C-H BDEs with a-OR

The broken bonds (boldface=dissociated atom), A(H (R), kcal/mol (kj/mol) BDEs (boldface = recommended data reference in parentheses) Methods (reference in  

Difluoromethyl trifluoromethyl ether CHF2OCF3 106 + 1 443.5+4.2 Kinetics 1995HSU/DEM 

Methyl trifluoromethyl ether CH3OCF3 102 + 1 426.8+4.2 Kinetics 1995HSU/DEM 

Ethyl tert-butyl ether CH3CH20-tert-C(CH3)3 96.9 405.4 Re-anal. 1998BRO/BEC 

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From this linear correlation, and entering the experimental E value determined for 4-Me0C6H4CH20H (Table 7), a BDE value of 77 1 kcalmoH could be extrapolated for the benzylic C—H bond bearing a geminal OH group. As a matter of fact, the BDEc h of benzyl alcohols was not experimentally available or reported with reasonable confidence the extrapolated value compares well with a BDEc h of 81 1 kcalmol" that could be extrapolated for PhCH20H from data of Espenson and coworkers. Because the BDEc h of toluene is 88.5 kcalmol", the extrapolated value... 

High-valent oxo-complexes, isolated or in situ-generated, interact most often with electron-rich n -systems 1 or suitable C-H bonds with low bond dissociation energy (BDE) in substrates 3 (Fig. 2). These reactions may occur concerted via transition states 1A or 3A leading to epoxides 2 or alcohols 4. On the other hand, a number of epoxidation reactions, such as the Jacobsen-Katsuki epoxidation, is known to proceed by a stepwise pathway via transition state IB to radical intermediate 1C [39]. Similarly, hydrocarbon oxidation to 4 can proceed by a hydrogen abstraction/S ... 

In many cases electrochemistry proved to be a powerful tool to activate the SnAt reactions. The cathodic reduction in aromatic compounds, prior to their interaction with nucleophilic reagents, is a new essential step of the S Ar reactions. Moreover, a basic thermodynamic study explains (BDEs values of C-Nu vs. C-H), why F, OH, OR, and SR nucleophiles do not react with arenes or heteroarenes. 

Resonance-stabilized radicals are even more stable than tertiary radicals, as can be seen by comparing the BDE values in Figure 11.7. A C—H bond at a benzyUc or allyUc position is more easily cleaved than a C—H bond at a tertiary position. These observed BDE values suggest that resonance-stabilized radicals are about 40-50 kj/mol more stable than tertiary radicals. This is a large difference when compared with the 8 kj difference between secondary and tertiary radicals. 

This chapter begins with an acknowledgement of the difficulties in writing this chapter. The problem is a matter of definition what, exactly, constitutes a radical-mediated C—H bond activation First, what qualifies as C—H activation If a C—H bond has bond dissociation energy (BDE) less than, arbitrarily, 95 kcal moH (see Chapter 1, Introdnction ), is it already activated and, thus, unable to undergo C—H activation. What if the C—H bond is cleaved via an acid/base step Should the Friedel-Crafts reaction or the Snieckns directed orf/jo-lithiation reaction be considered C—H activation Is it necessary for the C—H bond to be cleaved before or during the rate-determining step ... 

These questions may be pertinent to every chapter in this book. For this chapter specifically, some additional questions arise What does radical-mediated mean Can a radical hydrogen-atom abstraction meet the requirements of C—H activation Does it depend on the particular mechanism of the reaction, the C—H BDE, or the timing of the radical abstraction step How can a researcher be sure that absolutely no transition metals are present and active during the reaction What if a transition metal is an additive, but only serves as an electron shuttle and does not specifically cleave to or react with the C—H bond ... 

Consistent with the results of this study is the outcome of the oxidation of 4-X-substituted phenols by use of PINO, generated from HPI with Pb(OAc)4 at 25 °C in MeCN containing 1% AcOH . The reactivity (fcn) of PINO towards phenolic O—H bonds (BDE 85-90 kcal moC ) was about one order of magnitude higher than that measured towards the C—H bond of benzyl alcohols (cf. Table 4). A p value of —3.1 was obtained from plotting log kn vs. for this reaction, where removal of H-atom from the phenolic O—H bond (which is weaker than the O—H bond of aliphatic or benzyl alcohols) induces an oxidative phenolic coupling with the PINO moiety. In view of the low redox potential of the substituted phenols (in the 0.8-1.1 V/NHE range), and of the substantial value of the kinetic isotope effect = 3.1-3.7 measured, ... 

The HO-H bond dissociation energy (BDE) is 499 kj mol-1, while the C-H bonds in saturated hydrocarbons are much weaker (BDE = 376-410 kj mol-1 Berkowitz et al. 1994 for a compilation, see Chap. 6). Thus, there is a considerable driving force for H-abstraction reactions by -OH. On the other hand, vinylic hydrogens are relatively tightly bound, and an addition to the C-C double bond is always favored over an H-abstraction of vinylic or aromatic hydrogens. Hence, in the case of ethene, no vinylic radicals are formed (Soylemez and von Sonntag 1980), and with benzene and its derivatives the formation of phenyl-type radicals has never been conclusively established. 

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