The effect of substituents on the hydrogenation of an aldehyde catalyzed by Knölkerʹs catalyst

Knölkerʹs catalyst has prodigious applicability for the hydrogenation of aldehydes with various functional groups, such as organohalides, nitro groups and benzyl ethers. Density functional theory was used to explore the effect of the substituent on the hydrogen-transfer processes. The hydrogenation of a series of aldehydes with different substituents was analyzed using an outer-sphere concerted hydrogen transfer mechanism. Electron-donating substituents are kinetically and thermodynamically unfavorable for the hydrogenation of aldehydes whereas electron-withdrawing substituents are favorable for reductions. Furthermore, substituents with large steric hindrance have no significant influence on the hydrogenation of aldehydes. These conclusions drawn for the calculations are in good agreement with experimental results.