Asymmetric Anionic Polymerization of 1-Phenyldibenzosuberyl and 1-(2-Pyridyl)dibenzosuberyl Methacrylates and Chiral Recognition Ability of the Obtained Polymers

Abstract

ABSTRACT:Asymmetric anionic polymerization of 1-phenyldibenzosuberyl methacrylate [5-phenyldibenzosuberan-5-yl methacrylate, 5-phenyl-10, 11-dihydrodibenzo[a,d] conformation of main chain. In contrast, 2 gave one-handed helical polymers only by the polymerization with the Sp and PMP complexes. The polymerization of 2 with the DDB complex was sluggish. Monomers 1 and 2 showed higher resistance against methanolysis compared with triphenylmethyl methacrylate and diphenyl-2-pyridylmethyl methacrylate, respectively. The obtained optically active polymers resolved several racemic compounds. KEY WORDSOptically active polymers having helical conformation are obtained by asymmetric polymerization of achiral monomers including methacrylates, isocyanates, isocyanides, and chloral. 2 The importance of the helical polymers lies in application for chiral recognition of racemic compounds in addition to unique structural characteristics. Purely one-handed helical poly(triphenylmethyl methacrylate) [poly(TrMA)] is the first vinyl polymer of its kind. 3 -6 Poly(TrMA) has an almost perfectly isotactic configuration and its helical conformation is maintained by steric repulsion between the bulky side groups. One-handed helical polymers have been obtained by anionic polymerization with the complexes of organolithiums and chiralligands. Poly(TrMA) can resolve many classes of racemates when used as chiral stationary phase for high performance liquid chromatography (HPLC). 7 However, the poly(TrMA) has a shortcoming that the ester linkage is readily solvolyzed by methanol which is often used as an HPLC eluent. In order to overcome this drawback, we synthesized several TrMA-analogues including those having electron-withdrawing substituent on the phenyl groups 8 or a pyridyl group 9 -11 in place of a phenyl group. In the present work, we synthesized and polymerized 1-phenyldibenzosuberyl methacrylate (PDBSMA) and 1-(2-pyridyl)dibenzosuberyl methacrylate (2PyDBSMA) having a dibenzosuberyl moiety in the ester group in order to obtain novel, solvolysis-resistant optically active polymethacrylates. Solvolysis of triaryl esters is considered to involve a triaryl cation as an intermediate. The triaryl cation can be stabilized by delocalization of the cation by the planar resonance structures. We assumed that PDBSMA and 2PyDBSMA are more resistant against methanolysis than TrMA 330 and diphenyl-2-pyridylmethyl methacrylate (D2PyMA), their parent monomers, respectively, since the planar structures are difficult to attain for a 1-

Helix-Sense-Selective and Enantiomer-Selective Polymerization of a Chiral Methacrylate by Anionic and Free-Radical Mechanisms

“…This is supported by the results of the polymerization using DDB-DPEDA-Li (runs 3-6) which indicate that the polymer yield increases with the reaction time. The asymmetric anionic polymerization of PDBSMA 8 or TrMA 12 under similar conditions to those in the present work is known to complete within 24 h of reaction time: TrMA polymerization using PMP-9-fluorenyllithium under similar conditions to run 2 in Table I completes within I 0 min. 12 The sluggishness of the reaction may be due to the additional bulkiness of IDPDMA (acetonide moiety) compared with PDBSMA.…”

“…suggesting that this polymer may have a single-handed helicity induced by the chirality of the monomer during polymerization. The CD spectral pattern of the polymer was different from that of the monomer but similar to those of single-handed helical poly(PDBSMA) 8 and poly(TrMA) 12 especially in the wavelength range below 260 nm with an intense positive peak at around 240 nm (Figure 2). The peak intensity of the peak at 240 nm was smaller than that of the peak at 235 nm found for the single-handed helical poly(PDBSMA), 8 but this seems to be due to the overlapping by the negative peak at 230 nm of the (+)-monomer.…”

“…highly isotactic polymers by asymmetric anionic polymerization using chiral organolithium complexes as initiators (helixsense-selective polymerization), 1 · 2 and the polymers show chiral recognition ability toward a wide range of racemic compounds. · 1 · 4 PDBSMA is an especially interesting one among such bulky monomers because it gives an almost completely isotactic polymer (an equimolar mixture of right-and left-handed helices) even by free-radical polymerization regardless of reaction conditions'-" as well as by anionic polymerization 8 while free-radical polymerization of common vinyl monomers in solution generally has poor control of stereochemistry.9·1 ° Taking advantage of this high stereospecificity of the PDBSMA polymerization, we have succeeded in obtaining poly(PDBSMA)s with excess right-or left-handed helicity by free-radical mechanism using optically active initiators. chain transfer agents, solvents, or a free-radical metallic species.…”

Asymmetric Polymerization of 1-(3-Pyridyl)dibenzosuberyl Methacrylate and Chiral Recognition by the Obtained Optically Active Polymer Having Single-Handed Helical Conformation

“…8 • 11 • 12 3PyDBSMA was more durable compared with PDBSMA probably because of the electron withdrawing effect of the 3-pyridyl group which destabilizes 1-aryldibenzosuberyl cation which is an intermediate of the solvolysis reaction. However, 3PyDBSMA was more readily solvolyzed than 2PyDBS-MA.…”

“…These initiators are known to lead PDBSMA to singlehanded helical polymers. 8 The results of anionic polymerization are shown in Table II (runs 1-3). The obtained poly(3PyDBSMA)s had highly isotactic configuration as revealed by 1 H NMR spectra of PMMA derived from the original polymers and exhibited high specific rotation, suggesting that the polymers have helical conformation with excess helicity.…”