Four-center metathesis

Aside from two-center (Patterns 1 and 2) and three-center (Patterns 3, 4, 11, and 12) processes, most of the processes shown in Scheme 1.3 are four-center processes involving either addition (Patterns 5—10) or 0-bond metathesis (Pattern 13). In this context, it should be noted that addition is simply a four-center metathesis in which one molecule happens to be multiply-bonded. In addition to these metathetical processes, there is yet another fundamentally important four-center metathetical process termed migratory insertion and deinsertion (Patterns 14 and 15). It should be clear from Patterns 14 and 15 shown in Scheme 1.3 that distinction between insertion and deinsertion is only a relative and semantic issue. In the current discussion, a process involving cleavage of the C—Zr bond is termed migratory insertion, while the reverse process is termed migratory deinsertion. 

OCOH)(PH3)2 and a dihydrogen molecule [44], The introduction of a NH3 molecule in the system converts this single-step process in a three-step process, with three transition states. The energy change occurs smoothly and the highest barrier is only 2.1 kcal/mol (MP2//MP2) which is much smaller than the barrier in the absence of NH3 (11.4 kcal/mol, Table 4). This means that the four-center metathesis occurs with nearly no barrier in the presence of base. However, the base effects are likely overestimated in this calculations. The experimental triethylamine system cannot do all the interactions ammonia does, and polar or protic solvents should weaken the interaction between the base and the complex. 

Four center metathesis reactions. Ann. Rev. Phys. Chem. 30, 271. 

In contrast to the extensive body of work on the preparation of these zinc carbenoids, few investigations are on record concerning the mechanism of the Furu-kawa method for carbenoid formation. Two limiting mechanisms can be envisioned - a concerted metathesis via a four-centered transition structure or a stepwise radical cleavage-recombination (Scheme 3.11). 

Figure 13. Schematic representation of the transition states for the two different pathways in the four-centered (left) and six-centered metathesis (right) of RuHfn -OCOHXPIBh with a dihydrogen molecule [41], Bond distances are in A.

Another factor to be investigated in the metathesis process is the effect of bases in the reaction media. Bases such as triethylamine are added in the experimental conditions to stabilize the formic acid product because otherwise the product is thermodynamically less stable than the separate carbon dioxide and dihydrogen reactants. As discussed above, the o-bond methathesis involves the heterolytic H-H bond fission, which would be accelerated by the presence of the base. This effect was theoretically investigated in the four-center o-bond metathesis between RhOn1-... 

As proposed in the case of molecular complexes of early transition metals, the mechanism of such C-H bond activation is expected to be a <7 bond metathesis process with four-center intermediates but this has not yet been substantiated by any theoretical approach. 

High enantioselectivity (ca. 95-99% ee) is observed in this system, better than that revealed in previous reports of the hydrosilylation of imines. The mechanism is as yet unclear however, the authors propose that an active catalyst may be formed by cleavage of the Ti-F bond and generation of a Ti(III) hydride species. Insertion of an imine into the Ti-H bond, followed by a (r-bond metathesis with the silane in a four-centered transition state, may lead to the observed products. Another report on the activity of titanocene complexes as catalysts for the hydrosilylation of aid- and keti-mines also indicates formation of a Ti-H species as catalyst.188 Hydrosilylation proceeds to yield silylamines, with dependence on substitution at nitrogen and on the nature of the ligand bound to the metallocene precursor. 

Neutral dimethyl metallocenes of group 4, Cp2MMe2, are effective catalysts for the dehydropolymerization of primary silanes, via a a bond metathesis pathway.234 Given the topological similarities between the four centered transition states for olefin insertion into M-C bonds and a bond metathesis reactions, it was postulated that activation of these metallocenes by B(C6F5)3 might enhance dehydropolymerization of silanes in... 

Although the heterolytic process here is formally a concerted ionic splitting of H2 as often illustrated by a four-center intermediate with partial charges, the mechanism does not have to involve such charge localization. In other words, the two electrons originally present in the H H bond do not necessarily both go into the newly-formed M H bond while a bare proton transfers onto L or, at the opposite extreme, an external base. The term a-bond metathesis is thus actually a better description and may comprise more transition states than the simple four-center intermediate shown above, e.g., initial transient coordination of H2 to the metal cis to L and dissociation of transiently bound H- L as the final step. Examples of this type of activation will be given in this Section. 

It has been speculated (5) that the olefin metathesis reaction mech-nism involves a four-centered quasi-cyclobutane transition state. The three basic steps postulated for the reaction, namely, formation of a bis-olefin-tungsten complex, transalkylidenation and olefin exchange, may account, in general, for the initiation and propagation steps in the ringopening polymerization of cycloolefins. Several modes of termination have been considered, but suitable data to test these are not yet available. 

SCHEME 18.4 Mechanism suggested by Don Tilley et al. for the dehydrocoupling of RSiHj using CpCp HfHCl (Cp = T15-C5H5 Cp = Ti -CjMej ) as a catalyst. The catalytic cycle involves two o-bond metathesis reactions that pass through four-center transition states. " ... 

A wide variety of neutral, unsaturated, d° and d°f metal alkyls undergo bond metathesis reactions in which H2 or substrate C—H bonds are activated 22,23,73,87,105-107). These reactions are believed to proceed via four-center transition states that are accessed by initial coordination of the H—H or C—H bonds to the electrophilic metal center [Eq. (34)]. The... 

In addition, the (=Si-0)2Tani-H center catalyzed the metathesis of alkanes, an unprecedented process affording higher and lower homologs. For example, in the metathesis of propane to ethane and butanes, formation of four-centered intermediates was postulated (Scheme 51).708 In the first step, the C—11 bond is activated to yield (=Si-0)2Ta I-Prn and (=Si-0)2Tam-Pr ... 

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