Catalytic Transfer Hydrogenation by a Trivalent Phosphorus Compound: Phosphorus-Ligand Cooperation Pathway or PIII/PV Redox Pathway?

Main‐group‐element catalysts are a desirable alternative to transition‐metal catalysts because of natural abundance and cost. However, the examples are very limited. Catalytic cycles involving a redox process and E‐ligand cooperation (E=main‐group element), which are often found in catalytic cycles of transition‐metal catalysts, have not been reported. Herein theoretical investigations of a catalytic hydrogenation of azobenzene with ammonia–borane using a trivalent phosphorus compound, which was experimentally proposed to occur through PIII/PV redox processes via an unusual pentavalent dihydridophosphorane, were performed. DFT and ONIOM(CCSD(T):MP2) calculations disclosed that this catalytic reaction occurs through a P‐O cooperation mechanism, which resembles the metal‐ligand cooperation mechanism of transition‐metal catalysts. Team work: The title reaction of azobenzene with ammonia–borane using 1 occurs through a P‐O cooperation mechanism, where the active species is 3. This mechanism is new for main‐group‐element compounds, and resembles the metal‐ligand cooperation observed in transition‐metal complexes. When azobenzene is absent, the PIII/PV redox process occurs during the isomerization of 3 into 2, but it is not involved in the catalytic transfer hydrogenation.