Potassium phosphate as a solid base catalyst for the catalytic transfer hydrogenation of aldehydes and ketones

Potassium triphosphate is a surprisingly active catalyst for the catalytic transfer hydrogenation of carbonyl compounds. A pretreatment at 600 °C is critical to obtain the active material. Temperature programmed desorption of CO₂ indicates that sites of lower basicity were created after heat treatment for 5 h as compared to 1 h. With 2-propanol as hydride source, aromatic aldehydes were reduced to the corresponding alcohols with high rates. The presence of electron withdrawing chloro- and nitro-substituents at the benzene ring increases the rate of reaction compared with unsubstituted benzaldehyde, while electron donating methyl or 2-propyl groups has the opposite effect. Ketones were reduced with a lower rate of reaction than aldehydes. The role of K₃PO₄ as a heterogeneous catalyst is indicated by a complete lack of reaction in the absence of the solid base and by studies with competing substrates. The latter studies suggest that the catalytic transfer hydrogenation of benzaldehyde and chlorinated benzaldehydes takes place at different surface sites.