A. 2,2-Spirocyclohexane-4,5-diphenyl-2H-imidazole (Note 1). A 2-L, three-necked, round-bottomed flask equipped with a mechanical stirrer and a reflux condenser is charged with 1.0 L of glacial acetic acid (Note 2), 158 g (0.75 mol) of benzil (Note 2), 400 g of ammonium acetate (Note 2) and 80 mL (0.77 mol) of cyclohexanone (Note 2). The mixture is stirred and heated at reflux temperature for 1.5 hr (Note 3) and then, while hot, poured into 3 L of vigorously stirred water. The mixture is left overnight to cool to ambient temperature, the crystals are collected by filtration, washed 4 times with 300 mL of water, crushed in a mortar and dried under reduced pressure to give 205–210 g (95–97%) of the imidazole as yellowish-green crystals, mp 105–106°C, lit.³ mp 107–108°C (Note 4).

B. (±)-1,2-Diphenyl-1,2-ethylenediamine. A 2-L, four-necked, round-bottomed flask equipped with a mechanical stirrer, thermometer and dry ice condenser is charged with 72.0 g (0.250 mol) of 2,2-spirocyclohexane-4,5-diphenyl-2H-imidazole. The flask is flushed with argon, and 400 mL of tetrahydrofuran (Note 5) is added. The mixture is stirred until all solids dissolve, cooled to −78°C (dry ice/acetone bath) and treated with a stream of gaseous ammonia (Note 6) until the volume of liquid increases by about 400 mL (Note 7). One of the side necks is then equipped with a solids addition funnel and 6.94 g (1.00 mol) of lithium (Note 8) is slowly introduced by cutting the wire with scissors in a gentle stream of argon. The rate of lithium addition is such that the temperature does not rise above −65°C. Following the addition of lithium, the mixture is stirred for 30 min and 30 mL (1.0 mol) of ethanol (Note 9) is slowly added. The mixture is stirred for an additional 20 min and 70 g of ammonium chloride is added. The cooling bath is removed, the mixture is allowed to warm to 0°C, 400 mL of water is carefully introduced, and the phases are separated. The aqueous phase is washed 3 times with 300 mL of ether and the combined organic extracts are washed with brine, dried over anhydrous sodium sulfate, filtered and concentrated with a rotary evaporator to about 200 mL. The solution is transferred to a 1-L, one-necked, round-bottomed flask equipped with a mechanical stirrer, cooled to 0°C and treated with 300 mL of 2 N aqueous hydrochloric acid. The biphasic mixture is vigorously stirred at ambient temperature for 1 hr, 500 mL of water is added and phases are separated. The organic phase is washed with 150 mL of water and the combined aqueous phases are extracted with 300 mL of dichloromethane. The aqueous solution is then carefully treated with 300 mL of 2 N aqueous sodium hydroxide and the mixture is extracted 4 times with 150 mL of methylene chloride. The combined organic extracts are washed with brine, dried over anhydrous sodium sulfate, and filtered. Removal of volatile material under reduced pressure (water aspirator) gives 47–50 g (89–94%) of racemic diamine as a pale yellow solid, mp 81–82°C, lit.⁴ mp 82°C corr. (Note 10).

C. (1S,2S)-(−)- and (1R,2R)-(+)-1,2-Diphenyl-1,2-ethylenediamine (Note 11). A 1-L, round-bottomed flask equipped with a mechanical stirrer is charged with 42.5 g (0.200 mol) of the racemic diamine and 230 mL of ethanol (Note 9). The solids are dissolved by heating the mixture to 70°C whereupon a hot (70°C), homogeneous solution, of 30.0 g (0.200 mol) of L-(+)-tartaric acid (Note 12) in 230 mL of ethanol is added (Note 13). The tartrate salts precipitate immediately, and after the mixture is cooled to ambient temperature, the crystals are collected by filtration, washed twice with 60 mL of ethanol, and dried under reduced pressure. The solids are dissolved in 230 mL of boiling water, 230 mL of ethanol is added and the homogeneous solution is allowed to cool slowly to room temperature. The crystals are collected by filtration, washed with 40 mL of ethanol and dried under reduced pressure. The recrystallization procedure is then repeated twice using the same volumes of solvents (230 mL of water and 230 mL of ethanol) to give 23–25 g (63–69%) of the tartrate salt as colorless crystals, [α]²³D −10.8 ± 0.2° (H₂O, c 1.3), lit.⁵ [α]²³D −11° (H₂O).

The salt is transferred to a 1-L, one-necked, round-bottomed flask equipped with a magnetic stirring bar and suspended in 300 mL of water. After the mixture is vigorously stirred and then cooled to 0–5°C, 23 mL of 50% aqueous sodium hydroxide is added dropwise followed by 150 mL of dichloromethane, and stirring is continued for 30 min. The phases are separated, the aqueous phase is washed twice with 50 mL of dichloromethane and the combined organic extracts are washed with brine, dried over anhydrous sodium sulfate and filtered. Removal of the volatile material under reduced pressure gives a colorless solid that is recrystallized from hexane to yield 12–14 g (57–66%) of (S,S)-(−)-diamine as colorless crystals, [α]²³D −106 ± 1° (MeOH, c 1.1) lit.⁶ [α]²³D −106.5° (MeOH, c 1.09) (Note 14).

The filtrates from all crystallizations are combined and the solvent is evaporated on a rotary evaporator under vacuum (water aspirator). The residual solid is transferred to a 1-L, one-necked, round-bottomed flask equipped with a magnetic stirring bar, and suspended in 250 mL of water. To this vigorously stirred mixture is slowly added 25 mL of aqueous 50% sodium hydroxide followed by 200 mL of dichloromethane and the stirring is continued for 30 min. The phases are separated, the aqueous phase is washed twice with 50 mL of dichloromethane and the combined organic extracts are washed with brine, dried over anhydrous sodium sulfate and filtered. Removal of volatile material under reduced pressure gives 24–27 g of the enriched (R,R)-diamine as pale yellow crystals. This material is treated with D-(−)-tartaric acid (Note 12) and the resulting salt is recrystallized in exactly the same manner as described for the other enantiomer to give 29–31 g (80–85%) of colorless crystals, [α]²³D +4 ± 0.5 ° (H₂O, c 1.3). (The checkers found that the salt from (−)-tartaric acid was optically impure even after five recrystallizations. However, this did not affect the optical purity of the (R,R)-(+)-diamine.) Treatment with sodium hydroxide, as described above, followed by crystallization from hexane gives 11.5–13 g (54–61%) of (R,R)-(+)-diamine as colorless crystals, [α]²³D +106 ± 1° (MeOH, c 1.1) (Note 14).