Practical experiences with the synthesis of [¹¹C]CH₃ trough gas phase iodination reaction using a TRACERlabFX_C synthesis module

The results of [¹¹C]CH₃I synthesis through hydrogen gas reduction of [¹¹C]CO₂ on different nickel catalysts (HARSHAW-nickel, SHIMALITE-nickel, nickel on silica/alumina, nickel nanosize 99.99%) followed by gas phase iodination using a TRACERlab FX_C synthesis unit are reported. Further reaction parameters such as furnace temperatures, flow rate of hydrogen gas and reduction time were optimized. It was found that reduction of [¹¹C]CO₂ proceeded in 28-83% yield depending on the nickel catalyst and temperature. The gas phase iodination (methane conversion) gave 31-62% of [¹¹C]CH₃I depending on temperature and amount of iodine in the iodine furnace. [¹¹C]CH₃I was used for heteroatom methylation reactions exemplified by a piperazine and a phenol (1 and 3). The specific activity of the ¹¹C-labelled products 2 and 4 was determined after HPLC purification and solid-phase extraction. Compounds 2 and 4 were obtained in 8-14% radiochemical yield (decay-corrected, based upon trapped [¹¹C]CH₄) within 30 minutes. The specific activity was determined to be in the range of 20-30 GBq/µmol at the end-of-synthesis. Nickel catalyst nanosize was found to be superior compared with other Ni catalysts tested. The relatively low specific activity may be mainly due to carbon contaminations originating from the long copper tubing (500m) between the cyclotron and the radiochemistry facility.