Belleau's Reagent is primarily used in organic synthesis as a sulfurating agent. It is particularly effective in converting alcohols, amines, and other nucleophiles into their corresponding thiols or sulfides. This reagent is valuable in the synthesis of sulfur-containing compounds, which are often found in pharmaceuticals, agrochemicals, and materials science. Its ability to introduce sulfur atoms into organic molecules makes it a key tool in the development of drugs, where sulfur-containing functional groups can enhance biological activity or stability. Additionally, Belleau's Reagent is utilized in the preparation of sulfur-based polymers and ligands for catalysis, contributing to advancements in industrial chemistry and material engineering.