Substituent effect and ligand exchange control the reactivity in ruthenium(II)-catalyzed hydroacylation of isoprenes and aldehydes ‖ A DFT study

Abstract

Density functional theory (DFT) was used to investigate the reaction mechanisms of ruthenium (II)-catalyzed hydroacylation of isoprene with benzaldehyde, and o-methoxyl, m-methoxyl and p-methoxyl benzaldehyde. All intermediates and transition states were entirely optimized at the B3LYP/6-31G(d,p) level (LANL2DZ(f) for Ru). The results demonstrated that the hydroacylation had two di®erent catalytic cycles (path I and II), path II was more favorable than path I. Ru(II)-catalyzed hydroacylation began from the¯rst catalytic cycle, and the nucleophilic reaction was the rate-determining step. The activation barriers of hydrogen migration were the highest in two catalytic cycles, so the hydrogen migration was the rate-determining step. The activation barrier of hydrogen migration could be broken down to two parts: the free energy of exchange (ÁG ex ) and the relative free energy of transition state (ÁG). The ligand exchange energy (ÁG ex ) had more contribution to the activation barrier than the relative free energy of transition state (ÁG), so the ligand exchange would control these hydroacylation. Furthermore, our calculations also described the substituent e®ect, and the results indicated that four aldehydes showed di®erent chemical reactivity, and benzaldehyde and m-methoxyl benzaldehyde were predicted to have the best reactivity in ruthenium hydride-catalyzed hydroacylation.