Synthesis Of Chiral SALEN Complexes And Their Catalytic Performance On Asymmetric Reactions

A new salicylaldehyde derivative with a quaternary imidazole moiety was synthesized bythe formylation of 2-tert-butylphenol first, then, followed by chloromethylation, andquaternarization with the N-methylimidazole. The salicylaldehyde derivative condensed withtwo chiral diamines, called (1S, 2S)–diphenylethylenediamine and (1R, 2R)-diamnocyclohexane,to afford two corresponding novel chiral Salen ligands. The chiral Salen ligands respectivelycoordinated with Mn(Ⅲ), Fe(Ⅲ) and Cu(Ⅱ) to give six chiral complexes. All the compoundswere characterized by 1H NMR, FT-IR, UV-Vis, ESI-MS and elementary analysis.These Salen-Mn(Ⅲ) complexes were then used as catalysts in the asymmetric epoxidationof unfunctionalized alkene such as styrene, 1, 2-dihydronaphthalene,α-methylstyrene andcis-β-methylstyrene with NaClO and m-CPBA as oxidants. Under the optimized reactionconditions, which were styrene 0.5 mmol, NaClO 1.0 mmol,PyNO 0.1 mmol,CH2Cl2 2.5 mL,9b 8 mmol%, running at 0℃, in 2 h, 30% of enantiomeric excess was obtained. It was found thatboth the steric hinerance and electron donating effect of the quaternary imidazole at C5(5') in thecomplex are smaller than those of tert-butyl group at C5(5') in the Jacobsen complex, which leadto the lower e.e. in the asymmetric epoxidation.The complex 9b was also used as catalyst for the oxidative kinetic resolution (OKR) of1-phenylethanol. Under the optimized reaction conditions, which were 1-phenylethanol 1.0mmol , KBr 0.08 mmol, 9b 0.5 mol%, CH2Cl2 2.0 mL, H2O 4.0 mL, PhI(OAc)2 0.7 mmol,running at 0℃,in 30 minutes, 40% of enantiomeric excess was obtained. It was found that thequaternary imidazole at C5(5') of salicylaldehyde moieties could enhance the solubility of thecomplex and made the catalyst contact with Br+ efficiently. The complex also exhibited as aphase-transfer catalyst in the reaction which can increase the transmission rate of the substrate and the oxidiant in the two phases. All these properties of the complex can increase the reactionrate and lead to the reaction proceding smoothly at lower catalyst loading.