Alkyl halides are versatile alkylating agents in organic chemistry. Methyl halides are particularly popular as alkylating agents. Representative of the methyl halides is methyl iodide, which in a pure state is a clear liquid that over time becomes brown as a result of decomposition to form various iodine-containing species. Methyl iodide is often stabilized through the addition of a solid metal such as mercury or copper to the storage vessel. As alkyl halides including methyl iodide are susceptible to actinic degradation and free radical decomposition, alkyl halide storage is often problematic. Nonetheless, aged alkyl halides are readily restored to usable form through a distillation process.
The handling of isotopically enriched alkyl halides is made all the more difficult by radioisotope emissions creating free radicals that speed the chemical decomposition of the alkyl halide. Distillation to purify usable alkyl halides from a decomposing isotopically enriched alkyl halide is both technically challenging to perform and highly wasteful of radioisotopes.
Owing to the complexities of handling radioisotopes, isotopically labeled reagents tend to be small molecules that can be synthesized and used quickly. [Methyl-3H]methyl iodide is a common methylating reagent used in the synthesis of methyl-labeled radiochemicals. Unfortunately, the rapid degradation of tritiated methyl iodide and other isotope-enriched alkyl halides means that these reagents must be used rapidly after synthesis. The requirement of rapid usage of isotopically labeled alkyl halides entails a scheduled batch production of the reagent followed by numerous reagent consumptive reactions being performed thereafter. The net result is that labeling reactions cannot be efficiently performed but instead are tied to the schedule of alkyl halide production. Additionally, an excess of isotopically enriched alkyl halide is necessarily produced to preclude the possibility of performing a second batch production to account for any shortfall. The resulting excess production of isotopically enriched alkyl halide is wasteful of materials and increases the waste disposal volume. Thus, there exists a need for an isotopically enriched alkylating reagent that has a longer shelf life than the corresponding alkyl halide without loss of specific isotope activity.