Hydrogenolysis

Hydrogenolysis is a chemical reaction in which a chemical bond is cleaved by molecular hydrogen in the presence of a catalyst. It is distinct from hydrogenation, which adds hydrogen to unsaturated substrates, and from hydrolysis, which cleaves bonds by reaction with water. In practice, hydrogenolysis is typically catalyzed by metals such as palladium, platinum, ruthenium, nickel, or other transition metals supported on carbon or another material, and it often proceeds under hydrogen gas.

Most common hydrogenolysis reactions involve the cleavage of carbon–heteroatom bonds, especially carbon–oxygen (C–O) and carbon–nitrogen (C–N) bonds, though carbon–sulfur (C–S) and, less frequently, carbon–carbon (C–C) bonds can also be broken under suitable conditions. Mechanistically, hydrogenolysis usually involves adsorption of hydrogen on the metal surface, activation of the targeted bond, and reductive elimination of the bond with hydrogen to give two products.

In organic synthesis, hydrogenolysis is widely used to remove protecting groups, such as benzyl ethers and the Cbz (benzyloxycarbonyl) group, to reveal the free functional groups. Industrially, hydrogenolysis plays a role in the upgrading of biomass-derived materials and in various refinery processes where ether or other heteroatom-containing linkages are cleaved in the presence of hydrogen.

Conditions vary with substrate and catalyst but commonly employ hydrogen at modest pressures and moderate temperatures, for example 1–50 bar of H2 and temperatures from ambient to around 100–150°C. Limitations include possible over-reduction of sensitive functionalities, catalyst deactivation by poisons, and selectivity challenges when multiple reducible bonds are present.