C-H bond activation mechanisms

Fig. 2.5. C-H bond activation mechanism by Pt(n) complex psoposed by Shilov.

Later, Dixneuf, Jutand, and co-workers reported kinetic data on the reaction of [Ru(OAc)2(p-cymene)] with 2-phenyipyridine in acetonitrile that revealed a different C-H bond activation mechanism. " This involves an intermolecular deprotonation pathway via Se3 mechanism which is an autocatalytic process catalysed by the acetic acid produced during the C-H activation (Scheme 2). Kinetic analysis of the reaction between 6 and 2-phenylpyridine to give 9 showed that this process was considerably accelerated by the presence of acetic acid and water. A drastie retarding effect in the presence of 3 equivalents of K2CO3 was also observed which is consistent with the autocatalytic process, as K2CO3 neutralises the required AcOH. In order to explain the importance of the base, " it is essential to take into account the lasts steps to close the catalytic cycle. In most cases the C-H activation is followed by oxidative addition and reductive elimination to recover the active catalytic species. The C-H activation of 2-phenylpyridine was much faster (27 °C) than the following oxidative addition (120 °C) that becomes rate-determining. 

FIGURE 25.2 Schematic representation for an assisted C—H bond activation mechanism. 

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