Catalytic Deuteration of Benzene Combinations of oxidative additions and reductive eliminations have many applications in the synthesis of organic molecules using an organometallic reactant. An example is the deuteration of benzene (Fig. \(\PageIndex{1}\)). Deuterated benzene is an important solvent in NMR spectroscopy. Industrially, the deuteration is done using a dicyclopentadienyltrihydridotantalum(V) catalyst starting out from benzene and D 2 . The D 2 is provided in excess to drive the chemical equilibrium to the right side. How does this reaction work mechanistically? This 18e Cp 2 TaH 3 is actually a precatalyst that is in chemical equilibrium with H 2 and the 16e dicyclopentadienylhydrido tantalum(III) which is the actual catalyst (Fig. \(\PageIndex{2}\)). Elimination of H 2 from the Ta(V) catalyst is a reductive elimination (RE) and the re-addition of H 2 to the Ta(III) species an oxidative addition (OA). In the presence of benzene, the Ta(III) species can oxidatively add benzene to form an 18e Ta(V) complex. This complex can reductively eliminate H 2 to form a 16e Cp 2 Ta(III)-Ph complex. In the presence of deuterium this complex can add D 2 oxidatively to form an 18e Cp 2 D 2 Ta(V)-Ph complex. This species can then reductively eliminate a monodeuterated benzene molecule under formation of Cp 2 Ta(III)-D. In the presence of enough D 2 this monodeuterated benzene can be further deuterated in subsequent catalytic cycles.