3-Methylbutanal → 1-Isopentyl-4-(4-nitrophenyl)piperazine

A suspension of 1-(4-nitrophenyl)piperazine (5.00 g, 24.1 mmol, 1.0 eq), sodium triaectoxyborohydride (10.23 g, 48.3 mmol, 2 eq), and isovaleraldehyde (4.16 g, 5.18 mL, 48.3 mmol) is stirred at RT for 48 h. The reaction mixture is transferred to a separatory funnel and the solution washed with 10% aqueous NaCl solution (2×50 mL) (gas evolution). A yellow precipitate formed in the separatory funnel DCM (50 mL) was added followed by a saturated aqueous solution of NaHCO3 (50 mL) and the precipitate dissolved. The phases are separated and DCM phase is again washed with saturated aqueous solution of NaHCO3 (50 mL). The combined aqueous NaHCO3 extracts are extracted with DCM (2×50 mL), the combined DCM extracts are dried over MgSO4, filtered through a filtration tube, and the solvent is evaporated to provide a viscous oil that partially solidified on standing. The residue is triturated with hexanes (25 mL) and resulting slurry stirred at room temperature for 48 h. The precipitate which forms is collected by filtration, washed with hexanes (2×25 mL), dried briefly on the frit, and then dried under vacuum for 48 h to provide 1-isopentyl-4-(4-nitrophenyl)piperazine as a yellow solid (5.15 g, 77% recovery, 92% purity as determined by LC/MS analysis @ UV 254 nm detection). The crude product was used in the next step without further purification. LCMS-ESI (m/z): calcd for C15H23N3O2, 277; [M+H]+ found, 278.