Benzene meta dialkylation

When sodium formate is used as a terminating agent, hydrogenolysis of aryl-palladium occurs [10]. Reaction of iodobenzene, n-propyl bromide, and sodium formate afforded 1,3-di-n-propylbenzene (35) in 78% yield. In this case, the 2,6-di-n-propylphenylpalladium 30 was hydrogenolyzed with sodium formate, and 35 was obtained via Pd formate 33 and hydridopalladium 34. This is not only very interesting, but also useful, because meta dialkylation of benzene can be achieved in this way, which is difficult to carry out by conventional methods. 

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. 

However, there are some contradictory reports on the composition of the products of toluene alkylation or benzene dialkylation at high conversions. In some cases, compositions corresponding to the thermodynamic equilibrium between ortho, meta and para isomers were found, and in other cases, kinetic control of orientation, giving mostly the ortho + para substitution, prevailed. Consecutive isomerisation of the ortho and para isomers to the more stable meta isomer seems to be the cause of the disagreement. More active catalysts gave more meta derivatives than the less active ones [343] and increasing the temperature has the same effect [351]. 

Aromatic hydrocarbons substituted by alkyl groups other than methyl are notorious for their tendency to disproportionate in Friedel-Crafts reactions. This tendency has previously limited the application of the isomerization of para- or ortho-)dialkyl-benzenes to the corresponding meta compounds. At the lower temperature of the present modification, disproportionation can be minimized. 

There are directly measured enthalpy of formation data for the ortho, meta, and para isomers of the disubstituted benzenes R, R = Me, Me (1, g) Me, Et (1, g) Me, n-Pr (1) Me, i-Pr (1) Me, t-Bu (g) Et, Et (1). For all the meta and / ara-disubstituted species and for o xylene, the /7(g, 1) are less than the root-mean-square (rms) of the experimental uncertainty levels. All of these compounds are, therefore, unstrained and so too presumably would be the meta and para isomers of other dialkyl-substituted benzenes. Enthalpies of formation for those compounds where /7( ) is about 0 can be estimated from eqs 16 and 17 and the parameters in Table 3. 

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