Isotopically labeled compounds have unique properties and are used pervasively in medicinal chemistry. We are developing new chemical reactions that incorporate isotopes into molecules of biological interest. In particular, the hydrogen isotopes deuterium and tritium have a number of specific applications including as radiotracers for absorption, distribution, metabolism and excretion studies that are critical to develop a toxicology profile of pharmaceutical compounds. Deuterated compounds are also finding uses as new drugs due to resistance of in vivo metabolic processes. The tools we develop can be broadly applied for hydrogen isotope labeling and involve collaborative efforts with scientists at Merck. The bond-forming processes are facilitated by the unique reactivity of phosphorus compounds and can be leveraged to introduce heavy isotopes such as ¹¹C, ¹⁵N and ¹⁸F.