Metallacyclobutane, intermediate metathesis

Crowe proposed that benzylidene 6 would be stabilised, relative to alkylidene 8, by conjugation of the a-aryl substituent with the electron-rich metal-carbon bond. Formation of metallacyclobutane 10, rather than 9, should then be favoured by the smaller size and greater nucleophilicity of an incoming alkyl-substituted alkene. Electron-deficient alkyl-substituents would stabilise the competing alkylidene 8, leading to increased production of the self-metathesis product. The high trans selectivity observed was attributed to the greater stability of a fra s- ,p-disubstituted metallacyclobutane intermediate. 

The final stereochemistry of a metathesis reaction is controlled by the thermodynamics, as the reaction will continue as long as the catalyst is active and eventually equilibrium will be reached. For 1,2-substituted alkenes this means that there is a preference for the trans isomer the thermodynamic equilibrium at room temperature for cis and trans 2-butene leads to a ratio 1 3. For an RCM reaction in which small rings are made, clearly the result will be a cis product, but for cross metathesis, RCM for large rings, ROMP and ADMET both cis and trans double bonds can be made. The stereochemistry of the initially formed product is determined by the permanent ligands on the metal catalyst and the interactions between the substituents at the three carbon atoms in the metallacyclic intermediate. Cis reactants tend to produce more cis products and trans reactants tend to give relatively more trans products this is especially pronounced when one bulky substituent is present as in cis and trans 4-methyl-2-pentene [35], Since the transition states will resemble the metallacyclobutane intermediates we can use the interactions in the latter to explain these results. 

Olefin metathesis provides the principal synthetic context for metallacyclobutane reactivity this catalytic reaction proceeds by the transient, and reversible, formation of a metallacyclobutane intermediate from a metal alkylidene and an alkene. The olefin metathesis reaction has been exhaustively reviewed and is not directly discussed here... 

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. 

Metathesis, which is reversible and can be catalyzed by a variety of organometallic complexes, has been the subject of considerable investigation, and many reviews on this topic have been published.In 1970, Herisson and Chauvin proposed that these reactions are catalyzed by carbene (alkylidene) complexes that react with alkenes via the formation of metallacyclobutane intermediates, as shown in Figure 14-20. This mechanism, now known as the Chauvin mechanism, has received considerable support and is believed to be the pathway of the majority of transition metal-catalyzed olefin metathesis reactions. 

In any chain reaction, apart from initiation steps, the termination steps are also important. In metathesis there are many possibilities for termination reactions. Besides the reverse of the initiation step, the reaction between two carbene species is also a possibility (eq. (17)). The observation that, when using the Me4SnAVCl6 system, as well as methane traces of ethylene are also observed [26] is in agreement with this reaction. Further reactions which lead to loss of catalytic activity are (1) the destruction of the metallacyclobutane intermediate resulting in the formation of cyclopropanes or alkenes, and (2) the reaction of the metallacycle or metal carbene with impurities in the system or with the functional group in the case of a functionally substituted alkene (e. g., Wittig-type reactions of the metal carbene with carbonyl groups). 

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. 

The olefin metathesis story started with the advent of well-defined catalysts, most notably the Grubbs ruthenium catalysts.These are ruthenium carbenes that undergo a [2 + 2] cycloaddition to olefins to produce a metallacyclobutane intermediate. Cycloreversion of the metallacyclobutane in the opposite sense leads to olefin metathesis (Scheme 1.35). 

Studies on these carbene complexes, especially those of the Schrock type, have attracted special interest in connection with the mechanism of catalytic olefin metathesis reactions. The formation of metallacyclobutane intermediate from the oxidative cycloaddition reaction between carbene complex and olefin was found to be an important key step in the catalytic cycle (eq. (5)). 

Olefin metathesis is a catalytic process in which alkenes are converted into new products via the rupture and reformation of carbon-carbon double bonds. The key step in this process is the 2 + 2 reaction between an olefin and a transition metal alkylidene (carbene) complex, generating an unstable metallacyclobutane intermediate. This intermediate can either revert to the starting material, or open productively to regenerate a metal carbene and produce a new olefin (Eq. 1). 

One problem in catalysis is gradual loss of activity of the catalyst. There are many reasons underlying the deactivation of heterogeneous metathesis catalysts [42]. The most important causes of catalyst deactivation are (i) intrinsic deactivation reactions, such as the reductive elimination of metallacyclobutane intermediates,... 

The interaction of metallocarbene (or alkylidene) complexes with unsaturated compounds via the formation of a metallacyclobutane intermediate plays an important role in several stoichiometric and catalytic reactions, e.g., metathesis, cycloaddition and polymerization. 

In the 1970 s an intense interest in the olefin metathesis reaction [35] served as a driving force for research concerning metal-carbon double bonds. In its simplest form the olefin metathesis reaction consists of a redistribution of alkylidene components of olefins (Eq. 6). It was known to be promoted by tungsten, molybdenum, and rhenium, although at the time the catalytic reactions were black boxes, with nothing known about the mechanism or the detailed nature of the catalyst. Among the proposals was one that consisted of a reaction between an alkylidene complex (or carbene complex at that time) and an olefin to give a metallacyclobutane intermediate, from which an olefin could be lost and a new alkylidene complex formed, [36] as shown in... 

Fig. 4.9. Olefin metathesis and cyclopropanation pathways for a metallacyclobutane intermediate.

The factors that direct a metallacyclobutane intermediate along one of these pathways are only partially understood. For example, in the case shown in Fig. 4.10, both metathesis and cyclopropanation are mediated by the same tungsten-carbene complex when the reaction is performed in a non-coordinating solvent [31]. These conditions are consistent with reaction via a metallacyclobutane because they allow the olefin to coordinate to the metal center, a prerequisite for [2 -I- 2] cycloaddition. In contrast, only cyclopropanation occurs when the reaction is performed in a coordinating solvent, presumably because formation of a metal-solvent adduct prevents olefin coordination and leaves only a direct carbene transfer mechanism available. 

Catalyst deactivation and regeneration. The activity of a rhenium-based catalyst in the metathesis of unsaturated esters is unavoidably limited by the complexation of the ester group to the active site [8]. Moreover, there are many routes that lead to deactivation of the catalyst. Polar compounds such as H2O or free acids, alcohols and peroxides, which might be present as an impurity in the substrate(s), can act as catalyst poisons. Other possible routes for the deactivation of rhenium-based catalysts include (i) reduction of the rhenium below its optimum oxidation state (ii) adsorption of (polymeric) product molecules on the surface of the catalyst, blocking the active sites (iii) reductive elimination of the metallacyclobutane intermediate [59]. Even when the greatest care is taken, deactivation of the rhenium catalyst cannot be avoided. 

Scheme 18 Stability of metallacyclobutane intermediates in propane metathesis reaction...

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