Concerted metathesis

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). 

A similar pattern of reactivity has been reported for platinum(II) complexes [39d]. Ab initio calculations demonstrated that an oxidative addition-reductive elimination mechanism is more likely than the concerted metathesis [39e]. 

Metathesis is a catalyzed reaction that converts two olefin molecules into two different olefins. It is an important reaction for which many mechanistic approaches have been proposed by scientists working in the fields of homogenous catalysis and polymerization. One approach is the formation of a fluxional five-membered metallocycle. The intermediate can give back the starting material or the metathetic products via a concerted mechanism ... 

Application of the forbidden-to-allowed concept to the metathesis reaction implies that the metathesis is considered to be a concerted cyclo-... 

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 ... 

It is clear that a detailed mechanism for the metathesis reaction of alkenes cannot yet be given with certainty. In view of the fact that, for similar reactions which are formally cyclobutane-dialkene transformations, a nonconcerted reaction pathway has been demonstrated, a concerted fusion of two alkenes to form a cyclobutane complex and its decomposition in the same way with a change in the symmetry plane is less probable. On the basis of the information on the two other mechanisms to date, the mechanism involving a metallocyclic intermediate is more plausible than a mechanism involving carbene complexes. 

Osborn and Green s elegant results are instructive, but their relevance to metathesis must be qualified. Until actual catalytic activity with the respective complexes is demonstrated, it remains uncertain whether this chemistry indeed relates to olefin metathesis. With this qualification in mind, their work in concert is pioneering as it provides the initial experimental backing for a basic reaction wherein an olefin and a metal exclusively may produce the initiating carbene-metal complex by a simple sequence of 7r-complexation followed by a hydride shift, thus forming a 77-allyl-metal hydride entity which then rearranges into a metallocyclobutane via a nucleophilic attack of the hydride on the central atom of the 7r-allyl species ... 

E) Sigma-bond metathesis. Dihydrogen is observed to react with transition-metal-alkyl bonds even when the metal lacks lone pairs. In this case the reaction cannot be explained in terms of the oxidative-addition or reductive-elimination motif. Instead, we can view this reaction as a special type of insertion reaction whereby the ctmr bond pair takes the donor role of the metal lone pair and donates into the cthh antibond. When the M—R bonds are highly polarized as M+R, the process could also be described as a concerted electrophilic H2 activation in which R acts as the base accepting H+. 

Cleavage of Zr—C a bonds occurs readily on treatment with H20 or dilute acids, while the Zr—Cp bond usually survives mild protonolysis conditions. The use of D20 or DC1/D20 permits the replacement of Zr with D. Deuterolysis provides a generally reliable method for establishing the presence of Zr—C bonds. Protonolysis or deuterolysis of Zr—Csp bonds proceeds with retention of configuration [97]. In the hydrozirconation of terminal alkynes, deuterium can be introduced at any of the three positions in the vinyl group in a completely regio- and stereoselective manner, as shown in Scheme 1.18. Although relatively little is known about the mechanistic details, the experimental results appear to be consistent with concerted c-bond metathesis (Pattern 13) between C—Zr and H— X bonds. 

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. 

Olefin metathesis proceeds via reversible formation of metallacyclobutanes by [2 + 2] cycloaddition (Figure 1.7). The precise pathway for such a cycloaddition has been calculated for molybdenum complexes such as 1 (Figure 1.6) [9]. These calculations suggest that although Mo-C and C-C bond formation is concerted the Mo-C bond is formed more quickly than the C-C bond. It was also found, beautifully consistent with experimental results, that the activation barrier for [2 + 2] cycloaddition is lowered by increasingly electron-withdrawing alkoxy ligands. 

Some of the colleagues with whom I discussed my intention to write on this topic expressed polite scepticism. There is good historical reason for this. Several previous writers had amid general admiration used frontier orbital theories to rationalise such processes as concerted trans-addition, square four-carbon arrays in olefin metathesis, chelation of a metal by two strained carbon-carbon a-bonds, and trigonal bipyramidal Fe(II) alkyls. Belief in these is less widespread than it was five or ten years ago, and... 

The alkene metathesis reaction was unprecedented - such a non-catalysed concerted four-centred process is forbidden by the Woodward-Hoffmann rules - so new mechanisms were needed to account for the products. Experiments by Pettit showed that free cyclobutane itself was not involved it was not converted to ethylene (<3%) under the reaction condition where ethylene underwent degenerate metathesis (>35%, indicated by experiments involving Di-ethylene) [10]. Consequently, direct interconversion of the alkenes, via an intermediate complex (termed a quasi-cyclobutane , pseudo-cyclobutane or adsorbed cyclobutane ) generated from a bis-alkene complex was proposed, and a detailed molecular orbital description was presented to show how the orbital symmetry issue could be avoided, Scheme 12.14 (upper pathway) [10]. 

