Friedel-Crafts reactions induced by heteropoly acids. Regioselective adamantyl substitution of aromatic compounds

Heteropoly acid catalyzed adamantylation of aromatic compounds (toluene, anisole, fluoro- and bromobenzene) was studied. The catalysts applied were various dodecaheteropoly acids of Keggin-type (H_n[XM₁₂O₄₀]; n = 3, 4; X = Si, P; M = Mo, W) and some of their derivatives (Cs_2.5H_0.5[PW₁₂O₄₀] and heat-treated H₄[SiMo₁₂O₄₀]/SiO₂). The reaction in each case took place in excellent yield, however, the selectivity strongly depended on the experimental conditions applied. The para-isomer was shown to be formed under kinetic control, whereas para to meta isomerization occurred in the presence of acids with high acid strength (H₀ ≤ - 13) during longer reaction time. As a rule, strong acidity dependence was observed for the regioselectivity: the Mo-containing acids yield the para-substituted isomers with high selectivity, whereas the W-containing catalysts result in high meta/para ratios. In addition, using halobenzenes as reactants the formation of adamantane was observed as a product of reductive side reaction. This transformation was found to be significant in the presence of Mo-containing acids due to the redox character of these compounds.