Asymmetric Catalysis with Helical Polymers

Chiroptical molecular propellers based on hexakis(phenylethynyl)benzene through the complexation-induced intramolecular transmission of local point chirality

We designed hexakis(phenylethynyl)benzene derivatives with a tertiary amide group on each blade to achieve a helically biased propeller arrangement.

Optically active helical polyacetylene/Fe3O4 composite microspheres: prepared by precipitation polymerization and used for enantioselective crystallization

The precipitation polymerization for constructing chiral, magnetic microspheres based on substituted polyacetylene and Fe₃O₄ nanoparticles: preparation and application in enantioselective crystallization.

Tunable asymmetric catalysis through ligand stacking in chiral rigid rods

Chiral benzene-1,3,5-tricarboxamide (BTA) ligands, comprising one diphenylphosphino group and one or two remote chiral 1-methylheptyl side chains, were evaluated in the rhodium-catalyzed asymmetric hydrogenation of dimethyl itaconate. Despite the fact that the rhodium atom and the chiral center(s) are separated by more than 12 covalent bonds, up to 82% ee was observed. A series of control and spectroscopic experiments confirmed that the selectivity arises from the formation of chiral helical polymers by self-association of the BTA monomers through noncovalent interactions. The addition of a phosphine-free chiral BTA, acting as a comonomer for the chiral BTA ligands, increases the level of enantioselectivity (up to 88% ee). It illustrates how the selectivity of the reaction can be increased in a simple fashion by mixing two different BTA monomers. The concept was further probed by performing the same experiment with an achiral BTA ligand, i.e. a phosphine-functionalized BTA that contains two remote octyl side chains. It afforded an encouraging 31% ee, thus demonstrating the catalytically relevant transfer of chirality between the self-assembled units. It constitutes a unique example of the sergeants-and-soldiers principle applied to catalysis.

Complementary Induction of Right- and Left-Handed Helical Structures by the Positioning of Chiral Groups on the Monomer Units: Introduction of (-)-Menthol as Side Chains of Poly(quinoxaline-2,3-diyl)s

Poly(quinoxaline-2,3-diyl) bearing menthyloxymethyl side chains derived from (-)-menthol at the 6- and 7-positions of the quinoxaline ring showed a single, right-handed helical structure in chloroform. Upon introduction of the same (-)-menthol-derived side chains into the 5- and 8-positions, a single, left-handed helical structure was formed in chloroform. The former poly(quinoxaline-2,3-diyl)s showed solvent-dependent inversion of the helical sense in 1,1,2-trichloroethane to form a left-handed helical structure with high screw sense purity. Copolymers bearing both menthyloxymethyl and o-(diphenylphosphino)phenyl groups in their side chains served as highly enantioselective chiral ligands in the palladium-catalyzed hydrosilylation of styrene.

Preparation and application of abietic acid-derived optically active helical polymers and their chiral hydrogels

A novel chiral monomer N-propargyl abietamide, M1, was synthesized from abietic acid and catalytically polymerized with (nbd)Rh+B-(C6H5)4 (nbd=norbornadiene), providing polymer [poly(1)] with a molecular weight of 13,000-36,000 at a yield of 59-84%. Poly(1) did not form stable helices in tetrahydrofuran at room temperature whereas copolymerization of M1 and the achiral N-propargylamide monomer, M2, led to the formation of helical optically active copolymers as indicated by circular dichroism studies, UV-vis spectroscopy, and specific optical rotation measurements. Hydrogels were prepared based on an optically active helical copolymer, poly(M1(0.32)-co-M2(0.68)) that exhibited enantioselective recognition toward l-alanine. The novel chiral polymers derived from abietic acid are expected to find applications in such areas as chiral recognition, chiral resolution, and chiral catalysis.