Non-Pincer-Type Manganese Complexes as Efficient Catalysts for the Hydrogenation of Esters

Abstract

Catalytic hydrogenation of carboxylic acid esters is essential for the green production of pharmaceuticals, fragrances, and fine chemicals. Herein, we report the efficient hydrogenation of esters with manganese catalysts based on simple bidentate aminophosphine ligands. Monoligated Mn PN complexes are particularly active for the conversion of esters into the corresponding alcohols at Mn concentrations as low as 0.2 mol % in the presence of sub-stoichiometric amounts of KO^t Bu base.

Keywords: alcohols; esters; homogeneous catalysis; hydrogenation; manganese.

Scheme 1

Mn‐based (de)hydrogenation catalysts.

Figure 1

ORTEP diagrams of 1 (left) and 2 (right). Thermal ellipsoids are set at 30 % probability. Hydrogen atoms have been omitted for clarity.

Scheme 2

Hydrogenation of various esters with 2. Conditions: 1 mmol substrate, 75 mol % KO^tBu, 0.2 mol % 2, 2 mL 1,4‐dioxane, 100 °C, 50 bar H₂, 16 h. [a] 0.5 mol % 2, 6 h.

Figure 2

Effect of ester alkoxy group and KO^tBu amount on the degree of hydrogenation (equal to sum of benzyl alcohol, methyl benzyl ether, and $1/2$ benzyl benzoate yields).

Figure 3

Kinetic traces of methyl benzoate hydrogenation with 2. Conditions: 15 mmol methyl benzoate, 10–75 mol % KO^tBu, 0.5 mol % 2, 28 mL THF, 100 °C, 50 bar H₂.

Figure 4

Proposed catalytic cycle for methyl acetate hydrogenation by H₂ and 2 (ΔG and G ^≠ stand for the reaction and activation Gibbs free energy changes in kJ mol⁻¹ at 373 K).