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 comprising TFAsoluble (Pb(CH3CO2)4, Mn(CH3CO2)3, KMnO4, Tl(CF3CO2)3, AgCF3SO3) and TFA-insoluble (Ce(CF3SO3)4, RuCl3, FeBr3, K2Cr2O7) salts. Optimization 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 yield of 82% and 64% using Pb(CH3CO2)4 and Ce(CF3SO3)4, respectively. Radioiodination of weakly activated monosubstituted benzene derivatives led to high radiochemical yields of about 80% and 60% of the corresponding orthoand para-radioiodo-isomers using both salts. Weakly deactivated chlorobenzene could only be radioiodinated with Ce(CF3SO3)4 as oxidant, forming exclusively the para-product with a radiochemical yield of about 35%. Using the optimized reaction parameters for the other TFA-soluble and -insoluble metal salts with benzene and toluene good radiochemical yields were obtained in all cases except for the manganese and silver salts. Apparently their oxidation power was not strong enough for the radioiodination of the non-activated benzenes. In situ formed trifluoroacetyl [I]hypoiodite is discussed with regard to the reaction mechanism.