In 1970, Chauvin and Herisson presented a study of the co-metathesis of cycloalkene/alkene mixtures using a WOCLj/SnBuj pro-catalyst mixture [12]. Whilst the fully quantitative analysis of the product mixtures was made complicated by the range of techniques that were required for the low, medium and high molecular weight products (mono alkenes, telomers and polymers), it became clear that product ratios were not consistent with what would be predicted by either mechanism in Scheme 12.14. The analysis and associated mechanistic interpretation were seminal and worthy of consideration in some detail here. The key point is that both mechanisms in Scheme 12.14 are pairwise, i.e. each turnover of the catalyst cycle involves two alkenes that undergo concerted alkylidene exchange. When a single alkene, e.g. pent-2-ene (C5), is considered, the products of alkylidene metathesis... 

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. 

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. 

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-... 

These observations, along with kinetic isotope effect studies and Hammett correlation studies, support a concerted elimination by a-bond metathesis involving a four-membered transition state (Eq. 2) [23]. A large kinetic isotope effect is observed for the loss of methane from methyl amide complexes lb and lh (Eqs. 3 and 4), comparable to those observed by Buchwald and coworkers for formation of zirconium rj2-thioaldehyde complexes [25] and by Bercaw and coworkers for formation of tantalum rf- imine complexes [5a] through similar transition states. 

Several reactions in organometaUic chemistry also appear to contravene the rule, but which can be explained in a somewhat similar way. Hydrometallation [5.45, see (Section 5.1.3.4) page 162], carbometallation, metallo-metallation, and olefin metathesis reactions are all stereospecifically suprafacial [2 + 2] additions to an alkene or alkyne, for which the all-suprafacial pathway is forbidden. Hydroboration, for example, begins with electrophilic attack by the boron atom, but it is not fully stepwise, because electron-donating substituents on the alkene do not speed up the reaction as much as they do when alkenes are attacked by electrophiles. Nevertheless, the reaction is stereospecifically syn—there must be some hydride delivery more or less concerted with the electrophilic attack. The empty p orbital on the boron is the electrophilic site and the s orbital of the hydrogen atom is the nucleophilic site. These orbitals are orthogonal, and so the addition 6.126 is not pericyclic. 

Using the triptycene-based homotritopic host 32, Chen et al. observed the tris[2] pseudorotaxane with 15 using ESI-MS <2005JA13158>. After metathesis, MALDI-TOF (TOE = time of flight) MS, X-ray crystallography (summarized in Section 14.22.3.1), and H and NMR were used in concert to prove synthesis of the [4]catenane. 

A further hypothesis, the Rice-Teller hypothesis of least motion, is a powerful tool in understanding the chain kinetics. This states that in a metathesis, that path is most likely which involves least concerted motion of nuclei. 

The reaction of PhSiHs with the Ni-Me precursors has been shown to resnlt in the formation of ohgosilanes (PhSiH) . The initial step of this reaction is beheved to go throngh a concerted a-bond metathesis (Kh/Kd 10), bnt mnltiple intermediates are formed in the later stages of the catalysis, some of which result from the reductive elimination see Reductive Elimination) of the indenyl ligand (Scheme 8). ... 

The activation of C-H bonds by d° metal centers can be investigated very effectively with complexes of the type t-Bu3SiNH)3ZrR, where R is an alkyl or aryl group, as these compounds undergo reversible elimination of hydrocarbon. In these examples, concerted mechanisms involving R-H bond interaction at d° metal imido complexes, either isolable or transient, give metathesis products via R-H elimination. ... 

A reaction which is relatively new is the so-called a-bond metathesis [39,40]. Tlie word metathesis is used because of the now well-known metathesis of alkenes and alkynes. It is a concerted 2+2 reaction immediately followed by its retrograde reaction giving metathesis. The transition state is strongly polarized, such that in the reaction of M-R with H-H (Fig. 4.34), the transition state contains a [FI...H...R]"M unit with large negative charges at the terminal groups. 

Mechanism 3 shows a pathway that was strongly influenced by the results of Herisson and Chauvin and is outlined in Scheme 11.2. Two key intermediates in this pathway are an alkene-metal carbene complex (5) and a metallacyclobu-tane (6), formed through concerted cycloaddition of the M=C and C=C bonds. A highly significant feature of the mechanism, caused by the unsymmetrical structure of 6, is its explanation of randomization early in the course of reaction. The Herisson-Chauvin mechanism does not require a specific pair of alkenes to interact directly for metathesis to occur, hence the name non-pairwise mechanism. 

Transmetalation may proceed via a concerted cr bond metathesis, involving a four-center transition state (38) (or even a metallacyclobutane intermediate) that leads to transfer of the organic group R to M with retention of configuration. 

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