A new, convenient method for the preparation of 4-[18F]fluorobenzyl halides

A convenient method suitable for automated preparation of 4-[18F]fluorobenzyl halides from no-carrier-added [18F]fluoride has been developed. 4-[18F]Fluorobenzaldehyde, synthesized from [18F]fluoride by aromatic nucleophilic substitution on 4-trimethylammoniumbenzaldehyde triflate, was first retained on a C18 cartridge and there efficiently reduced to 4-[18F]fluorobenzyl alcohol simply by flowing an aqueous solution of NaBH4. The conversion of 4-[18F]fluorobenzyl alcohol to 4-[18F]fluorobenzyl halide was investigated using PBr3, PI3, P2I4, Ph3PBr2 and Ph3PI2 in CH2Cl2. 4-[18F]Fluorobenzyl halides were purified by passing through a disposable silica cartridge. The conversion rapidly proceeded in radiochemical yields of nearly 90% at 40°C with P2I4 and almost quantitatively at room temperature with Ph3PBr2. With this last reagent 4-[18F]fluorobenzyl bromide was obtained in overall radiochemical yields of 50–60% within 30 min from EOB.