Metathesis dissociative mechanism

Based on the insight that a dissociative mechanism plays the major role along the metathesis pathway [11], these catalysts have been designed such that only one bulky phosphine, one chloride and one cumulenylidene ligand are attached to a Ru(II) center. Because arene ligands are known to be labile on such a metal fragment, they will easily liberate free coordination sites ( ) for the interaction with the alkene substrate. Although the precise mode of action of such allenyli-... 

As mentioned in the introduction, early transition metal complexes are also able to catalyze hydroboration reactions. Reported examples include mainly metallocene complexes of lanthanide, titanium and niobium metals [8, 15, 29]. Unlike the Wilkinson catalysts, these early transition metal catalysts have been reported to give exclusively anti-Markonikov products. The unique feature in giving exclusively anti-Markonikov products has been attributed to the different reaction mechanism associated with these catalysts. The hydroboration reactions catalyzed by these early transition metal complexes are believed to proceed with a o-bond metathesis mechanism (Figure 2). In contrast to the associative and dissociative mechanisms discussed for the Wilkinson catalysts in which HBR2 is oxidatively added to the metal center, the reaction mechanism associated with the early transition metal complexes involves a a-bond metathesis step between the coordinated olefin ligand and the incoming borane (Figure 2). The preference for a o-bond metathesis instead of an oxidative addition can be traced to the difficulty of further oxidation at the metal center because early transition metals have fewer d electrons. 

That initiation involves dissociative substitution of a phosphine ligand from catalysts 23 and 24 (rather than associative displacement) is indicated by several experimental observations. Metathesis activity for these catalysts is severely depressed in the presence of excess PR3. The values for degenerate phosphine exchange in these systems are 13eu, indicative of a dissociative mechanism (via the 14-electron alkylidene 25) for this process. Furthermore, the rate of this exchange is identical to the rate of initiation (when corrected for temperature), as measured by the irreversible reaction of 23 and 24 with ethyl vinyl ether (Scheme 13). Thus, the propensity for PR3dissociation is directly related to a given catalyst s ability to initiate and enter the catalytic cycle shown within the dotted lines of Scheme 12. 

With the general mechanism of olefin metathesis established by experimental work, early theoretical studies focused on the details of several of the steps outlined above. Ligand exchange to form the initial olefin complex could occur by either an associative or dissociative mechanism. Experimental evidence from Grubbs and coworkers [2] pointed to a dissociative process. The structure of the active olefin complex was also a matter of uncertainty, as both bottom-bound (trans to L) and side-bound (cis to L) complexes have been reported (Chapter 8). Finally, the detailed structure and reactivity of the metallacyclobutane have been the focus of several theoretical investigations, as this intermediate was not initially experimentally observed (Chapter 8). 

Scheme 7.8 Acetylene insertion into the Ru-methylidene bond in intermolecular enyne metathesis via a dissociative mechanism. Gibbs free energies (in kcal/mol) were computed...

It is generally accepted that the Ru-catalyzed alkene metathesis reaction proceeds via a dissociative mechanism, which is initiated by the dissociation of a phosphine ligand from RuX2(PR3)L(=CHR) to form a 14-electron species ( B ). A i in this sense, catalyst initiation involves the dissociation of PCyj for both I and n. However,... 

In this chapter, theoretical studies on various transition metal catalyzed boration reactions have been summarized. The hydroboration of olefins catalyzed by the Wilkinson catalyst was studied most. The oxidative addition of borane to the Rh metal center is commonly believed to be the first step followed by the coordination of olefin. The extensive calculations on the experimentally proposed associative and dissociative reaction pathways do not yield a definitive conclusion on which pathway is preferred. Clearly, the reaction mechanism is a complicated one. It is believed that the properties of the substrate and the nature of ligands in the catalyst together with temperature and solvent affect the reaction pathways significantly. Early transition metal catalyzed hydroboration is believed to involve a G-bond metathesis process because of the difficulty in having an oxidative addition reaction due to less available metal d electrons. 

Such cases are not uncommon, but full quantitative treatments are rare, since often relatively large amounts of Y must be added to obtain measurable effects. Complications may then arise from the effects of the added Y on the nature of the medium (see Chapters 2 and 3). These are particularly notable when Y and I are charged, as is often the case. Under those circumstances, maintenance of the constant ionic strength of the medium with a known non-participating ionic species is essential. The classic case of common ion depression in solvolysis of benzhydryl chloride is dealt with in Chapter 2. A more recent example of this kind of treatment with neutral reactants occurs in the elucidation of the mechanism of olefin metathesis [20], catalysed by the ruthenium methylidene 9, Scheme 9.6. With ca. 5% of 9, disappearance of diene 10 was clearly not first order. However, reactions run in the presence of large excesses of phosphine 11 were much slower and showed first-order kinetics. The plot of kQ K against 1/ [ 11 ] was linear, consistent with dissociation of 9 to yield an active catalytic species prior to engagement with the diene, with k t [11] 3 > fc2[diene]. Because first-order kinetics were observed under these conditions, determination of order with respect to the catalytic species (as well as the diene) was simplified, and an outline for the mechanism could be constructed (see also Chapter 12 for more detailed consideration of catalysed olefin metathesis). 

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 accepted mechanism for olefin metathesis proceeds through formation of a metallacyclobutane after olefin coordination to the 14e species. Piers et al. have collected the first evidence for the metallacyclobutane intermediate 19 in the condensed phase [52], The proposed C2V symmetry of this key structure has been predicted by calculations [53] (for related theoretical investigations on olefin metathesis, see [54-57]). Metallacyclobutane formation is likely to determine the regio- and stereochemical outcome of the metathesis reaction, and insight into its geometry is therefore critical in the development of new, selective catalysts. Cycloreversion and olefin dissociation complete the catalytic cycle to re-form the catalytically active species ([Ru] = CH2) which can bind phosphine to re-form the precatalyst or olefin for a subsequent metathesis transformation. 

An exception to the general rule that 17-electron complexes react by strictly associative mechanisms has been reported. The alkyl for iodide metathesis in the 17-electron complex CpCr(NO)(PPh3)I is inhibited by excess phosphine. The reaction requires formation of a THE solvate CpCr(NO)(THF)I, which has been characterized. Although the formation of this intermediate takes place by an associative pathway, the requirement that a two-electron ligand dissociate prior to the replacement of iodide by the alkyl group puts this transformation of a 17-electron complex into a different mechanistic class. 

Almost all of the reactions of metals can be classified into just a few typical reactions, and the reactions that metals promote in organic chemistry are simple combinations of these typical reactions. If you learn these typical reactions, you will have no trouble drawing metal-mediated mechanisms. The typical reactions of metal complexes are ligand addition/ligand dissociation/ligand substitution, oxidative addition/reductive elimination, insertion/j8-elimination, a-insertion/ a-elimination, cr-bond metathesis (including transmetallations and abstraction reactions), [2 + 2] cycloaddition, and electron transfer. 

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