Isomer distributions electrophilic aromatic

Finally, it is customary to compare the partial rate factors obtained under different conditions to indicate the reactivity of the electrophile. Unfortunately, the medium, nitric acid in sulphuric acid, in which the nitronium ion is most clearly established as the electrophile, is such a poor solvent for aromatics that meaningful competitive nitrations are impossible and kinetic studies are hampered by the difficulties noted above. However, since the isomer distribution is a function of the rate factors, inspection of these distributions (Table 15) shows a very... 

Aromatic compounds can be hydroxylated, benzoxylated and acetoxylated and the isomer distributions and substituent effects indicate that the reaction is an electrophilic substitution132. Very little kinetic work has been carried out so that the nature of the electrophile is in some doubt. 

Olivier and Berger335, who measured the first-order rate coefficients for the aluminium chloride-catalysed reaction of 4-nitroben2yl chloride with excess aromatic (solvent) at 30 °C and obtained the rate coefficients (lO5/ ) PhCI, 1.40 PhH, 7.50 PhMe, 17.5. These results demonstrated the electrophilic nature of the reaction and also the unselective nature of the electrophile which has been confirmed many times since. That the electrophile in these reactions is not the simple and intuitively expected free carbonium ion was indicated by the observation by Calloway that the reactivity of alkyl halides was in the order RF > RC1 > RBr > RI, which is the reverse of that for acylation by acyl halides336. The low selectivity (and high steric hindrance) of the reaction was further demonstrated by Condon337 who measured the relative rates at 40 °C, by the competition method, of isopropylation of toluene and isopropylbenzene with propene catalyzed by boron trifluoride etherate (or aluminium chloride) these were as follows PhMe, 2.09 (1.10) PhEt, 1.73 (1.81) Ph-iPr, (1.69) Ph-tBu, 1.23 (1.40). The isomer distribution in the reactions337,338 yielded partial rate factors of 2.37 /mMe, 1.80 /pMe, 4.72 /, 0.35 / , 2.2 / Pr, 2.55337 339. 

The triflic acid catalyzed electrophilic hydroxylation of aromatics with BTSP gives the corresponding phenols in high yields without apparent polyhydroxylation or secondary oxidation. Thus, treatment of CeHe with CF3SO3H followed by BTSP gave 11% PhOH. The isomer distributions are in accord with the electrophilic nature of the reaction. The observed ortho/para ratio in the case of toluene agrees with the expected trends (Scheme 4 and Table ll) . 

It is well-recognized that phenols are completely protonated in superacidic solutions.420 This raised the possibility that protonated phenols, once formed in these media, might resist further electrophilic attack. Electrophilic hydroxylations of aromatics with hydrogen peroxide (98%) in superacidic media has been achieved by Olah and Ohnishi617 in Magic Acid, which allows clean, high-yield preparation of monohydroxylated products. Benzene, alkylbenzenes, and halobenzenes are efficiently hydroxylated at low temperatures. The obtained yields and isomer distributions are shown in Table 5.36. Subsequently, Olah et al.618 found that benzene and... 

The same workers590,591 also showed that, in the Pd(II)-catalyzed acetoxylation of substituted arenes, a complete reversal of the usual pattern of isomer distribution for electrophilic aromatic substitution or anodic oxidation of aromatics is observed. To explain these results it was suggested that acetoxylation by Pd(OAc)2 takes place via the following addition-elimination sequence ... 

Aromatic compounds are the only type of i-nucleophiles involved in reactions with organoxenonium salts. While pentafluorobenzenes, CeFsZ (Z = H, F, CN, SiMej), are not reactive towards [C6F5Xe][AsF6] in MeCN at 20 °C, a series of 2,3,4,5,6-pentafIuorobiphenyls was obtained from monosubstituted benzene derivatives, CeHsZ, under these conditions. The reaction rates diminish in the sequence Z = CH3 > F > CF3 CN > NO2, which is consistent with the electrophilic nature of the process. Nevertheless, the isomer distribution in the C6F5C6H4Z products shows unambiguously the radical character of the pentafluorophenylation reaction (eq 17) 46). 

The fluorination of aromatic compounds with xenon difluoride has been extensively investigated. The fluorination of benzene with xenon difluoride in the presence of hydrogen fluoride as a catalyst results in the formation of fluorobenzene in 68% yield. Monosub-stituted aromatic systems are reported to give high yields of monofluorinated compounds, the isomer distributions of which arc similar to those observed in electrophilic substitution (Table... 

The isomer distribution for anodic acetoxylation of a number of monosubstituted benzenes has been determined [122]. The reaction closely resembles ordinary electrophilic aromatic substitution processes, perhaps on the side of low-selectivity reactions. The isotope effect, A h//cd, for nuclear acetoxylation in anisole was found to be 1.0, whereas for a-substitution in ethylbenzene a value of 2.6 was observed. The interpretation of these values is not straightforward [126]. 

The partial rate factors and the isomer distribution in the amination by di-methylamino radical cation of toluene, isopropylbenzene, -butylbenzene, biphenyl and naphtalene are reported in Table 7. These partial rate factors are far the highest ever observed in homolytic substitutions so that the general character of the homolytic amination allows a more relevant analogy to be drawn with the electrophilic substitutions than with the homol5rtic arylation, the only homol5rtic substitution for which numerous and accurate quantitative data exist in homo-cyclic aromatic series. 

The Introduction of Fluorine Into Aromatic Rings.— The direct fluorination (F2-N2) of nitrobenzene, benzotrifluoride, benzoic acid, and toluene, usually in trifluoroacetic acid at 5 C, has been described by Russian workers. Where direct comparisons can be made, the isomer distributions of the monofluoroarenes obtained are qualitatively similar to those formed in acetonitrile during concurrent American work, sununarized in Vol. 1 (p. 189). However, the Russians favour a homolytic mechanism, the preference for attack at the /wc/o-position in nitrobenzene being attributed to the electrophilic character of the attacking fluorine atom. Electrophilic displacement of a proton by F, via the usual addition-elimination mechanism, was preferred in the American paper, and this view has been emphasized in a recent review of the field by the same author. ... 

The isomer distribution for 5-ArDMU s was analogous to those typically observed in aromatic photosubstitution reactions. On the other hand, few reports have appeared on the meta-oriented aromatic photosubstitution reaction, the primary example being the photoinduced proton exchange in benzenes that proceeds via an electrophilic aromatic photosubstitution reaction. In the case of either the 5-chloro-or 5-fluoro-l,3-dimethyluracil (5-ClDMU, 5-FDMU), > addition of TFA simply promoted 5-arylation (photo-Friedel-Crafts reaction), while the addition of a bromide (p-dibromobenzene, 1,2-dibromoet-hane, benzyl bromide, HBr) to the solution induced the meta-oriented 6-arylation (Scheme 2)."... 

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