Type A Hydrogen Abstraction

Strengths of the substrate C-H bond which is broken and that of the O-H bond which is being formed. The successful application of Evans-Polanyi correlations of the type shown in Fig. 17.6 for transition metal oxidants has led to the generalization that these reactions proceed by a synchronous PCET process that is mechanistically identical to hydrogen abstraction by an organic radical oxidant [52, 54]. 

We note however that the reactivity summarized in Table 17.1 for transition metal acceptors fundamentally differs from their organic counterparts in one important aspect - the proton and electron accepting sites for the reactions in Table 17.1 are distinct. It has been well documented that transition metal complexes that are capable of abstracting hydrogen atoms from substrates do not need to have unpaired spin density at the abstracting atom [52]. With the unpaired spin residing mainly at the transition metal center, upon completion of a PCET event, the electron is transferred to the metal M ) while the proton comes to 

The ambiguity in the data presented in Figs. 17.7 and 17.8 arises from the relation between IE and BDE shown in Fig. 17.9. As illustrated by the figure, the BDE. h and IE for most simple hydrocarbon substrates correlate with one another. This is especially true of the hydrocarbons used in the aforementioned thermo-kinetic correlations (shown within the small box in Fig. 17.9). Even methane, which is difficult to oxidize in a controlled fashion, fits the overall trend. 

Given the parallel between IE and BDE for most typical hydrocarbon substrates lacking polar functional groups, correlations between log k vs. BDE, and log k vs. 

IE produce very similar plots and cannot uniquely distinguish between synchronous (HAT) and asynchronous PCET For this reason, outliers to the correlation in Fig. 17.9 are the most interesting from a mechanistic perspective. These substrates are either aromatic (i.e., benzene and anthracene) or contain either polar functional groups (acetonitrile, formaldehyde and ethanol) or double and triple bonds (i.e., acetylene and cydohexadiene). 

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