DFT calculations on the Friedel-Crafts benzylation of 1,4-dimethoxybenzene using ZnCl2 impregnated montmorillonite K10 — inversion of relative selectivities and reactivities of aryl halides

Abstract

AbstractZinc was bound on montmorillonite K10 by cation exchange to obtain a catalyst named clayzic. In the Friedel-Crafts benzylation of 1,4-dimethoxybenzene, this catalyst was used for the synthesis of substituted diphenylmethanes using 4-chlorobenzyl chloride and 4-bromobenzyl bromide. During the reaction, sub-products from a second benzylation reaction process were observed. For a better understanding of their formation, reactions were carried out at different times to obtain data on the progress of benzylation and the relative ratio of each product was calculated using two different analytical methods. It was shown that the selectivity and reactivity of both aryl halides were reversed under these experimental conditions contrary to those obtained using the more conventional catalyst, zinc dichloride. These results were explained by geometrical and electronic considerations. It was found that the formation of transition states and Wheland intermediates from aryl bromide and chloride in the presence of clayzic can be explained in terms of preferential absorption. Moreover, the high percentage of 4-chlorobenzyl chloride conversion was attributed to its covalent radius, which is smaller than that of Br. At the same time it was shown that the presence of a Brönsted acid, due to the liberation of HCl during the benzylation, is responsible for the poisoning of the clayzic catalyst. Moreover, poisoning effect of the bromine anion could not be excluded.