Polymerization Amplified Stereoselectivity (PASS) of Asymmetric Michael Addition Reaction and Aldol Reaction Catalyzed by Helical Poly(phenyl isocyanide) Bearing Secondary Amine Pendants

Thermo-responsive chiral micelles as recyclable organocatalyst for asymmetric Rauhut-Currier reaction in water

Developing eco-friendly chiral organocatalysts with the combined advantages of homogeneous catalysis and heterogeneous processes is greatly desired. In this work, a family of amphiphilic one-handed helical polyisocyanides bearing phosphine pendants is prepared, which self-assembles into well-defined chiral micelles in water and showed thermo-responsiveness with a cloud point of approximately 38.4 °C. The micelles with abundant phosphine moieties at the interior efficiently catalyze asymmetric cross Rauhut-Currier reaction in water. Various water-insoluble substrates are transferred to target products in high yield with excellent enantioselectivity. The yield and enantiomeric excess (ee) of the product generated in water are up to 90% and 96%, respectively. Meanwhile, the yields of the same R-C reaction catalyzed by the polymer itself in organic solvents is <16%, with an ee < 72%. The homogeneous reaction of the chiral micelles in water turns to heterogeneous at temperatures higher than the cloud point, and the catalyst precipitation facilitates product isolation and catalyst recovery. The polymer catalyst is recycled 10 times while maintaining activity and enantioselectivity.

Achiral organoiodine-functionalized helical polyisocyanides for multiple asymmetric dearomative oxidations

Immobilizing organocatalyst onto helical polymers not only facilitates the catalyst recycling from homogeneous reactions, but also boosts enantioselectivity. In this work, achiral organoiodine-functionalized single left- and right-handed helical polyisocyanides were prepared from the same monomers, which catalyzed three asymmetric oxidations gave the desired products in high yields and excellent enantioselectivity. The enantiomeric excess of the target products was up to 95%. Remarkably, the enantioselectivity can be switched by reversing the helicity of the polymer backbone. The polymer catalysts can be facilely recovered and recycled in different asymmetric oxidations with maintained excellent activity and enantioselectivity.

Recent advances in asymmetric organocatalysis based on helical polymers

The significant research progress (from 2011 to 2021) in artificial helical polymers, such as polyacetylenes, polyisocyanides, polycarbenes, etc., in the fields of asymmetric organocatalysis is described.

Helical Polycarbenes Bearing D-Prolinol Ester Pendants: An Efficient Catalyst for Asymmetric Michael Addition Reaction

A novel diazoacetate monomer (1) carrying tert-butyloxycarboryl (Boc) protected D-prolinol ester was designed and synthesized successfully. Molecular weight-controlled polymerization of 1 using the complex of π-allylPdCl coordinated Wei-phos (LR) ligand gives a series of helical polycarbenes (poly-1ms) with well-defined molecular weights (Mns) and low polydispersity (Mw/Mns). Removing the protecting Boc groups on the D-prolinol ester pendants leads to the formation of helical poly-1m-As, which showed high optical activity. Furthermore, the poly-1m-As showed high catalytic ability on asymmetric Michael addition reaction (up to 76% ee and 94/6 dr). Both the enantioselectivity and diastereoselectivity of the Michael addition reaction were increased comparing to D-prolinol as catalyst. Moreover, the helical polycarbene catalyst can be easily recovered and reused at least four times without significant loss of its enantioselectivity and diastereoselectivity.

Alkyne-Palladium(II)-Catalyzed Living Polymerization of Isocyanides: An Exploration of Diverse Structures and Functions

