Oxidative addition concerted additions

Finally, C—H bond activations that proceed through organometallic intermediates with M—C bonds are common mechanistic pathways toward C—H functionalizations. Several distinct mechanisms for C—H cleavage (o-bond metathesis oxidative addition concerted metalation deprotonation through 4- or 6-membered transition states) have been elucidated for this step. The exact pathway of C—H bond activation typically depends on the identity of the metal, its oxidation state, and the ancillary ligands. 

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 ... 

The familiar standard de carbonyl at ion mechanism ( 3, 5) involving a concerted oxidative-addition of aldehyde, CO migration (with subsequent elimination), and reductive-elimination of product, would seem with metalloporphyrins to require coordination numbers higher than six, and in this case Ru(IV) intermediates. Although this is plausible, the data overall strongly suggest a radical mechanism and Ru(III) intermediates. 

E) Sigma-bond metathesis. Dihydrogen is observed to react with transition-metal-alkyl bonds even when the metal lacks lone pairs. In this case the reaction cannot be explained in terms of the oxidative-addition or reductive-elimination motif. Instead, we can view this reaction as a special type of insertion reaction whereby the ctmr bond pair takes the donor role of the metal lone pair and donates into the cthh antibond. When the M—R bonds are highly polarized as M+R, the process could also be described as a concerted electrophilic H2 activation in which R acts as the base accepting H+. 

Cationic ruthenium complexes of the type [Cp Ru(MeCN)3]PF6 have been shown to provide unique selectivities for inter- and intramolecular reactions that are difficult to reconcile with previously proposed mechanistic routes.29-31 These observations led to a computational study and a new mechanistic proposal based on concerted oxidative addition and alkyne insertion to a stable ruthenacyclopropene intermediate.32 This proposal seems to best explain the unique selectivities. A similar mechanism in the context of C-H activation has recently been proposed from a computational study of a related ruthenium(ll) catalyst.33... 

This concerted process may operate in the case of d° early metal complexes where the oxidative addition is forbidden [194]. Nevertheless, it was postulated also in the interaction of a dihalo-ruthenium(II) intermediate and a hydrosilane... 

FIGURE 4.78 Mechanistic pathways for aromatic hydroxylation by concerted addition of oxene, pathway 1, or by stepwise addition of oxene, pathway 2. Pathways 2, 3, and 4 describe the formation of phenol that bypasses the arene oxide intermediate. 

As with an isolated double bond, epoxide formation in an aromatic ring, i.e., arene oxide formation, can occur mechanistically either by a concerted addition of oxene to form the arene oxide in a single step, pathway 1, or by a stepwise process, pathway 2 (Fig. 4.78). The stepwise process, pathway 2, would involve the initial addition of enzyme-bound Fe03+ to a specific carbon to form a tetrahedral intermediate, electron transfer from the aryl group to heme to form a carbonium ion adjacent to the oxygen adduct followed by... 

A concerted mechanism has also been discussed [29,30], involving either a 2+2+1 or 3+2 mechanism. To avoid trimolecular reactions this requires an interaction between Rh(I) and silanes prior to the reaction with a ketone. Interaction of silanes not leading to oxidative addition usually requires high-valent metals as we have seen in Chapter 2. The model is shown in Figure 18.16 it proved useful for the explanation of the enantiomers formed in different instances. The formation of a rhodium-carbon bond is included and thus formation of silyl enol ethers remains a viable side-path. 

Some years ago, Dr. Cross and I put forward a description of concerted reductive elimination (and, by implication, concerted oxidative addition) processes at transition metal centres, assuming the conservation of orbital symmetry, within a single dominant configuration, for the most obvious reaction path This picture had unexpected implications which some recent work has rendered quite explicit, and which are discussed in Part II of this article. 

Whether this condition can be fulfiUed depends on the electron count of the metal, and the stereochemistry of the elimination. For instance, in m-elimination from octahedral d , or square planar d , systems, metal ndipP -y ) acts as acceptor, and this should be a facile process ( e Fip. 1, 2). For /rans-elimination, on tiie other hand, the lowest empty orbital of correct symmetry is (n + l)p. Such elimination Kerns energetically less Ukely, unless a non-concerted pathway (such as successive anionic and cationic loss) is available. The same arguments apply, of course, to oxidative additions. It foUows that the many known cases of traits oxidative addition to square planar t/ systems are unlikely to take place by a concerted mechanism, and this conclusion is now generally accepted There are special complexities in reductive elimination from trigonal systems, and these are discussed furdier in Part III. 

