Sigma cleavage mechanism

The results can be rationalized in terms of an oxidative addition-reductive elimination mechanism as illustrated with XXII. A similar mechanism has been proposed for the acid cleavage of Pt(II)-C sigma bond (57, 58). 

These results indicate that proton transfer occurs in the ratedetermining step and there is little carbon—carbon bond cleavage in the transition state. Clear-cut evidence for a two-step mechanism is supplied by Lynn and Bourns results of variable carbon-13 isotope effects in the decarboxylation of 2,4-dihydroxybenzoic acid in acetate buffers [249] (Table 22). Slow proton transfer occurs in the first step and C—C bond cleavage takes place in the second step. At high concentrations of the buffer base, the rate of reversal of the first step becomes comparable to the (relatively fast) rate of the second step and, consequently, the second step becomes partially rate-determining which causes a weak carbon isotope effect. The most reasonable structure of the intermediate is that of the sigma complex. 

If the radical is formed by the loss of an electron from a given a bond, such as a bond to a highly substituted carbon, the cleavage at this location will be favored sigma electron ionization mechanism). 

The rules indicated above seem to not have much connection to the fragmentation results obtained in pyrolysis. For example, Stevenson s rule, the charge site ionization mechanism, and the sigma electron ionization mechanism are not applicable to pyrolysis products, as the molecules in pyrolysis are not ionized. On the other hand, the a cleavage and certain rearrangements may be similar for the two processes. Also the fact that small molecule elimination is favored in mass spectrometry makes possible that, with a certain frequency, pyrolysis products are similar to mass fragments obtained in mass spectrometry. 

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. 

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