Optically active poly(diphenyl-2-pyridylmethyl methacrylate): Asymmetric synthesis, stability of helix, and chiral recognition ability

Abstract

Dipheny-2-pyridylmethyl methacrylate (D2PyMA) was anionically polymerized with several organolithium complexes of chiral ligands such as (-)-sparteine (Sp), (S,S)-( + )- and (R,R)-(-)-2,3-dimethoxy-1,4-bis(dimethylamino)butane [(+)- and (- )-DDB], (S)-(-)-1 - methyl-2-(piperi-dinomethyl)pyrrolidine (MPP), (S,S)-( + )-3,4-dimethoxy-N-(2-(dimethylamino)ethyl)pyrrolidine (DDEP), and (S,S)-(+ )-N-benzyl-3,4-dimethoxypyrrolidine (BDP) in toluene at low temperature. Sp complexes gave a polymer of low optical activity. DDB was a better chiral ligand, and its complex, particularly the complex with N,N'-diphenylethylenediamine monolithium amide (DPEDA-Li), was effective in preparing a polymer of high optical rotation. A THF-soluble portion of the polymer obtained with (-)-DDB-DPEDA-Li showed [α]^25/365 about - 1400°, which was comparable to that of poly(triphenylmethyl methacrylate) (PTrMA) with pure one-handed helical structure. However, in the polymerization with DDB-DPEDA-Li, optical rotation of the polymer depended greatly on polymerization conditions, such as monomer and initiator concentrations, reaction temperature, and polar additives. Fractionation by THF and/or gel permeation chromatography of the obtained polymers revealed that poly(diphenyl-2-pyridylmethyl methacrylate) (PD2PyMA) consisted of a mixture of (+)- and (-)- polymers which probably were not enantiomeric to each other. DDEP-DPEDA-Li and DDEP-fluorenyllithium complexes were also efficient chiral initiators that gave polymer of the same or higher optical rotation in comparison with DDB complexes. The optical rotation of PD2PyMA in CHCl₃-2,2,2-trifluoroethanol (90:10) slowly changed with time. Optically active PD2PyMA coated on macroporous silica gel resolved racemic compounds as a chiral stationary phase for high-performance liquid chromatography (HPLC).