14.3.1: Catalytic Deuteration

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Catalytic Deuteration of Benzene

Combinations of oxidative additions and reductive eliminations have many applications in the synthesis of organic molecules using an organometallic reactant.

Figure \(\PageIndex{1}\) Deuteration of benzene

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 D₂. The D₂ is provided in excess to drive the chemical equilibrium to the right side.

Figure \(\PageIndex{2}\) Mechanism of the catalytic deuteration of benzene

How does this reaction work mechanistically? This 18e Cp₂TaH₃ is actually a precatalyst that is in chemical equilibrium with H₂ and the 16e dicyclopentadienylhydrido tantalum(III) which is the actual catalyst (Fig. \(\PageIndex{2}\)). Elimination of H₂ from the Ta(V) catalyst is a reductive elimination (RE) and the re-addition of H₂ 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 H₂ to form a 16e Cp₂Ta(III)-Ph complex. In the presence of deuterium this complex can add D₂ oxidatively to form an 18e Cp₂D₂Ta(V)-Ph complex. This species can then reductively eliminate a monodeuterated benzene molecule under formation of Cp₂Ta(III)-D. In the presence of enough D₂ this monodeuterated benzene can be further deuterated in subsequent catalytic cycles.