Inspired by the perfect helical structures and the resulting exquisite functions of biomacromolecules, helical polymers have attracted increasing attention in recent years. Polyisocyanide is well known for its distinctive rodlike helical structure and various applications in chiral recognition, enantiomer separation, circularly polarized luminescence, liquid crystallization, and other fields. Although various methods and catalysts for isocyanide polymerization have been reported, the precise synthesis of helical polyisocyanides with controlled molecular weight, low dispersity, and high tacticity remains a formidable challenge. Owing to a limited synthesis strategy, the controlled synthesis of topological polyisocyanides has barely been realized. This Accounts highlights our recent endeavors to explore novel catalysts for the living polymerization of isocyanides. Fortunately, we discovered that alkyne-Pd(II) catalysts could initiate the living polymerization of isocyanides, resulting in helical polyisocyanides with controlled structures, high tacticity, and tunable compositions. These catalysts are applicable to various isocyanide monomers, including alkyl isocyanides, aryl isocyanides, and diisocyanobenzene derivatives. Incorporating chiral bidentate phosphine ligands onto alkyne-Pd(II) complexes formed chiral Pd(II) catalysts, which promoted the asymmetric living polymerization of achiral isocyanide, yielding single left- and right-handed helices with highly optical activities.Using alkyne-Pd(II) catalysts, various topological polyisocyanides have been facilely prepared, including hybrid block copolymers, bottlebrush polymers, core cross-linked star polymers, and organic/inorganic nanoparticles. For instance, various hybrid block polyisocyanides were easily produced by coupling alkyne-Pd(II)-catalyzed living isocyanide polymerization with controlled radical polymerization and ring-opening polymerization (ROP). Combining the ring-opening metathesis polymerization (ROMP) of norbornene with Pd(II)-catalyzed isocyanide polymerization, bottlebrush polyisocyanides and core cross-linked star polymers were easily prepared. Pd(II)-catalyzed living polymerization of poly(lactic acid)s with isocyanide termini resulted in densely grafted bottlebrush polyisocyanides with closely packed side chains. Moreover, the surface-initiated living polymerization of isocyanides produced a family of polyisocyanide-grafted organic/inorganic hybrid nanoparticles using nanoparticles with alkyne-Pd(II) catalysts anchored on the surfaces. Surprisingly, the nanoparticles and star polymers with helical polyisocyanide arms performed exceptionally well in terms of chiral recognition and resolution. Incorporated organocatalysts such as proline and prolinol units onto the pendants of optically active helical polyisocyanides, a family of polymer-based chiral organocatalysts, were generated, which showed significantly improved stereoselectivity for the asymmetric Aldol reaction and Michael addition and can be easily recycled.Using a chiral alkyne-Pd(II) catalyst, single-handed helical polyisocyanides bearing naphthalene and pyrene probes were produced from achiral isocyanide monomers. These polymers showed excellent self-sorting properties as revealed using a fluorescence resonance energy transfer (FRET) investigation and were self-assembled into two-dimensional (2D) smectic nanostructures driven by both helicity and chain length. Incorporating helical poly(phenyl isocyanide) (PPI) onto semiconducting poly(3-hexylthiophene) (P3HT) induced the asymmetric assembly of the resulting P3HT-b-PPI copolymers into single-handed cylindrical micelles with controlled dimensions and tunable photoluminescence.

Emergence of Highly Enantioselective Catalytic Activity in a Helical Polymer Mediated by Deracemization of Racemic Pendants

Any polymers composed of racemic repeating units are obviously optically inactive and hence chiral functions, such as asymmetric catalysis, will not be expected at all. Contrary to such a preconceived notion, we report an unprecedented helical polymer-based highly enantioselective organocatalyst prepared by polymerization of a racemic monomer with no catalytic activity. Both the right- and left-handed helical poly(biarylylacetylene)s (PBAs) composed of dynamically racemic 2-arylpyridyl-N-oxide monomer units with N-oxide moieties located in the vicinity of the helical polymer backbone can be produced by noncovalent interaction with a chiral alcohol through deracemization of the biaryl pendants. The macromolecular helicity and the axial chirality induced in the PBAs are retained ("memorized") after complete removal of the chiral alcohol. Accordingly, the helical PBAs with dual static memory of the helicity and axial chirality show remarkable enantioselectivity (86% ee) for the asymmetric allylation of benzaldehyde. The enantioselectivity is slightly lower than that (96% ee) of the homochiral PBAs prepared from the corresponding enantiopure (R)- and (S)-monomers, but is comparable to that (88% ee) of the helical PBA composed of nonracemic monomers of ca. 60% ee.

Amino-acid-substituted polyacetylene-based chiral core–shell microspheres: helix structure induction and application for chiral resolution and adsorption

The random coil polymer was first compounded with substrate and induced it into helical structure subsequently.

