Reaction sigma-bond metathesis

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+. 

Sigma-bond metathesis at hypovalent metal centers Thermodynamically, reaction of H2 with a metal-carbon bond to produce new C—H and M—H bonds is a favorable process. If the metal has a lone pair available, a viable reaction pathway is initial oxidative addition of H2 to form a metal alkyl dihydride, followed by stepwise reductive elimination (the microscopic reverse of oxidative addition) of alkane. On the other hand, hypovalent complexes lack the... 

Even the late metals can show this reaction, although in such cases it is hard to completely eliminate the possibility of oxidative addition/reductive elimination as an alternative redox pathway to the same products. Bergman proposed a sigma bond metathesis pathway for the reaction of alkanes including methane with the dichloromethane complex, [Cp IrR(PMe3)(ClCH2Cl)]+. 

Marks has reviewed this area. Basset has bound organometallic species to a silica surface and seen interesting alkane reactions that go via initial CH bond activation this step probably proceeds by a sigma bond metathesis pathway since the metals involved are d . Snbseqnent reactions inclnde CC bond cleavage and carbon skeletal rearrangements. 

In 1984, Tremont described a procedure for palladium acetate-promoted anilide alkylation by alkyl iodides [45, 46], He suggested that the reaction proceeds by either a Pd(II)-Pd(IV) catalytic cycle or a sigma-bond metathesis mechanism. A possible Pd(II)-Pd(IV) catalytic cycle is presented in Scheme 8. 

In this reaction, a C—H bond in methane is made, and a C—H bond of benzene is cleaved, but the oxidation state of the scandium center remains +3. This class of reaction, which is not hmited to early transition metals, is called sigma-bond metathesis. In this mechanism, the metal is first postulated to coordinate the bond to be activated in an rf- fashion, followed by formation of a four-centered transition state that leads to an exchange of ligands at the metal (Figure 14.8). 

The scope of this chapter does not permit a detailed discussion of proposed mechanisms for oxidative addition reactions. The reader is encouraged to consult references at the end of Chapter 13 and 14 for mechanistic details. A challenge in many cases of bond activation is to distinguish between a mechanism of sequential oxidative addition/reductive elimination or sigma-bond metathesis. 

As aheady mentioned, it was observed that one mole of hydrogen is liberated when methane is reacted with the tantalum hydride with the formation of tantalum methyl. The reaction with methane above 150°C leads to the formation of the Ta-methyl, Ta-methylene, and Ta-methylidyne species plus H2 (M=Ta) [40-42, 54]. These observations are a proof that the first step of alkane metathesis is the formation of metal alkyl intermediate via cleavage of the C-H bond of the alkane likely by sigma bond metathesis. Further, detailed mechanistic [22, 55] and experimental kinetic studies revealed that the alkenes and hydrogen are the primary products [56]. Initially, it was believed that the active site was a bis-siloxy tanta-lum-monohydride, but progressively, evidence came in favor of an equilibrium between bis-siloxy tantalum-monohydride d and bis-siloxy-tantalum-tris-hydride d° [57], and the mechanism would fit much better with a bis-siloxy-tantalum-tris-hydride [58]. 

Cross metathesis between two different alkanes represent one of the most difficult challenges in organic chemistry [53]. In 2001, Basset et al. first demonstrated the possibilities of sigma bond metathesis between two different alkanes [55]. In 2004, this same group has reported the cross metathesis between ethane and toluene [81] and methane and propane [82]. Silica-supported tantalum hydride catalyst [(=SiO)2TaH] [(=SiO)2TaH3] was employed for cross-metathesis reaction between toluene and ethane at 250°C. Under static condition, it produced mainly ethyl benzene and xylenes as major product along with propane and methane (Scheme 24). 

In Chapter 2, Diaconescu and coworker have focused on processes of C—H bond activation of hydrocarbons induced by some rare-earth metals and actinides. In addition to the well-known sigma-bond metathesis and 1,2-addition reactions, special attention is devoted to new mechanistic pathways for such transformations. 

Actinide bond disraption enthalpies of the complexes Cp jMR M = actinide, R H, Me have been measured using iodinolysis and alcoholysis titration calorimetry and biallyl-monocyclopentadienyl lanthanum(III) complexes have been prepared for use as butadiene polymerisation catalysts. Finally, the reaction of [ Cp 2LnH 2] Ln = Y, Sm with [CpjWH2] results in the formation of the sigma bonded metathesis products via dehydrocoupling. ... 

Labile H2 complexes are likely intermediates when protonation of a metal hydride liberates H2. Sigma complexes can also be involved in sigma bond metathesis (Eq. 3.41). For example, the reaction of hydrogen with the 12e alkyl WMee (Eq. 3.32) cannot go by OA because, as a d° species, W is already at the maximum permitted oxidation state, yet H2 reacts readily to liberate MeH. Weak H2 binding as a a complex (without back donation or metal oxidation) allows protonation of the methyl groups without needing OA (Eq. 3.41, X = Me Y = H). 

Scheme 1 gives a postulated reaction pathway for all these metathesis like reactions. The heteroallenes or the heteroalkenes are supposed to react in a 2 + 2 cycloaddition reaction with Cl3(dme)WCtBu to form metalla cyclobutene derivatives. Electrocyclic ringopening reactions of these (not isolated) metallacycles yield tungsten imido or 0X0 complexes with sigma bonded vinyl, iminyl, ketenyl or keteniminyl ligands. 

Moreover complex 7 alone, in absence of any cocatalyst such as MAO, is able to polymerize ethylene, when dichloromethane is used as solvent. All these facts are consistent with the recent evidences for the ion-pair nature of the active species in soluble Ziegler-Natta polymerizations. It is very interesting to observe that this complex can be considered as a borderline catalytic complex between metathesis and Ziegler-Natta catalysts, having the possibility to behave as a carbene or as a Ti-C sigma bond depending on the reaction conditions. 

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