Oxidative addition concerted

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... 

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. 

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. 

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. 

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. 

All of these reactions have recently been reviewed (206), and it is clear that the combination of the properties of both trans-pairs of ligands can be as decisive in determining the geometry of concerted oxidative addition as they can for associative nucleophilic ligand replacement. 

Examples of electrophilic interactions at the metal ions (as well as at the leaving and entering groups) have been demonstrated. These represent a formal oxidation of the metal and can lead to oxidative additions to the metal or, by transfer to a ligand, to a dissociation. On the other hand, at least some concerted oxidative additions, often described as electrophilic interactions, are seen in reality to resemble nucleophilic attack in their early stages and some of the two-step oxidative additions (most of which do conform to electrophilic attack) probably resemble the concerted mechanism in their initial interactions, the boundary being indistinct. Nucleophilic attack can, in some cases, precede electrophilic attack. 

The metal inserts into the C—H bond and the reaction may be considered as a concerted oxidative addition. This is a rare example of an allyl-hydrido-metal complex being isolated from allylic C—H bond cleavage by the metal. Other examples are the reaction of Mo(0) and Ru(0) complexes with olefins where isolable hydrido-7t-allyl-metal(II) complexes are characterized (vide infra). 

It may be assumed, that the reaction of the chloroaquoplatinum(ll) complex with atkanes begins as oxidative addition, proceeds through a three-center transition state, and terminates by the synchronous formation of a platinum-carbon bond with elimination of a proton (which can be transferred to a molecule of water). A similar mechanism has been proposed [35a] for the cyclometalation of 8-alkylquinolines by palladium(II). It has also been suggested that the reverse process (the protolysis of the platinum-carbon bond in alkyl complexes) involves a three-center transition state [35b], and the concerted oxidative addition to Ir(I) complex has been proposed [35c]. 

Oxidative addition of fluorinated aromatics to give metal(aryl) fluoro complexes has been observed frequently for complexes of group 10 metals, but rarely otherwise. The reactions include the insertion of an Ni(PEt3)2 unit into a carbon-fluorine bond in hexafluorobenzene (Scheme 5). The very slow conversion (4 weeks) was initially reported by Fahey and Mahan full spectroscopic and crystallographic characterization of the product was performed by Perutz et al " Mechanistic studies and density functional theory (DFT) calculations give strong evidence for precoordination of the aromatic substrate followed by a concerted oxidative addition. " " Oxidative addition has also been... 

Galculations based on DPT suggest that the hypothetical activation of a carbon-fluorine bond in GF4 at trans-[IrGl(PH3)2] would occur via a concerted oxidative addition mechanism and not via a single electron transfer or an SN2-type mechanism. The calculations are not related to any known reaction, but it has also been concluded that a TT-donor ligand and the heavier transition metal allow more facile oxidative addition for the complexes trans-[M(X)(PH3)2] (M = Ir, Rh X = H, GH3,... 

Route I (oxidative addition) involves a concerted oxidative addition process with the formation of metal-hydride species A. Alternatively, an electrophilic attack by the metal center on the aryl ip o-carbon may afford a metal arenium (Wheland) complex B followed by proton loss. In the agostic C-H bond activation route, the six-membered transition state C including a hydrogen-metal interaction has been found to initiate the C-H activation process, leading to an agostic intermediate D and acting simultaneously as an intramolecular base for deprotonation. 

Little mechanistic information is available for the carbene-mediated dihydrogen activation shown above. In the absence of such information, a concerted oxidative addition seems as plausible as anything else. Sketch out such a pathway using conventional arrow pushing. 

Concerted Oxidative Additions of Reagents with C-X Bonds of Medium Poiarity... 

Moreover, endo-dig cyclization products 136 or 137 are obtained by cationic ruthenium-based catalysts (Scheme 66) (118). In this catalytic system, a mechanistic pathway involving concerted oxidative addition-hydride insertion and the formation of ruthenacyclopropene intermediate is thought to be operative (118,119). 

Further supported by extensive ab initio calculations (G. Mignani, Rhodia Silicones, personal communication, 2002), the concerted oxidative addition-1,2 migratory insertion mechanism might be substantiated by the observation of a large primary kinetic isotope effect. Accordingly, we performed the hydrosilylation or 1-octene (2) in the standard conditions but using the deuterated derivative of silane 3 [16]. Unfortunately, the error related to the extracted hydrosilylation rate constant is too high to allow any quantitative conclusions ( D-hydro = 7 159 mol s ). However, this experiment disclosed a... 

Both concerted oxidative addition to and reductive elimination... 

Therefore, the mechanism must be concerted. However, in contrast to the concerted oxidative addition of H2, the addition of vinyl and phenyl C-X bonds occurs more rapidly with complexes than with d complexes. The reactions are still second order, and the normal order for leaving group reactivity is foimd. 

Aryl iodides add cleanly to zero-valent PI13P (orL) complexes of palladium. The kinetics and activation parameters support a simple concerted oxidative addition process as in (126), preceded by (125). A... 

In catalysis reactions (Chapter 9), a reductive elimination is often the last step in a catalytic cycle, and the resulting L M fragment must be able to survive long enough to react with the substrates for the organic reaction and so reenter the catalytic cycle. The eliminations of Eqs. 6.31-6.35 are analogous to the concerted oxidative additions in that they are believed to go by a nonpolar, nonradical three-center transition state, such as 6.10. Retention of stereochemistry at carbon is a characteristic feature of this group of reactions. 

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