O-bond metathesis pathway

The activation of C—H bonds by a o-bond metathesis pathway involves coordination of the C—H group followed by concerted C—H bond breaking of the activated substrate and C—H bond formation with a hydrocarbyl ligand. Thus, the reaction proceeds through a four-centered transition state (Scheme 11.8). 

Alkane metathesis is a reaction that leads to the formation of lower and higher alkane homologs via an apparent o-bond metathesis pathway (Scheme 6.22 Chapter 2) [91-93]. Overall, two o-bonds are broken and formed. This reaction... 

When a metal monohydride is considered as the active catalyst, the commonly accepted reaction mechanism suggests that the first step is COj insertion into the metal hydride bond to give a metal formate intermediate. The formate intermediate then undergoes metathesis with H2 to give the formic acid product and regenerate the metal hydride species. Two pathways are possible for the metathesis between the formate intermediate and H2, namely, the o-bond metathesis pathway and oxidative addition of H2 followed by the reductive ehmination pathway (Scheme 6.1). 

In 2002, Sakaki and coworkers carried out a theoretical study on the mechanism of hydrogenation of CO2 catalyzed by the three-coordinate, 14e d monohydride model complex RhH(PH3)2 [13]. It was found that CO2 insertion into the Rh-H bond is almost barrierless. This result suggests that the 14e rhodium(I) hydride is very reactive toward CO2. The COj insertion leads to formation ofthe rhodium (I) formate intermediate Rh(PHg)2(0C(0)H). To the rhodium(I) formate intermediate, oxidative addition of H2, which leads to formation of a dihydride complex, requires a barrier of 7.3 kcal mol". Then the reductive elimination of formic acid in the dihydride complex via a five-membered ring transition state (1) was calculated to have a very small barrier of 1.9 kcal mol . In the o-bond metathesis pathway, a barrier of only 6.1 kcal mol" was calculated. The o-bond metathesis... 

The initially proposed mechanism [14], and one that continues to be considered as the likely pathway for most variants, involves the oxidative cyclization of a Ni(0) complex of an aldehyde and alkyne to a metallacycle (Scheme 18). Metallacycle formation could proceed independently of the reducing agent via metallacycle 19, or alternatively, metallacycle 20a or 20b could be formed via promotion of the oxidative cyclization transformation by the reducing agent. Cleavage of the nickel-oxygen bond in a o-bond metathesis process generates an alkenyl nickel intermediate 21. In the variants involv-... 

A reaction which is rather new and not mentioned in older textbooks is the so-called o-bond metathesis. It is a concerted 2+2 reaction immediately followed by its retrograde reaction giving metathesis. Both late and early transition metal alkyls are prone to this reaction, but for d° early transition metals there is no other mechanism than o-bond metathesis at hand. Many similar reactions such as the reaction of metal alkyls with other HX compounds could be described as if they would follow this pathway, but the use of the term o-bond metathesis is restricted to those reactions in which one reacting species is a metal hydrocarbyl or metal hydride and the other reactant is a hydrocarbon or dihydrogen. In Figure 2.30 the reaction has been depicted. 

Computational studies performed on model complexes in collaboration with Hall and coworkers suggest that alkane borylation may occur by a ej-bond metathesis pathway (Scheme 3) [48]. The proposed mechanism for the borylation of alkanes by 1 begins with elimination of HBpin to generate the 16-electron complex Cp Rh(Bpin)2. This complex then forms a <7-complex (3) with the alkane. The vacant p-orbital on boron then enables cr-bond metathesis to generate a o-borane complex (4). Reductive elimination of the alkylboronate ester product and oxidative addition of B2pin2 then regenerate 1. 

The recognition that electron-rich late transition metals can activate C—H bonds via oxidative addition or by O-bond metathesis reactions poses a challenging mechanistic question. That is, for late transition metals with d-electron counts greater than zero, overall metathesis between M—R and R -H substrates to give M—R and R—H could proceed via concerted o-bond metathesis or through stepwise oxidative addition and reductive elimination sequences (e.g., see Scheme 11.29). If an oxidative addition intermediate is not observed, the experimental distinction between the two pathways is quite subtle and challenging. 

Periana et al. have reported a mercury system that catalyzes the partial oxidation of methane to methanol.81 Hg(II) is typically considered to be a soft electrophile and is known to initiate electrophilic substitution of protons from aromatic substrates. The catalytic reaction employs mercuric triflate in sulfuric acid, and a key step in the catalytic cycle is Hg(II)-mediated methane C—H activation. For methane C—H activation by Hg(II), an oxidative addition reaction pathway via the formation of Hg(IV) is unlikely. Thus, an electrophilic substitution pathway has been proposed, although differentiation between proton transfer to an uncoordinated anion versus intramolecular proton transfer to a coordinated anion (i.e., o-bond metathesis) has not been established. Hg(II)-based methane C H activation was confirmed by the observation of H/D exchange between CH4 and D2S04 (Equation 11.9). 

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. 

As the deuterium labeling studies suggested that the 2-picoline C-H activation chemistry proceeded by cr-bond metathesis for both the thorium and uranium Cp 2 AnMe2 complexes, we examined the role of the actinide metal center in hydrogen atom migration. We identified the transition states for the sp and sp C-Hbond activation pathways if the proposed o-bond metathesis mechanism was operative. 

Mechanistic interpretations of the copper-catalyzed aromatic nucleophilic substitution reactions remain unsettled even after half-a-century of debate [19, 20]. Possible pathways involve an S Ar reaction mediated by copper complexation to the pi-system (Scheme 4a), an electron transfer reaction followed by halide dissociation (Scheme 4b), four-centered c-bond metathesis reaction (Scheme 4c) and Cu(l) oxidative addition to the Ar-X bond, followed by the nucleophile exchange and reductive elimination in the resulting Cu(lll) system (Scheme 4d). There is presently a considerable body of experimental and theoretical data for and against each of the proposed mechanisms [21]. While the mechanistic studies were mostly related to the formation of C-C, C-O and C-N bonds, it is likely that the copper-catalyzed halogen exchange reactions follow a similar trend. 

P and R have similar coordination stractures with an Mo-O-Si bond in terms of both their comparable NMR data and initial TOFs for olefin metathesis. The chemical bonding between the Si02 surface and the Mo imido alkylidene complex is suggested to prevent some deactivation pathways such as dimerization of reactive intermediates, resulting in longer Hfetime under metathesis reaction conditions [57]. The SiO -supported Mo imido alkylidene complex (P) was also efficient for alkane metathesis [58]. 

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