Self-assembly and fluorescence emission of UV-responsive azobenzene-containing helical poly(phenyl isocyanide) copolymers

UV-responsive azobenzene-containing helical copolymers were obtained, and their self-assembly and fluorescent properties were investigated.

Helical poly(phenyl isocyanide)s grafted selectively on C-6 of cellulose for improved chiral recognition ability

Cellulose graft copolymers are an effective way to endow new properties to cellulose substrate, as well the rigidity, regularity, and helicity of the cellulose backbone could induce the self-assembly of supramolecular structures. In this work, right-handed helical poly(phenyl isocyanide)s (PPI_n) were grafted selectively onto C-6-cellulose. Alkyne-terminated PPI_n was synthesized by living polymerization of right-handed phenyl isocyanide monomer using an alkyne-terminated palladium(II) complex as an initiator/catalyst, and were grafted onto the C-6 of the cellulose backbone (Cell-6-g-PPI_n) at various chain lengths using copper-catalyzed alkyne-azide cycloaddition (CuAAC) "click" chemistry. We confirmed the successful grafting by liquid ¹H NMR and ¹³C NMR, as well as solid ¹³C NMR, FTIR, and GPC. After grafting onto cellulose, the right-handed chirality of PPI_n was significantly increased by 111.2%. Additionally, the Cell-6-g-PPI_n exhibited better chiral recognition of L-Phe-DNSP than PPI_n alone. Therefore, the helical cellulose backbone has enhanced effect on preferred helix of PPI_n.

bola-Type Ala-Ala Dipeptides: Odd-Even Effect in Molecular Packing Structures

A series of bola-type Ala-Ala dipeptides with different alkylene bridges (n = 10-15) were synthesized. In methanol, the molecules with even-numbered carbon bridges self-assembled into twisted nanoribbons, while those with odd-numbered carbon bridges self-assembled into straight belts. The morphology displays a pronounced odd-even dependence upon the number of carbons (n) in the connecting alkylene bridge. The circular dichroism spectra of the self-assemblies showed that molecules with even- and odd-numbered carbon bridges stacked in different structures. FT-IR spectra indicated that the dipeptides with even-numbered carbon bridges formed hydrogen bonds between the amide group and carboxyl ester group, while those with odd-numbered carbon bridges formed hydrogen bonds only between the amide groups. X-ray diffraction patterns revealed that molecules with odd- and even-numbered carbon bridges stacked in monoclinic and triclinic structures, respectively.

Telechelic Helical Poly(quinoxaline-2,3-diyl)s Containing a Structurally Defined, Circularly Polarized Luminescent Terquinoxaline Core: Synthesis by Core-Initiated Bidirectional Living Polymerization

We have designed and synthesized divalent initiators that contain a fluorescent terquinoxaline unit with two palladium groups for the living polymerization of 1,2-diisocyanobenzenes. Using these divalent initiators, the bidirectional living polymerization of a monomer bearing (S)-butoxymethyl side chains afforded telechelic helical poly(quinoxaline-2,3-diyl)s (PQXs), which consist of a terquinoxaline unit at the center of the polymer chain and chiral oligomeric blocks on both sides. The location of the core unit was confirmed by NMR spectroscopy and photoluminescence measurements. Upon changing the solvent from CHCl₃ to 1,1,2-trichloroethane, these PQXs exhibit both left- and right-handed circularly polarized luminescence with dissymmetry factors of approximately 1.0 × 10^-3.

Chiral helical substituted polyacetylene grafted on hollow polymer particles: preparation and enantioselective adsorption towards cinchona alkaloids

Hollow polymer particles tethering chiral helical polymer chains and functional carboxyl groups were prepared and applied in enantioselective adsorption.

Macromolecular helicity control of poly(phenyl isocyanate)s with a single stimuli-responsive chiral switch

The introduction of a single chiral substituent with a switchable conformer equilibrium at the poly(phenyl isocyanate) terminus allowed the control of the helical sense of the polymer backbone using various external stimuli. Helical sense enhancement was achieved through a domino effect, where the helical sense was assigned by VCD spectroscopy and DFT calculations.