Electrophilic aromatic substitution proton exchange

Hydrogen exchange can occur under either acid- or base-catalyzed conditions. Both can be considered electrophilic aromatic substitutions, the latter involving attack of the electrophile upon an aromatic anion, zwitterion, or ylide. The former reaction is aided by electron supply, the latter by electron withdrawal (particularly by -/ effects) as the ratedetermining step is the initial proton loss. Steric hindrance, negligible in virtually all cases under acid-catalyzed conditions, appears to be of slightly greater importance under base-catalyzed conditions. 

In a proton transfer to an aromatic carbon atom, a so-called sigma complex is formed in which the configuration of the valence electrons of the carbon has been changed from sp2 to sp3. In the next step, the other electrophilic atom or group bonded to the same carbon may be split off. This leads to an electrophilic aromatic substitution. Examples are aromatic hydrogen isotope exchange, aromatic decarboxylation, deboro-nation, and deiodination (see Sect. 9 Chap. 2, and Vol. 13, Chap. 1). 

A class of reactions in which the variation of with ApK has been extensively investigated is electrophilic aromatic substitution, in which the rate-determining step is proton loss from a phenonium-ion intermediate. This includes diazo-coupling [56], nitrosation [63] and aromatic hydrogen exchange [47,51,64,65]. Aromatic hydrogen... 

Substituted 1,2,3-triazole 1-oxides 448 have been reported to undergo electrophilic and nucleophilic aromatic substitution and are subject to debromination, proton-metal exchange, and halogen-metal exchange followed by electrophilic addition. Transmetallation and cross-coupling have not been described. 3-Substituted 1,2,3-triazole 1-oxides 448 can be proton-ated or alkylated at the O-atom and they can be deoxygenated and deal-kylated. The individual reactions are described in Section 4.2.7.1-4.2.7.14. 

The rate of hydrogen exchange with a base depends, as we know, on the proton mobility of the hydrogen atoms in aromatic CH bonds, which is very dependent on the inductive shift of cr-electrons in the carbon skeleton of the aromatic ring. In hydrogen exchange with an acid (just as in other electrophilic substitution reactions in compounds... 

Periana et al. have reported a mercury system that catalyzes the partial oxidation of methane to methanol.81 Hg(II) is typically considered to be a soft electrophile and is known to initiate electrophilic substitution of protons from aromatic substrates. The catalytic reaction employs mercuric triflate in sulfuric acid, and a key step in the catalytic cycle is Hg(II)-mediated methane C—H activation. For methane C—H activation by Hg(II), an oxidative addition reaction pathway via the formation of Hg(IV) is unlikely. Thus, an electrophilic substitution pathway has been proposed, although differentiation between proton transfer to an uncoordinated anion versus intramolecular proton transfer to a coordinated anion (i.e., o-bond metathesis) has not been established. Hg(II)-based methane C H activation was confirmed by the observation of H/D exchange between CH4 and D2S04 (Equation 11.9). 

Hydrogen in the stable bonds of aliphatic and aromatic compounds can be exchanged only under particularly energetic conditions. It is favored, for instance, by solid catalysts, by strong bases which ease the removal of protons in such cases, or by strong acids which act as effective deuteron-donors. An acid-base-catalysed ionic mechanism and an electrophilic substitution mechanism have both been discussed as explaining the formation of deuterated compounds.70,71... 

---