The mechanism for the hydrogenation of ketones catalyzed by Knölker's iron-catalyst

Knölker's iron-based catalysts have some value in "green" transformations given the relatively low toxicity of iron compared to more commonly used precious-metal catalysts. Density functional methods have been used to extensively explore the use of an iron catalyst for hydrogen-transfer processes. Several inner- (with substrate coordination) and outer-sphere mechanisms (with no substrate coordination) were analyzed using a semi-simplified system. An outer-sphere route involving a concerted hydrogen-transfer to the substrate was found to be the most kinetically feasible mechanism, with a free energy barrier of only 10.2 kcal mol(-1). This mechanism was then used to explore the complete system at the same theoretical level. The real system had a higher free energy barrier (20.7 kcal mol(-1)) which is due to the steric effect of the substituent group. All the calculated results were in good agreement with the experimental results.