Dialkylbenzenes, isomerization

A good deal of experimental care is often required to ensure that the product mixture at the end of a Friedel-Crafts reaction is determined by kinetic control. The strong Lewis acid catalysts can catalyze the isomerization of alkylbenzenes, and if isomerization takes place, the product composition is not informative about the position selectivity of electrophilic attack. Isomerization increases the amount of the meta isomer in the case of dialkylbenzenes, because this isomer is thermodynamically the most stable. ... 

Notable examples of general synthetic procedures in Volume 47 include the synthesis of aromatic aldehydes (from dichloro-methyl methyl ether), aliphatic aldehydes (from alkyl halides and trimethylamine oxide and by oxidation of alcohols using dimethyl sulfoxide, dicyclohexylcarbodiimide, and pyridinum trifluoro-acetate the latter method is particularly useful since the conditions are so mild), carbethoxycycloalkanones (from sodium hydride, diethyl carbonate, and the cycloalkanone), m-dialkylbenzenes (from the />-isomer by isomerization with hydrogen fluoride and boron trifluoride), and the deamination of amines (by conversion to the nitrosoamide and thermolysis to the ester). Other general methods are represented by the synthesis of 1 J-difluoroolefins (from sodium chlorodifluoroacetate, triphenyl phosphine, and an aldehyde or ketone), the nitration of aromatic rings (with ni-tronium tetrafluoroborate), the reductive methylation of aromatic nitro compounds (with formaldehyde and hydrogen), the synthesis of dialkyl ketones (from carboxylic acids and iron powder), and the preparation of 1-substituted cyclopropanols (from the condensation of a 1,3-dichloro-2-propanol derivative and ethyl-... 

In this connection three oases may be considered 1) the strength of aprotic centres is sufficient for the activation of olefin and the following alkylation by interaction with aromatic hydrocarbon frem gaseous phase, but insufficient for aromatic ring activation. Para-dialkylbenzene formed in this case does not undergo further isomerization on these centres. The group of centres... 

The AlCl3-catalyzed isomerization of dialkylbenzenes was studied in detail by Allen and coworkers76-79 and Olah and coworkers.80-82 The equilibrium isomer... 

As pointed out (see Section 5.1.4) alkyl group migration (i.e., isomerization) in these carbocationic intermediates can take place with ease, even under conditions where the product dialkylbenzenes themselves do not undergo further isomerization. Kinetically controlled alkylation free of thermodynamically controlled alkyl group migration therefore cannot be judged on absence of product alkylbenzene isomerization. 

When dialkylbenzenes are passed over Nafion-H at 160°C, both isomerization and disproportionation take place [Eqs. (5.102) and (5.103)]. Monoalkylbenzenes also disproportionate under these conditions268-271 [Eq. (5.104)]. 

Compared with the parent system and those with identical substitution in all four carbons, the structure of other derivatives should be affected by the substitution pattern and by the nature of the substituents. For 1,2-disubstituted derivatives, structure type C, in which the doubly substituted cyclobutane bond is weakened (and lengthened), or a related structure type in which the bond is cleaved, should be favored. This is born out by several observations mentioned earlier. For example, the geometric isomerization of 1,2-diaryloxycyclobutane (Sect. 4.1) can be rationalized by one-bond rotation in a type C radical ion. Similarly, the fragmentation of the anti-head-to-head dimer of dimethylindene (Sect. 4.4) may involve consecutive cleavage of two cyclobutane bonds in a type C radical ion. The (dialkylbenzene) substituents have a lower ionization potential (IP 9.25 eV) [349] than the cyclobutane moiety (IP 10.7 eV) [350] hence, the primary ionization is expected to occur from one of the aryl groups. 

Prolonged exposure of 167 to aluminum chloride also gives product 172 via 171, which is a thermodynamic process that favors the most stable isomer, and polyalkylation is also possible. Exposure of sec-butyl-benzene (173) to the mixed-acid HF—BF3 led to a mixture of 36.7% of benzene, 10.9% of n-butylbenzene, 21.8% of scc-butylbenzene, and 30.6% of di-scc-butylbenzene. Similarly, when n-butylbenzene was heated with aluminum chloride (100°C, 3 h), 45.2% of butylbenzene (99.4% n-butylbenzene and 0.6% sec-butyl-benzene), 15.2% of benzene, and 27.7% of dibutylbenzene (>90% meta) along with 11.9% of polyalkylated benzene products were obtained.lO Lewis acid induced isomerization of dialkylbenzenes usually leads to an increase in the relative percentage of meta isomer, as seen with n-butylbenzene, based on the greater thermodynamic stability of the meta product than that of the ortho and para products. ... 

Allqrl groups attached to the benzene ring are known to direct entering substituents into the ortho and para positions. Substitution in the meta position, therefore, was an unexpected finding when alkylbenzenes were monoalkylated (or benzene was dialkylated). It was subsequently established that such meta substitution is generally not caused by a kinetic aU lation of low selectivity, but thermodynamically controlled isomerization. The relative amovmts of the three isomeric dialkylbenzenes that are produced depend on the aromatic hydrocarbon, the alkylating agent, the catalyst, and the reaction conditions. In mechanistic interpretations, therefore, it is important to differentiate between direct alkylation controlled by the kinetics of the reaction and subsequent or concurrent isomerization of the alkylated product controlled by thermodynamic factors. 

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