CT-Bond Metathesis

We saw earlier that binuclear OA is important for first row metals that prefer to change their oxidation state by one rather than two units. The same holds for RE as shown in Eq. 6.24 (L = PBU3) and Eq. 6.25. 

These reductive eliminations tend to have a higher kinetic barrier than for C-H or C-C. In RE of C-X (X = F, OR, NR2), the tt donor X group prefers to locate at the base of the Y in the Y-shaped intermediate mentioned earlier, and thus is remote from the RE partner. Numerous cases of such REs have been reported in recent years, however, notably in connection with the Buchwald-Hartwig coupling procedure to form C-X bonds (Section 9.7 and 14.1). 

Apparent OA/RE sequences can in fact go by a different route, o-bond metathesis or a-bond complex-assisted metathesis These are most easily identified for d early metal complexes, such as Cp2ZrRCl or WMeg, where OA is forbidden, since the product would have to be d (Section 2.4). When a dP complex reacts with H2 (Eq. 3.32), path a of Fig. 6.3 is therefore forbidden and path b ox c must take over. Path b 

FIGURE 6.3 Sigma bond metathesis (paths b and c) and OA/RE (path a) are hard to distinguish for cf-cf complexes, but for cf cases, only sigma bond metathesis is allowed because OA would produce a forbidden oxidation state. 

FIGURE 6.4 Protonation has similar limitations. Protonation at the metal (path a) and at the M-R bond (path b) are hard to distinguish for complexes, but for d cases, only path a is allowed because protonation at the metal would produce a forbidden oxidation state. 

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As shown in Figure 1, the next step in the catalytic cycle of carbon dioxide hydrogenation is either reductive elimination of formic acid from the transition-metal formate hydride complex or CT-bond metathesis between the transition-metal formate complex and dihydrogen molecule. In this section, we will discuss the reductive elimination process. Activation barriers and reaction energies for different reactions of this type are collected in Table 3. 

Carbometalation), oxidative addition see Oxidative Addition) and reductive elimination see Reductive Elimination), CT-bond metathesis see a Bond Metathesis, and migratory insertion see Migratory Insertion). 

It has been concluded that the interaction of an analogous vanadium complex with hydrocarbons does not occur by a simple ct bond metathesis mechanism [35b], The alkylidene tantalum(V) complexes undergo intermolecular cyclo-metalation of the aryloxide hgand [35c] (Scheme VIII.5). 

This is a common mechanism for P-C bond formation using lanthanide and alkaline earth metal catalysts instead of oxidative additions, as in Sect. 2.1, CT-bond metathesis is a key step. 

The reaction of a late-transition-metal-alkyl complex containing d-electrons with hydrogen or a hydrocarbon to extrude alkane or arene and generate a new transition-metal-alkyl, -aryl, or -hydride complex usually occurs by a sequence of oxidative addition and reductive elimination. However, this overall transformation could also occur by a CT-bond metathesis-(Sclieme 6.5). It is difficult to determine by experiment if the reaction of a- late-transition-metal-alkyl complex occurs by the sequence of oxidative addition and reductive elimination or by o-bond metathesis. Therefore, theoretical studies have addressed this issue. In most cases, even when the process would involve a relatively high oxidation state, the process has been calculated to occur by oxidative addition and reductive elimination. ... 

It is interesting that neopentyl platinum compounds tend to decompose by -y elimination (Eq. 7.45), in contrast to the a elimination found for the Ta complexes shown in Eq. 7.39. This may imply that the mechanism in the two cases is different for example, in the Ta case, a ct bond metathesis is possible in which one alkyl might be deprotonated at the activated a-H by a second alkyl group, rather than undergo an oxidative addition of a C—H bond, which is more favorable for low-valent Pt. Related examples of -y, 8, and e elimination are shown in Eq. 7.46. 

CT-Bond metathesis gives the same outcome as oxidative addition/ reductive elimination the two situations are hard to tell apart. 

A process that involves an unprecedented B—B bond cleavage in B2cat2 promoted by two Ni(I) centers in the dimer [Ni(p -PNP)]2 has been described by Meyer and Mindiola to proceed plausibly via a binuclear oxidative addition reaction (Scheme 20A). The B NMR spectrum of [Ni(Bcat)(PNP)] clearly reveals the formation of a rare example of a nickel-boryl (47 ppm). Alternatively, the [Ni(Bcat)(PNP)] complex can be prepared by o-bond metathesis from [Ni(0 Bu)(PNP)] and B2cat2 (Scheme 20B) A proposed intermediate could be detected by NMR spectroscopy and confirmed by DFT calculations as the isomer that conducts the CT-bond metathesis. The weak interaction between the nickel center and the tethered borane fragment in B2cat2 is rationaHzed as a consequence of the high energy required to access the empty dj(2 y2-orbital in a stericaUy encumbered square planar environment. 

The essential aspect of the u-bond metathesis mechanism is the concerted exchange of a metal-ligand a-bond with one of an incoming substrate where the reaction proceeds via a [2o-+2o-] cycloaddition, as illustrated in the transitions state in Figure 14.6. The key concept is that CT-bond metathesis is a one-step reaction with two u-bonds breaking (M-C and C -H) and two u-bonds forming (M-C and C-H). Therefore, there is no change involved in the oxidation state of the metal center. This reaction mechanism has been proposed for the d° and d°P metal compounds, and has been recently reviewed for transition metals. [58,59]... 

The basic difference between OA and cr-bond metathesis lies in the difference between the structures of the two transition states. Note that in cT-bond metathesis, unlike OA, there is no change in the oxidation... 

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