Protonation of aromatics

The acidity of benzylic protons of aromatics complexed to transition-metal groups was first disclosed by Trakanosky and Card with (indane)Cr(CO)3 [61]. Other cases are known with Cr(CO)3 [62], Mn(CO)3 [63], FeCp+ [64, 65], and Fe(arene)2+ [31, 66] but none reported the isolation of deprotonated methyl-substituted complexes. We found that deprotonation of the toluene complex gives an unstable red complex which could be characterized by 13C NMR ( Ch2 = 4.86 ppm vs TMS in CD5CD3) and alkylated by CH3I [58] Eq. (13) ... 

The sites of protonation of aromatic compounds, including the possible three mono fluoronitrobenzenes, have been studied by neutralization-reionization mass spectrometry (NRMS)22. The NRMS experiments on the MD+ species generated by D2 chemical ionization clearly indicated that the D+ attachment takes place to the nitro group rather than to the aromatic ring, as evidenced by the abundant losses of OD and DNO2 (NO + OD )22. 

Compcired to the aliphatic amines, the aromatic amines have lower values. This lower value indicates that the product of the protonation of aromatic amines is less stable. The decrease in stability is due to a loss in resonance stabilization of the protonated form. 

Assignment of diastereotopic groups (e.g., (3-protons of aromatic amino acids, methyl groups in Val or Leu, both a-protons of Gly, pro-R or pro-5) is necessary for reliable conformational analysis but it is normally not important for proof of the constitution (Section 7.5.3). 

In practice, extrapolations of p fR in water have usually used the older acidity function based method, for example, for trityl,61,62 benzhydryl,63 or cyclopropenyl (6) cations.66,67 These older data include studies of protonation of aromatic molecules, such as pKSi = —1.70 for the azulenium ion 3,59 and Kresge s extensive measurements of the protonation of hydroxy- and methoxy-substituted benzenes.68 Some of these data have been replotted as p fR or pKa against XQ with only minor changes in values.25,52 However, for more unstable carbocations such as 2,4,6-trimethylbenzyl, there is a long extrapolation from concentrated acid solutions to water and the discrepancy is greater use of an acidity function in this case gives pA 2° = —17.5,61 compared with —16.3 (and m = 1.8) based on X0. Indeed because of limitations to the acidity of concentrated solutions of perchloric or sulfuric acid pICs of more weakly nucleophilic carbocations are not accessible from equilibrium measurements in these media. 

Thus if we axe interested only in the relative values of equilibrium or rate constants for closely similar reactions, we need only concern ourselves with changes in delocalization energy. This of course is a useful simplification. A good illustration is provided by the protonation of aromatic hydrocarbons. Aromatic hydrocarbons react with strong acids to form salts. These are now known4 to be arenonium ions thus benzene is converted to benzenonium ion (I) by strong acids ... 

MO studies of aromatic nitration cast doubt on the existence of jt-complexes and electron-transfer complexes in liquid-phase nitrations.14 The enthalpy of protonation of aromatic substrates provides a very good index of substrate reactivity to nitration. Coulomb interaction between electrophile and substituent can be a special factor influencing regioselectivity. A detailed DFT study of the reaction of toluene with the nitronium ion has been reported.15 Calculated IR spectra for the Wheland intermediates suggest a classical SE2 mechanism. MO calculations of cationic localization energies for the interaction of monosubstituted benzenes with the nitronium ion correlate with observed product yields.16... 

Friedel-Crafts aromatic substitution reactions have been widely explored in polymer chemistry [29,30] and generally proceed with Lewis acids such as AICI3 with elimination of hydrogen halides. In superacid solutions, however, the Friedel-Crafts reactions take place with dehydration from the oxygen of the carbonyl group and the proton of aromatics. The reactivity of the pro-tonated carbonyl group in the superacid can be further increased by the... 

Protonation of aromatic rings adjacent to methyl-terminated chain ends (4) can result in the formation of alkyl benzenes, propene, and benzene, depending on how the macro cation decomposes. 

Fig, 9. Brdnsted plot for the protonation of aromatic substrates by hydronium ion and for the deprotonation of the benzenonium ions by water. Data are statistically corrected and the numbers correspond to the compounds in Table 10. For a description of the rate coefficients kT (open points) and krev (closed points) see text. Redrawn with permission from A. J. Kresge et al., J, Am. Chem. Soc., 93 (1971) 6181. Copyright by the American Chemical Society. 

Protonation becomes a rapid reaction in protic solvents and in the presence of acids, as demonstrated for, e.g., -butyl acrylate in aqueous solution [207], methyl acrylate in EtOH [208], cinnamates in the presence of phenol in DMF [209], and benzaldehyde in ethanolic buffer solution [210]. Rate constants for protonation of aromatic radical anions (anthracene [211], naphthalene, 2-methoxynaphthalene, 2,3-dimethoxynaphthalene) by a number of proton donors including phenols, acetic acid, and benzoic acids in aprotic DMF were found to vary from 5.0 X 10 M- s-> (for anthracene, in the presence of p-chlorophenol) to 6.2 x lO s (for anthracene, in the presence of pentachlorophenol) [212]. For dimedone, PhOH, or PhC02H the rate of protonation depends on the hydrogen-bond basicity of the solvent and increases in the order DMSO < DMF MeCN [213],... 

Faster protonation rates were observed in the self-protonation of aromatic carboxylic radical anions (kprot > 10 m s ) [216] and (R)-(—)-l,l -biphenyl-2,2 -diyl hydrogen phosphate radical anion (kprot = 5.7 x 10 m s ) [217]. 

C. The change in electron density during a titration is not transmitted throughout the carbon chain in the aliphatic amino acids and aliphatic protons of aromatic amino acids. 

Some novel schemes for protonation of aromatic systems with protons floating in w-electron clouds have been finally put to rest by the elegant work of MacLean and Mackor (1961). The proton exchange between the conjugate acid of the aromatic system and the acid used in the protonation can be slowed sufficiently at — 70°C if the acidic medium is an HF/BF3 mixture. Proton resonance spectra can be unequivocally assigned to a non-aromatic conjugate acid of the type... 

The effect of temperature on the equilibrium ratios of isomeric arenium ions was not studied, but it cannot be strong. The entropy of protonation of aromatic hydrocarbons and their methylated derivatives in liquid hydrogen fluoride was shown to be practically independent of the structure and basicity of hydrocarbon (ASp = = —21 e.u. This seems to be also valid for other acid systems so the... 

The reversibility of C-protonation of aromatic compounds explains the possibility of replacing the hydrogen located at the sp -hybridized ring carbon of the arenium ion by another strong electrophilic substituent. 

In recent years, however, large evidence indicates that the orientation effects in the reactions of aromatic compounds with electrophiles are not only determined by the thermodynamic characteristics of c-complexes. This is evidenced, in particular, by kinetic control in the protonation of aromatic compounds, i.e. cases of the primary formation of less stable arenium ions which are then rearranged into thermodynamically more favourable ions (sec the preceding sec-... 

Data on the relative basicity of a number of aromatic hydrocarbons and their alkyl derivatives were obtained by the Dutch researchers They measured the coefficients of hydrocarbon distribution between an inert solvent (n-hexane) and liquid hydrogen fluoride, containing additives of inorganic fluorides or saturated with BFj. The protonation of aromatic hydrocarbons in HF is described by the equation ... 

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