Optically active helical poly[( R or S )-3-(9-alkylfluoren-9-yl)propene oxide]s (poly-AFPOs) were used to induce the enantioselective addition of methyllithium to aldehydes. The addition of methyllithium to 4-methoxybenzaldehyde gave the highest enantioselectivity (88% ee) under the inducement of poly-AFPOs. Poly-AFPOs could be recovered simply by pouring the reaction mixture into methanol in the end of the reaction and separating the precipitated polymer via filtration. The recovered poly-AFPOs could be reused for many times to induce the enantioselective addition of methyllithium to aldehyde without losing their enantioselective inducement ability. The investigation of the optical rotation and the MALDI-ToF MS confirmed that, in the enantioselective addition induced by poly-AFPOs, the lithium cation was embedded in the hole of the helical column of poly-AFPOs, leaving the negative methyl ion outside the one-handed helical column. The methyl anion abutted tightly against the helical polymer and acquired a chiral environment. It was this helically chiral environment that resulted in the enantioselective inducement of the addition reaction.
Several highly modular chiral amino diol tridentate ligands were found to be effective for the asymmetric alkylation reaction of aromatic aldehydes in the presence of methyllithium reagent, providing up to 96% ee values and up to 94% yields under relatively mild conditions. The investigation of the catalytic activity of these ligands shows a correlation of enantioselectivities of ligands with steric properties of substituent N-alkyl and nucleophilic Li-alkyl. With the addition of Ti(O- i -Pr) 4 to the ligand 3c , the 1 H NMR spectrum reveals a hexa-coordinate Ti transition state complex with only one Ti metal center generated in solution.
Tridentate 2,6-bis(imino)phenol ( L1 − L4 ) and 2,6-bis(amino)phenol ( L5 − L8 ) ligands and their corresponding zinc chloride complexes ( 1 – 8 ) have been synthesized and well characterized by FT-IR, 1 H and 13 C NMR , elemental analysis and single-crystal X-ray diffraction analysis. It was found that zinc complexes ( 1 – 4 ) with 2,6-bis(imino)phenol ligands were inactive for the ring-opening polymerization (ROP) of ε-caprolactone (ε-CL) even if they were activated by methyllithium . Zinc complexes ( 5 – 8 ) with 2,6-bis(amino)phenol ligands were proved to be highly efficient initiators for the ROP of ε-CL in combination with 4 equiv. of methyllithium. The ROP of ε-CL in the absence or presence of benzyl alcohol , including the influence of monomer ratio, reaction temperature and the substituents in the ligands, were investigated. The structure and thermal properties of obtained polymers were analyzed by NMR and MALDI-TOF mass spectra, DSC and TGA , respectively.
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