A ruthenium catalyst for the reduction of esters by hydrogenation has been developed. Processes for the hydrogenation of esters have also been developed for (R)-1,2-propanediol and 2-(l-menthoxy)ethanol. The catalyst shows good catalytic activity for the hydrogenation of esters in methanol. Methyl lactate was reduced at 30 °C and gave turnover numbers (TON) up to 4000. The optical purity of the (R)-1,2-propanediol made by the hydrogenation of methyl (R)-lactate was higher than that via the asymmetric hydrogenation of hydroxyacetone. A hydrogenation process to replace the lithium aluminum hydride (LAH) reduction used in the production of 2-(l-menthoxy)ethanol was developed.

A ruthenium complex was found to catalyze the hydrogen reduction of esters under mild and neutral conditions. A variety of optically active esters can be reduced to the corresponding alcohols in excellent yield without loss of their optical purity or causing undesirable side reactions. Hydrogen reduction needs such simple operations -reaction, concentration, and purification -that the violent quench step and extraction step, which accompany conventional sodium borohydride or lithium aluminum hydride reduction, can be omitted.Keywords: chiral alcohols; esters; homogeneous catalyst; hydrogenation; reduction; ruthenium Chiral alcohols are versatile intermediates for physiologically active compounds.[1] A wide variety of chiral secondary alcohols having the hydroxy group on chiral centers are efficiently synthesized by asymmetric syntheses such as catalytic asymmetric hydrogenation or asymmetric biocatalytic reduction of the corresponding ketones.[2a-f] Another technique is dynamic kinetic resolution of racemic secondary alcohols by the combination of metal catalysts and enzymes.[2g] Chiral primary alcohols have often been obtained via stoichiometric metal hydride reduction of the corresponding chiral esters, which are generally easily available from the chiral pool or through asymmetric syntheses. Conventionally, sodium borohydride or lithium aluminum hydride reductions have been employed for this purpose. They are reliable in the laboratory, but an alternative method is desirable for large scale synthesis to avoid the use of extremely reactive metal hydride compounds, and to simplify the subsequent operations.To the best of our knowledge, there are few published articles that refer to catalytic reduction of optically active esters to the corresponding chiral alcohols. In a recent article, a Nishimura catalyst, which consists of Rh/Pt oxide, catalyzed the hydrogenation of chiral a-hydroxy or a-amino esters at 25 8C under 10 MPa hydrogen pressure without racemization. [3] However, application of the catalyst was limited, because at the same time a phenyl group was hydrogenated to a cyclohexyl group.More recently, homogeneous ruthenium-aminophosphine complexes were reported to demonstrate excellent performance in the catalytic reduction of esters. They showed high activity and selectivity under basic and relatively mild conditions.[4] However the article gave no description of chiral esters and optical purity.We began to develop a homogeneous catalytic system for the reduction of chiral esters because homogeneous catalysts seemed the most likely to have both activity and selectivity. Herein, we report a new ruthenium-catalyzed hydrogen reduction of various kinds of optically active esters to the corresponding chiral alcohols with virtually perfect retention of their optical purities.We first investigated whether the reported highly active catalytic system, which employed the complex RuCl 2 (aminophosphine) 2 and a base, [4a] could catalyze the reduction of chiral esters (Table 1). Unfortunately, the procedu...

The use of methanol for the selective methylation of aromatic amines with RuHCl(CO)(PNP) (PNP = bis(2-diphenylphosphinoethyl)amine) is reported. Various aromatic amines were transformed into their corresponding monomethylated secondary amines in high yields at 150 °C with a very low catalyst loading (0.02-0.1 mol %) in the presence of KO Bu (20-60 mol %). The catalyst precursor, RuHCl(CO)(PNP), was converted to [RuH(CO)(PNP)] under the catalytic conditions and also serves as a highly effective catalyst. The robustness of this catalyst contributes to its outstanding catalytic activity, even under reaction conditions, in which CO is liberated from methanol.

New pincer ruthenium complexes bearing a monodentate N-heterocyclic carbene ligand were synthesized and demonstrated as powerful hydrogenation catalysts. With an atmospheric pressure of hydrogen gas, aromatic, heteroaromatic, and aliphatic esters as well as lactones were converted into the corresponding alcohols at 50 °C. This reaction protocol offers reliable access to alcohols using an easy operational setup.

A ruthenium complex was found to catalyze the hydrogenation of chiral esters without the loss of their optical purities under mild and neutral conditions. This method can avoid a violent quench step and an extraction step which accompany conventional reduction using metal hydride reagents such as sodium borohydride and lithium aluminum hydride.