Sigma -bond metathesis mechanism

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... 

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. 

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

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... 

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... 

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. 

The results of the crossover experiments argue against a mechanism initiated by hy-drosilylation. An alternative mechanistic possibility, which is consistent with all of the data and which adequately accounts for the stereoselective formation of only diastere-omers 148 and 149, is given in Scheme 51. Complexation of the silane to an initially formed palladacycle such as 155 would afford a complex such as 156. Sigma bond metathesis to a mixture of 7r-allylpalladium complexes 157 (presumed major) and 158 (presumed minor) followed by reductive elimination from each of these would generate the observed products 148 (major) and 149 (minor). 

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]. 

A more appealing strategy for arene C—H functionalizations involves the use of simple benzene derivatives lacking DGs. The overall pathway for such transformations entails metal mediated C—H activation to afford the metal-aiyl intermediate Ai M followed by subsequent functionalization of Ai M to release the desired product (Scheme 24.1). The C—H activation step can proceed via oxidative addition, sigma bond metathesis, or concerted metalation deprotonation pathway. The exact mechanism of C—H cleavage is dependent on the nature of the metal and the ancUlaiy ligands [1,3]. 

Consistent with these results, as shown in Fig. 7.33, the DFT calculations of the CH activation mechanism show that the lowest energy pathway involves pre-equilibrium, dissociative loss of pyridine to generate a trans-5rate-determining trans-cis isomerization to generate the cis-benzene complex and fast CH bond cleavage by a sigma-bond metathesis transition state. 

---