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E. Mennicke, M.H. Holschbach, Heinz H. Coenen

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 TFA-soluble (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 ortho- and 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 [¹³¹I]hypoiodite is discussed with regard to the reaction mechanism.

Radiochimica Acta, Oldenbourg Wissenschaftsverlag

Print ISSN: 0033-8230
Volume: 88, 03/2000
Pages: 221

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