Direct n.c.a. radioiodination of weakly activated arenes using metal salts

The direct electrophilic no-carrier-added (n.c.a.) aromatic radioiodination was examined using various metal salts in trifluoroacetic acid (TFA) as in situ oxidation agents. Two different types of metal salts were used compri sing TFA-soluble (Pb(CH3CO2)(4), Mn(CH3CO2)(3), KMnO4, Tl(CF3CO2)(3), AgCF3 SO3) and TFA-insoluble (Ce(CF3SO3)(4), RuCl3, FeBr3, K2Cr2O7) salts. Optimi zation of both labelling systems has been performed using Pb(CH3CO2)(4), Ce (CF3SO3)(4) and benzene as a model substrate. At room temperature, the one- pot synthesis was completed within 15 min, resulting in a radiochemical yie ld of 82% and 64% using Pb(CH3CO2)(4) and Ce(CF3SO3)(4), respectively. Radi oiodination of weakly activated monosubstituted benzene derivatives led to high radiochemical yields of about 80% and 60% of the corresponding ortho- and para-radioiodo-isomers using both salts. Weakly deactivated chlorobenze ne could only be radioiodinated with Ce(CF3SO3)(4) as oxidant, forming excl usively the para-product with a radiochemical yield of about 35 %. Using th e optimized reaction parameters for the other TFA-soluble and -insoluble me tal salts with benzene and toluene good radiochemical yields were obtained in all cases except for the manganese and silver salts. Apparently their ox idation power was not strong enough for the radioiodination of the non-acti vated benzenes. In situ formed trifluoroacetyl [I-131]hypoiodite is discuss ed with regard to the reaction mechanism.