Transfer of Chirality from (−)‐Sparteine Zinc(II) (Meth)acrylate Complexes to the Main Chains of Their (Meth)acrylate Polymer Derivatives

Abstract

Optically active polymers represent a class of materials of importance because of their unique properties and potential application as chiral chromatographic stationary phases, as supports for or ligands in asymmetric catalysts, and as components in nonlinear optical and liquid-crystalline materials. However, vinyl and related polymers that are optically active as a result of the configuration of their main chains, rather than chiral side-chain substituents, are very rare as this requires very special stereoregular symmetries of consecutive repeat units.In general, symmetry operations are more difficult to recognize in a long polymer chain made up of uniformly repeating units than in low-molecular-weight compounds. Wulff, however, recognized that only one specific sequence of eight possible hexad sequences of a homopolymer of a monosubstituted vinyl monomer is chiral.[1] Not surprisingly it is extremely difficult to control a vinyl polymerization to introduce such a chiral hexad sequence in a polymer backbone. One of the methods proposed by Wulff to reduce this difficulty was to introduce a second repeat unit in the polymer backbone to help break down the overall symmetry. For such copolymers, triad sequences (Figure 1) in which a syndiotactic

Controlling monomer-sequence using supramolecular templates

“…14 (A) Cyclocopolymerisation of a supramolecular complex of two (meth)acrylic acid monomers with aryl monomers. 88,89 (B) Promotion of cyclopolymerisation of ethylene glycol containing bivalent monomers using potassium-cations. 92 (C) Promotion of the cyclopolymerisation of a heterobifunctional monomer containing both a styrene and a maleimide moiety using a potassium-cation.…”

“…87 Jana et al used metal-ligand complexation to create chiral metal complexes incorporating two polymerisable moieties ((meth)acrylates), Zn(II) and (−)-sparteine for the preparation of main-chain optically active polymers. 88,89 Racemic diads spaced by a single different monomer are needed to obtain optically active polymers. 90 The coordination of two (meth)acrylic acid monomers and optically active spartein to Zn(II), which is tetracoordinating, yielded a complex, the copolymerisation of which with aryl containing monomers (e.g.…”

Indolizidine and quinolizidine alkaloids

“…90 Another publication in which crystal structures of the sparteine complexes with zinc(II) acrylate or methacrylate were described also included interesting observations on the transfer of chirality from the complexes to the (meth)acrylate copolymers formed by solution polymerisation with styrene or 2-vinylnaphthalene in the presence of azoisobutyronitrile as initiator. 91 Monoprotonation of sparteine, like complexation with metals, is also accompanied by conformational inversion of ring C to give an all-chair arrangement, with the proton forming a hydrogenbonded bridge between the two nitrogen atoms. An NMR spectroscopic comparison of the monoperchlorate salts of sparteine, 2methylsparteine and a-isosparteine with the corresponding zinc(II) complexes containing a variety of counter-ions (Cl, Br, CN, OAc, methacrylate) revealed conformational similarities in solution that agreed well with reported solid-state conformations.…”

Synthesis of main chain chiral methacrylate copolymers via chirality transfer from polymerizable chiral metal complexes

“…S1, ESI †) as reported in our preliminary publication. 9 Polarimetric measurements at three different wavelengths (365, 436 and 589 nm) proved that both complexes ZMS and ZAS are optically active and levorotatory (Table 1). Although the structures of ZMS and ZAS are similar to each other, the specific optical rotation values of ZAS are about six to eight times higher than those of ZMS.…”

“…Previous positive results from our laboratory yielded main chain optically active 4-vinylpyridine copolymers 8 which encouraged us to investigate other polymerizable metal complex templates that might yield main chain chiral polymers with better prospects for subsequent exploitation. Indeed in our preliminary publication 9 we were able to disclose the syntheses of optically active copolymers of styrene (St) with both acrylic (AA) and methacrylic acids (MAA) (see List of abbreviations), where the sole source of asymmetry is the configuration of the carbon atoms along the polymer chain. The syntheses were achieved by free radical copolymerization of St with (À)-sparteine Zn(II) dimethacrylates, ZMS or ZAS, followed by cleavage of the (À)-sparteine Zn(II) templates, with chirality being transferred from these complexes to the copolymer main chains (Scheme 1).…”