrophilic pathway whereas the metal hydride-catalyzed deuterium labeling of arenes document the concerted oxidative addition mechanism [8, 17]. These two processes differ both in kinetic behavior and regioselectivity and thus we may appreciate the need to differentiate these two types of process. However, the choice of C-H bond activation to designate only one, the oxidative addition pathway, creates a similar linguistic paradox. Indeed, it is hard to argue that the C-H bond in the cationic cr-complex is not activated. 

Calculations show that for M = [CpIrin(PH3)(Me)]+ the oxidative addition mechanism A is the low-energy pathway, while Rhm may adopt path B.132 With complexes containing very labile ligands, such as the 171 -dichloromethane complex [Cp (PMe3)IrMe(ClCH2Cl)]+, methane activation takes place under very mild conditions and at temperatures as low as 10°C, while benzene adds rapidly at -30°C.133... 

The cleavage of the C—H bond by direct participation of a transition metal ion proceeds via an oxidative addition mechanism or an electrophilic substitution mechanism. Metals in low oxidation states undergo oxidative addition while high oxidation state metals take part in electrophilic substitutions. Another function of the metal complex in these reactions consists of abstracting an electron or a hydrogen atom from the hydrocarbon, RH. The RH radical ions or R radicals which are formed then interact with other species, such as molecular oxygen which is present in the solution or in one of the ligands of the metal complex (21). 

C6HjCH2Br (benzylbromide) Free-radical oxidative addition mechanism of 14.1.2.5.3. 

It is therefore not surprising that the reactivities of arenes and alkanes in electrophilic substitution reactions are very different, with the former being much more active. At the same time, the mechanism of the interaction (oxidative addition) of both saturated and aromatic hydrocarbons with complexes of metals in a low oxidation state is in principle the same. The reactivities of arenes and alkanes in oxidative addition reactions with respect to low-valent metal complexes therefore usually differ insignificantly. Furthermore, a metal complex via the oxidative addition mechanism can easily cleave the C-H bond in olefin or acetylene. 

Scheme IV. 12 (part 1). Activation of vinylic C-H bonds by metal complexes in a low oxidation state via an oxidative addition mechanism. Reference numbers are in brackets.

Theoretical calculations support a low-energy oxidative addition mechanism [26c], Reaction of the unsolvated cationic complex Cp Ir(PMe3)(CH3) with pentane, cyclohexane or benzene in the gas phase also gives Cp Ir(PMe3)(R) as the product. However, a mechanistic investigation of this process by electrospray tandem spectrometry has demonstrated that neither the oxidative addition-elimination mechanism nor the concerted a-bond metathesis mechanism is operative. Instead, the authors proposed a dissociative elimination-addition mechanism which proceeds through a series of 16-electron Ir(III) intermediates [26d]. 

Various other examples of the activation of C-H bonds by conventional, low-valent metal complexes have been described [27]. Not all of these reactions proceed via an unambiguous, simple oxidative addition mechanism. 

Metal complexes induce a whole series of reactions involving the substitution of hydrogen atoms in hydrocarbons by the atoms of other elements. The complex very often catalyzes such reactions. Considerable proportions of the processes proceed via the oxidative addition mechanism. After this first stage the alkyl... 

Compounds containing bonds between some other elements can react with transition metal complexes via oxidative addition mechanism to produce various derivatives. In some aspects, these reactions are related to the oxidative addition reaction of C-H bonds to low-valent metal complexes and consequently it would be interesting to consider them briefly in this section of the book. Among these processes, the activation of C-C bonds is the most important for us. 

Since silicon is an analog of carbon in the Periodic System, it is important to survey reactions of Si-H compounds with complexes of metals in low oxidation states. Similar to C-H compounds, derivatives containing Si-H bonds easily react with low-valent metal complexes via oxidative addition mechanism to afford hydridosilyl or silyl compounds. Typical examples of such reactions [70] are shown below. 

Metal complexes can add components of bonds between carbon and some other elements via oxidative addition mechanism to produce derivatives, which contain both metal-carbon and metal-element bonds. Here the element is Si, S, P, etc. [74]. Analogously, metal complexes split bonds between two atoms of an element [75] and between element and hydrogen [76]. Examples of such reactions published in recent years [77] are depicted in Scheme IV.37. 

The theoretical calculations using density functional theory showed that the intermolecular C-H activation of alkanes by the complex CpIr(PMc3)(CH3) (described by Bergman, vide supra) is a lower-energy process and that both inter- and (nfronolecular C-H activation proceed only through an oxidative-addition mechanism (Scheme VI.8) [64] (compare Scheme VI.7). 

In this chapter, we will consider the reactions of C-H compounds, such as alkanes, arenes as well as some others, with platinum complexes containing mainly chloride ligands. The reactions of alkanes with platinum(II) complexes have been the first examples of true homogeneous activation of saturated hydrocarbons in solution. Complexes of Pt(II) exhibit both nucleophilic and electrophilic properties, they do not react with alkanes via a typical oxidative addition mechanism nor can they be regarded as typical oxidants. Due to this, it is reasonable to discuss their reactions in a special chapter which is a bridge between previous chapters (devoted to the low-valent complexes) and further sections of the book that consider mainly complexes in a high oxidation state. Chloride cortplexes of platinum(IV) are oxidants and electrophiles and they will constitute the first subjects in our discussion of processes of electrophilic substitution in arenes and alkanes as well as their oxidation. 

The oxidative addition mechanism was proposed for the first step of the reaction in the original publication [2], It looked more probable in particular because of the lack of strong rate dependence on polar factors and on the acidity of the medium. Later, however, the mechanism of electrophilic substitution was proposed in some publications. 

There is no experimental evidence to support an oxidative addition mechanism followed by reductive elimination. 

The spectroscopic and magnetic data leave little doubt that a type of oxidative addition mechanism is operative in O2 binding by hemery-thrin. The two electrons necessary to reduce O2 are furnished by the two Fe(II) ions, which from the magnetic and spectral data are anti-ferromagnetically coupled Fe(III) ions in the [02 "-Fe(III)-Fe(III)] oxidative addition product. Two-electron reductive elimination of O2 by this unit completes the reversible process. The oxidative addition process is shown in Figure 17. 

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