Chiral 1,2-diols and 1-hydroxy-2-amino compounds are versatile building blocks for a variety of biological active ingredients. These products can be prepared catalytically via aminohydroxylation and dihydroxylation of terminal C=C bonds, and via hydrolytic resolution of terminal epoxides using Jacobsen's methodology. Another attractive approach is obviously the reduction of alpha-hydroxy- and alpha-amino acids or esters from the chiral pool. In most cases, this is carried out using metal hydrides which are reactive at low temperatures thereby guaranteeing the integrity of the stereogenic centre. Even though such reactions can be carried out at room temperature on a 100 - 150 g scale, the costs and the large amount of waste render this methodology less attractive for technical applications.

Taking these drawbacks into account, catalytic hydrogenation would be an attractive alternative, if a catalyst was available that could do the desired transformation under mild conditions without racemisation.

Solvias AG has succeeded in using the Evonik Nishimura catalyst FG 209 Y/D 2% Pt + 4 % Rh in the hydrogenation of chiral alpha-amino and alpha-hydroxy esters to the corresponding chiral alcohols under very mild conditions with reasonable yields.

The key findings were:

• Catalyst type: Only the Nishimura catalyst (Evonik grade FG 209 Y/D 2% Pt + 4 % Rh) works.
• Catalyst loading: A relatively high loading (10 % w/w) is needed.
• Substrate: Only the esters of alpha-hydroxy and alpha-amino acids are converted.
• Solvent: Methanol and ethanol work best, but THF, DMF or AcOH are also suitable.
• Conditions: Room temperature, 60-100 bar hydrogen, 15-20 h

For more details about this mild and selective catalytic system please see:

• M. Studer, S. Burkhardt, H.-U. Blaser, Adv. Synth. Catal. 2001, 343, 802-808.