DFT Mechanistic Investigation on Manganese Pincer Complex Catalysed Cross-Coupling of Methanol with Benzyl Alcohol to Afford Methyl Benzoate.

In this paper the density functional theory (DFT) method was employed to investigate the cross-coupling of methanol with benzyl alcohol to afford methyl benzoate, catalysed by Mn-PNN pincer complex. The whole reaction process mainly includes three stages: the dehydrogenation of benzyl alcohol to benzaldehyde, the coupling of benzaldehyde with methanol to hemiacetal and the dehydrogenation of hemiacetal to methyl benzoate. The calculated results indicated that two dehydrogenation processes are influenced by two competitive mechanisms of inner and outer spheres. Dehydrogenation of benzyl alcohol to benzaldehyde is the rate-determining step of the whole reaction, with the energy barrier of 22.1 kcal/mol. In addition, the regeneration of catalyst is also extremely important. Compared with direct dehydrogenation, the dehydrogenation mode assisted by formic acid is more advantageous. This work might provide theoretical insights and shed light on the design of cheap transition-metal catalysts for the dehydrogenation reaction.

The Origin of Stereoselectivity in the Hydrogenation of Oximes Catalyzed by Iridium Complexes: A DFT Mechanistic Study.

Herein the reaction mechanism and the origin of stereoselectivity of asymmetric hydrogenation of oximes to hydroxylamines catalyzed by the cyclometalated iridium (III) complexes with chiral substituted single cyclopentadienyl ligands (Ir catalysts A1 and B1) under acidic condition were unveiled using DFT calculations. The catalytic cycle for this reaction consists of the dihydrogen activation step and the hydride transfer step. The calculated results indicate that the hydride transfer step is the chirality-determining step and the involvement of methanesulfonate anion (MsO-) in this reaction is of importance in the asymmetric hydrogenation of oximes catalyzed by A1 and B1. The calculated energy barriers for the hydride transfer steps without an MsO- anion are higher than those with an MsO- anion. The differences in Gibbs free energies between TSA5-1fR/TSA5-1fS and TSB5-1fR/TSB5-1fS are 13.8/13.2 (ΔΔG‡ = 0.6 kcal/mol) and 7.5/5.6 (ΔΔG‡ = 1.9 kcal/mol) kcal/mol for the hydride transfer step of substrate protonated oximes with E configuration (E-2a-H+) with MsO- anion to chiral hydroxylamines product R-3a/S-3a catalyzed by A1 and B1, respectively. According to the Curtin-Hammet principle, the major products are hydroxylamines S-3a for the reaction catalyzed by A1 and B1, which agrees well with the experimental results. This is due to the non-covalent interactions among the protonated substrate, MsO- anion and catalytic species. The hydrogen bond could not only stabilize the catalytic species, but also change the preference of stereoselectivity of this reaction.