Example 2: Preparation of L-cyclohexylalanine
Dissolve or suspend 20 g (121 mmol) of L-phenylalanine in a mixture of 200 ml of deionized water, 200 ml of isopropanol and 12.2 ml (146 mmol) of 37% hydrochloric acid. 2 g of Pt/Rh catalyst (4% Pt + 1% Rh loaded on activated carbon with ~50% water content, corresponding to ~5 wt% by mass of L-phenylalanine) was added. The reaction mixture was transferred to an 11-gallon hydrogenation autoclave. It was displaced three times with nitrogen and then flushed twice with hydrogen, followed by establishing a hydrogen pressure of 8-10 bar and heating the reaction system to 50-60°C. After about 6 to 8 hours of reaction, the hydrogen uptake was completed (theoretical H2 uptake of 8.1 L). The hydrogenator was depressurized and replaced with nitrogen again three times. The reaction mixture was filtered while hot through a nut filter and the catalyst was washed with 50 ml of deionized water. The filtrate was first concentrated to a smaller volume under reduced pressure (mainly to remove isopropanol), followed by adjusting the pH of the residue to 5-6 with 50% sodium hydroxide solution. the solution was cooled to 0-10 °C, the product was collected with a nut filter, washed with 50 ml of deionized water and dried in a vacuum oven at 50-70 °C. The reaction mixture was then purified with nitrogen. Yield: 19.5 g (94.2%).
1H-NMR (500 MHz, D2O/NaOD): δ (ppm) = 0.85-1.0, 1.1-1.52, and 1.63-1.75 (all multiple peaks totaling 13H, cyclohexyl-H, and cyclohexyl-CH2), 3.3 (triple peak, 1H, α-H).
