Oxidative addition concerted mechanism

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]. 

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. 

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... 

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. 

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. 

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. 

The characteristics of the 1,3-dipolar cycloaddition mechanism of azides and other 1,3-dipoles (such as diazoalkanes, azo-methine imines, nitrones, nitrile imines, nitrile oxides) have been described in detail by Huisgen.191 19 According to the author, the addition of a 1,3-dipole (a b c) to a dipolarophile (d e) occurs by a concerted mechanism in which the two new a bonds are formed simultaneously although not necessarily at equal rates (32). As a consequence, a stereoselective cis addition is observed. Thus, the addition of p-methoxyphenyl azide to dimethyl fiynarate (33) yields l-(p-methoxyphenyl)-4,5-froiw-dicarbomethoxy-AMriazoline (34),194 and 4-nitrophenyl azide gives exclusively the respective cis-addition products 35 and 36 on addition to irons- and cis-propenyl propyl ether.196... 

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. 

The controversy between Huisgen and Firestone concerning the mechanism for 1,3-dipolar cycloaddition is longstanding.9,11 For nitrile oxide cycloadditions, experimental data have been interpreted either as supportive of a concerted mechanism9 or in favor of a stepwise mechanism with diradical intermediates.11 Theory has compounded, rather than resolved, this problem. Ab initio calculations on the reaction of fulmonitrile oxide with acetylene predict a concerted mechanism at the molecular otbital level,12,13 but a stepwise mechanism after inclusion of extensive electron correlation.14 MNDO predicts a stepwise mechanism with a diradical intermediate.13 The existence of an extended diradical intermediate such as (4 Scheme 2) has been postulated by Firestone in order to account for the occasional formation of 1,4-addition products such as the oxime (5).11 Of course, the intermediates (4) and (5) for the Firestone mechanism do not correspond to the initial transition states in Firestone s theory. These are attained prior to the formation of, and at higher energy than, the intermediates. 

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. 

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. 

There are a number of possible mechanisms for oxidative addition and the precise one followed depends on the nature of the reacting partners. Vaska s complex [lr(PPh3)2C0CI] has been extensively studied and it reacts differently with hydrogen and methyl iodide. Hydrogen is added in a c/s fashion, consistent with concerted formation of the two new iridium-hydrogen bonds. The... 

The gas-phase reactions of the cationic Irm complexes follow a previously unreported mechanism for their observed a-bond metathesis reactions. Previous discussions had considered a two-step mechanism involving intermolecular oxidative addition of either [Cp Ir(PMe3)(CH3)]+ or [CpIr(PMe3)(CH3)]+ to the C-H bond of an alkane or arene producing an Irv intermediate, followed by reductive elimination of methane, or a concerted a-bond metathesis reaction sim-... 

Other mechanisms for oxidative additions are also well-established for example, the concerted addition of H2 to Ir(PPh3)2(CO)Cl to give Ir(PPh3)2 (C0)(H)2C1 (Ir(I) to Ir(III), also (fjcf), and a variety of radical and radical chain processes, for example, in the addition of Mel to Pt(PPh3)3. 

Three mechanisms are usually considered for oxidative addition of alkyl and aryl halides Sn2, radical and concerted. In the Sn2 pathway, the metal acts as a nucleophile, displacing the hahde from RX, followed by coordination of the halide to the metal. The oxidation state of the metal rises by two units. 

In concerted oxidative addition, the C-X bond breaking occurs with concerted formation of M-C and M X bonds. Such is often the case for aryl halides where the Sn2 process is not applicable. In contrast with the trans product of equation (17), the initial concerted oxidative addition product is often cis. The concerted mechanism is also associated with negative AS (ca. — 20eu) because the transition state is highly organized. 

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