The Effect of Sodium Hydroxide on the Stereoselectivity of Hydrogenolytic Asymmetric Transaminations between (R)-2-Amino-2-phenylethanol and 2-Keto Acids

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Abstract
Hydrogenolytic asymmetric transaminations between (R)-2-amino-2-phenylethanol and 2-keto acids were carried out over Pd catalyst in the absence or presence of various amount of sodium hydroxide in water or alcohols. The stereoselectivity of the reaction was significantly influenced by the amount of sodium hydroxide in the reaction mixture. When pyruvic acid was used in water the optical purity of alanine was 22% ee (R) and in 1 M (1 M=1 mol dm−3) sodium hydroxide solution it was 62% ee (S). When 2-oxobutyric acid was used in ethanol in the absence and presence of sodium hydroxide (1M), the optical yields of 2-aminobutyric acid were 2% ee (R) and 81% ee (S), respectively. In contrast, the addition of sodium hydroxide had little or no effect on the stereochemistry of the reaction using (S)-α-methylbenzylamine as a chiral source in aqueous or ethanolic solution. The substituent, additive, temperature, and solvent effects on the asymmetric reactions were examined in aqueous or ethanolic alkaline solution. Based on these experimental results, a possible explanation for the change in stereoselectivity by using sodium hydroxide is discussed.

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