Alkene metathesis pathway

Alkene Metathesis Pathway for Well-Defined Catalysts... 

Scheme 6.12 Alkene metathesis pathway, with Mo(VI) and Mo(VI) alkylidenes supported on alumina.

The mechanism is suggested to proceed by a composite series of a-bond metatheses of C-H bonds and a- and (3-eliminations (rather than direct a-bond metathesis of C-C bonds). The a-elimination from a d metal-methyl or metal-alkyl species generates respectively HTa=CH2 or HTa=CHR, and the mechanism is proposed to then follow an alkene metathesis pathway with the olefins being generated by (3-elimination (including metallacyclobutane intermediates as in the Chauvin 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-... 

Scheme 8.1 Initial pathways for the synthesis of allenylidene-ruthenium alkene metathesis catalysts.

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]. 

Besides enyne metathesis [66] (see also the chapter Recent Advances in Alkenes Metathesis in this volume), which generally produces 1-vinylcyclo-alkenes, ruthenium-catalyzed enyne cycloisomerization can proceed by two major pathways via hydrometallation or a ruthenacycle intermediate. The RuClH(CO)(PPh3)3 complex catalyzed the cyclization of 1,5- and 1,6-enynes with an electron-withdrawing group on the alkene to give cyclized 1,3-dienes, dialkylidenecyclopentanes (for n=2), or alkylidenecyclopentenes (for n= 1) [69,70] (Eq. 51). Hydroruthenation of the alkyne can give two vinylruthenium complexes which can undergo intramolecular alkene insertion into the Ru-C bond. 

Opening of a strained ring system and the subsequent coupling with an acyclic alkene results in the formation of diene products. Because of many metathesis pathways available in the systems containing a cyclic and a linear olefin, the... 

Electron-rich alkenes also react with electrophilic carbene complexes, as equation 10.48 shows. Although reaction temperatures tend to be lower than necessary for cyclopropanation of electron-poor alkenes, the distribution of products is a function of CO pressure. In the absence of CO, alkene 62 forms predominantly.72 At 100 bar CO pressure, cyclopropane formation predominates. Presumably, at low CO pressure a metathesis pathway (Chapter 11) can occur. At high CO pressure, loss of CO is unlikely, so RE elimination to form cyclopropane predominates. 

W, and Re alkene metathesis catalysts. However, the constraints imposed by the bidentate ligand lead to a more complicated pathway that imposes a metallacyclobutane isomerization [105]. 

A common theme in many reactions is the generation of an equivalent of CP2M through the reduction of a tetravalent precursor metallocene with a metal alkyl such as n-butyllithium. The chemistry of olefin complexes of CP2M is characterized by a facile interconversion between formally M(II) and M(IV) manifolds, as shown in equation 34. Another central motif of metallocene chemistry, especially that of titanocenes, is the accessibility of pathways connecting metallacycles and metal alkylidene complexes, eg in alkene metathesis (eq. 35). 

Scheme 6.55. A representation of a possible pathway using metal (M = Ru or W) carbene-like intermediates to account for alkene metathesis.

This method is specific for metallacyclopentanes. The alkene-coupling process is favored by metal reduction. A typical synthetic strategy is the in situ reduction of a metal halide precursor in the presence of the alkene see, for example, the synthesis of 79 in Scheme 34.1 An alkylidene precursor may also lead to a metallacycle with elimination of the car-bene ligand as in the synthesis of 81, representing a deactivation pathway for alkene metathesis catalysts. Ilie two alkenes may be generated in situ in the coordination sphere by rearrangement processes, such as intramolecular hydrogen transfer from an alkyl-vinyl precursor. I ... 

A combined experimental and computational study of substrate-induced decomposition pathways for alkene metathesis catalysts reveals the importance of / -hydride transfer from ruthenacyclobutane intermediates. Subsequent steps afford allyl hydride species, which then eliminate H2 to give catalytically inactive unsaturated complexes of the form [RuL(77 -alkene)Gl2]. ... 

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. 

While diene metathesis or diyne metathesis are driven by the loss of a (volatile) alkene or alkyne by-product, enyne metathesis (Fig. 2) cannot benefit from this contributing feature to the AS term of the reaction, since the event is entirely atom economic. Instead, the reaction is driven by the formation of conjugated dienes, which ensures that once these dienes have been formed, the process is no longer a reversible one. Enyne metathesis can also be considered as an alkylidene migration reaction, because the alkylidene unit migrates from the alkene part to one of the alkyne carbons. The mechanism of enyne metathesis is not well described, as two possible complexation sites (alkene or alkyne) exist for the ruthenium carbene, leading to different reaction pathways, and the situation is further complicated when the reaction is conducted under an atmosphere of ethylene. Despite its enormous potential to form mul-... 

Another piece of mechanistic evidence was reported by Snapper et al. [14], who describe a ruthenium catalyst caught in action . During studies on ring opening metathesis, these authors were able to isolate and characterize carbene 5 in which a tethered alkene group has replaced one of the phosphines originally present in Id. Control experiments have shown that compound 5 by itself is catalytically active, thus making sure that it is a true intermediate of a dissociative pathway rather than a dead-end product of a metathetic process. 

Although the quasi-cyclobutane mechanism accounts fully for the products of the metathesis of alkenes at equilibrium, see above, some workers were uncomfortable with the unconventional nature of the bonding in the proposed intermediate. An alternative mechanism which brings about the same net alkylidene exchange (lower pathway in Scheme 12.14)... 

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