Metathesis, sigma bond

All the C—H bond activation examples given in Sections 14.1.2 and 14.1.3 exhibit the classic characteristics of oxidative addition and involve metal centers with eight valence electrons (Ir(I), Pd(II), Fe(0)). These metals have relatively low oxidation states and are predisposed toward being able to oxidize C—H bonds. Does this mean that complexes with metals in their formally highest oxidation states are inherently unable to effect C—H bond activation It is fascinating that some (f transition-metal complexes are able to activate C—H bonds without any change in oxidation state. For example, consider the following classic reaction  

R—H — L M—R r-Lh R FIGURE 14.8 General Sigma-Bond Metathesis Mechanism. 

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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... 

This difference in behavior suggests different mechanisms. As already observed with cyclic alkanes, the initiahon is a C-H bond achvahon (likely via sigma bond metathesis), leading to the corresponding metal-alkyls. With group 4 metals, a... 

Woo s combinative catalyst system of CpjMCE/Hydride is different from the catalyst systems using Cp2MCl2/2 alkyllithiums of Corey, Tanaka, and Harrod. Real catalytic species in the dehydrocoupling of hydrosilanes could be a metallocene hydride based on a sigma-bond metathesis mechanism.12,1315 Inorganic hydrides effectively produce a metallocene hydride whereas alkyllithium can produce a metallocene hydride via... 

Insoluble solid polymers were isolated in 82% yield for Ti, 95% yield for Zr, and 80% yield for Hf. TGA ceramic residue yields were 72% for Ti, 73% for Zr, and 74% for Hf. The weight average molecular weights of the oily polymers were 4120 for Ti, 9020 for Zr, and 5010 for Hf. The TGA ceramic residue yields of the soluble oily polymers were ca. 14%. The dehydrocoupling mechanism of 4 should be similar to the sigma-bond metathesis for the dehydrocoupling of phenylsilane.11,12... 

Scheme 4 Sigma bond metathesis of actinide-aUcyl bonds...

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. 

Still, it seems likely that an oxidative addition must be the first step. Oxidative addition of Pd(0) to the Bn-OR bond may give Bn-Pd(II)-OR. Sigma-bond metathesis may take place next to give Bn-H (toluene) and H-Pd(II)-OR, which undergoes reductive elimination to give ROH and regenerate the Pd(0). 

A qualitative approach will possibly be more fruitful. Fig. 12 illustrates how the dihydrogen addition step (late with respect to heavy atom locations, early with respect to dihydrogen) might appear for the two diastereomeric pathways of the 16-electron route, with CHIRAPHOS as the ligand. In the alternative 14-electron route, dissociation of the alkene is assumed to occur, followed by irreversible H2 addition. The process in then consummated by reformation of the alkene-rhodium bond, or by a sigma bond metathesis which bypasses the dihydride state. 

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. 

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