Several mechanisms for the peroxide oxidation of organosilanes to alcohols are compared. Without doubt, the reaction proceeds via anionic, pentacoordinate silicate species, but a profound difference is found between in vacuo and solvated reaction profiles, as expected. In the solvents investigated (CH2CI2 and MeOH), the most favorable mechanism is addition of peroxide anion to a fluorosilane used as starting material or formed in situ, followed by a concerted migration and dissociation of hydroxide anion. In the gas phase, and possibly in very nonpolar solvents, concerted addition-migration of H2O2 to a pentacoordinate fluorosilicate is also plausible. ... 

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. 

The reverse reaction is reductive elimination. No mechanism is implied in reaction (13.3). The addition may be stepwise, radical, electrophilic, or nucleophilic or concerted. Oxidative additions of H—H [reaction (13.1)] or H—R [reaction (13.2)] tend to be concerted. 

This reaction has the characteristics of an oxidative addition (page 689) The formal oxidation state of [r increases from + I to +3 and the coordination number increases from 4 to 6. The process is believed to proceed via a concerted mechanism ... 

Mechanisms for oxidative additions vary according to the nature of X—Y. If X—Y is nonpolar, as in the case of H3. a concerted reaction leading to a three-centered transition state is most likely. 

While fuels are usually the source of materials compatibility problems, they also can provide some of the solutions through the use of additives. Additives from several manufacturers are available to help prevent corrosion of fuel system components and to stabilize fuel properties from oxidation. While fuel additives are helpful, they can rarely do it all themselves and must be developed in concert with knowledge of fuel system materials properties. 

Oxidative additions are frequently observed with transition metal d8 systems such as iron(0), osmium(O), cobalt(I), rhodium(I), iridium(I), nickel(II), palladium(II) and platinum(II). The reactivity of d8 systems towards oxidative addition increases from right to left in the periodic table and from top to down within a triad. The concerted mechanism is most important and resembles a concerted cycloaddition in organic chemistry (Scheme 1.1). The reactivity of metal complexes is influenced by their... 

Both complexes are octahedral, 18-electron Mn(I) species. The formation of a seven-coordinate Mn(III) intermediate by oxidative addition is unlikely and the mechanism is probably a concerted process in which no Mn-H bond is ever formed. 

Intramolecular oxidative cyclizations in the appropriately substituted phenols and phenol ethers provide a powerful tool for the construction of various practically important polycyclic systems. Especially interesting and synthetically useful is the oxidation of the p-substituted phenols 12 with [bis(acyloxy)iodo]-arenes in the presence of an appropriate external or internal nucleophile (Nu) leading to the respective spiro dienones 15 according to Scheme 6. It is assumed that this reaction proceeds via concerted addition-elimination in the intermediate product 13, or via phenoxenium ions 14 [18 - 21]. The recently reported lack of chirality induction in the phenolic oxidation in the presence of dibenzoyltar-taric acid supports the hypothesis that of mechanism proceeding via phenoxenium ions 14 [18]. The o-substituted phenols can be oxidized similarly with the formation of the respective 2,4-cyclohexadienone derivatives. 

Besides these determinations of static structures, further studies of the kinetics of oxidative addition to Ir(I) complexes have been made. Beginning with Ir(CO)H(PPh3)3 (VIII), the first step is seen as dissociation to the four-coordinate planar intermediate (IX). This then deforms and undergoes concerted cis addition to yield the adduct (V) (179,224). In a comparison of group IV hydrides, PhsMH, the rates increased in the order Sibond energy but to the effects of differing polarizability and solvation of M-Ir products on the activation energy for addition. 

These reactions, shown in Table VII, are chiefly a special type of oxidative addition process. Most important are those involving insertion of "Fe(CO)4 this is thought to occur via electrophilic Si-C cleavage in an essentially concerted process. Experiments with substituted silacyclobutanes have shown that the process is both regio- and stereospecific (135). Other metals seem reluctant to undergo this reaction the manganese analog in entry 23 is thermally very unstable. 

Metallacyclobutene complexes of both early and late transition metals can, in some cases, be prepared by intramolecular 7-hydrogen elimination, although the intimate mechanism of the reaction varies across the transition series. For low-valent late metals, the reaction is generally assumed to proceed via the oxidative addition of an accessible 7-C-H bond (Scheme 28, path A), but for early metals and, presumably, any metal in a relatively high oxidation state, a concerted cr-bond metathesis is considered most probable (path B). In this process, the 7-C-H bond interacts directly with an M-X fragment (typically a second hydrocarbyl residue) to produce the metallacycle with the extrusion of H-X (i.e., a hydrocarbon). Either sp3- or spz-hybridized C-H bonds can participate in the 7-hydrogen elimination. 

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. 

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