To a well-stirred aqueous solution of 1,2-diaminocyclohexane (11.4 g, 0.1 mol) was added sodium hydroxide (8 g, 0.2 mol) followed by dropwise addition of carbon disulfide (8.36 g, 0.11 mol) at room temperature. The reaction mixture was heated to 100°C and stirred overnight. After completion of the reaction, the mixture was cooled to room temperature, acidified with dilute hydrochloric acid to pH ≈ 2, and then extracted with dichloromethane (100 mL × 3). The organic layers were combined, dried with anhydrous sodium sulfate, filtered and concentrated under reduced pressure to remove the solvent to give the off-white solid product octahydro-2H-benzimidazole-2-thione (12.7 g, 81% yield). The product was characterized by 1H-NMR (400 MHz, CDCl3): δ 1.12-1.31 (m, 2H), 1.33-1.52 (m, 2H), 1.57-1.91 (m, 2H), 1.99-2.08 (m, 2H), 3.26-3.35 (m, 2H), 6.2 (s, 1H). Mass spectrum (CI): m/z 157.2 ([M + 1]+).
