Remarkable Structure Effects on Chiroptical Properties of Polyisocyanides Carrying Proline Pendants

Abstract

Chiral polymers with simple chemical structures and high helical conformation stabilities are important for their applications as chiral supports and asymmetrical catalysts. We report herein the synthesis of a series of aliphatic polyisocyanides carrying proline pendants of different chiralities, and an investigation of the effects of the chemical structures of these pendants on the chiroptical properties of the polymers. The configuration of the chiral center at the 4-position of the proline pendants was changed from S to R to check its effect on the handedness of the helical conformation. To examine the effects of steric hindrance on the stabilities of the helical conformation for these aliphatic representatives, proline pendants with various substituents at both the carboxyl and amine terminals were designed. To further examine the steric effects of the proline pendants, aromatic counterparts were also prepared. In the latter case, the effects of hydrogen bonds between pendant units on the enhancement and stabilities of the helical conformation were investigated by switching from the ester to an amide linkage. The Cotton effects and signal intensities of both aliphatic and aromatic polyisocyanides from circular dichroism spectroscopy were compared based on the bulkiness of the pendant groups, solvent polarities, and solution temperatures. It was found that highly stable helical conformations of polyisocyanides could be imposed by small bulky monoproline pendants.

Synthesis of Unimolecular Micelles with Incorporated Hyperbranched Boltorn H30 Polyester modified with Hyperbranched Helical Poly(phenyl isocyanide) Chains and their Enantioselective Crystallization Performance

“…The resultant hybrid materials with covalently grafted chiral helical polyacetylene chains showed fascinating optical activity and were further used as chiral inducers toward enantioselective crystallization of alanine enantiomers, achieving a high enantiomeric excess value. Besides polyacetylenes, many other helical polymers, including polyisocyanate, polyguanidines, polyisocyanides, as well as sterically restricted poly(methacrylate ester)s and poly(aryl vinyl)s were synthesized and reported to be utilized in many areas one after another.…”

One-Pot Synthesis of Brush Copolymers Bearing Stereoregular Helical Polyisocyanides as Side Chains through Tandem Catalysis

“…Although some investigations have been carried out in recent decades, the number of artificial polymers with stable helical conformation is still limited. Examples of such artificial helical polymers include sterically restricted poly(methacrylate ester)s and poly(aryl vinyl)s, polyisocyanides, polyisocyanate, polyguanidines, and polyacetylenes . Among them, polyisocyanides have attracted considerable research interests in recent years owing to their unique fascinating rigid helical structures and wide applications in many fields .…”

“…Well-defined polyisocyanides can be obtained through the polymerization of appropriate isocyanide monomers with transition metal complexes as catalysts or initiators. The nickel(II) complex is by far the most used catalyst for the isocyanide polymerization; even the Ni(II) species reside on a polymer chain end. , We found that the Ni(II)-terminated regioregular poly(3-hexylthiophene) can initiate the living polymerization of isocyanide, affording well-defined conjugated block copolymers with controlled molecular weights and tunable compositions . However, polyisocyanides prepared with Ni(II) complexes usually possess low stereoregularity, which is important to the optical activity of a helical polymer .…”

Synthesis and Crystal Structure of 3D-Superamolecular Zinc Complex Derived from Pyridine-2,5-dicarboxylate and Oxalate

“…However, few examples of metal-pyridine-2,5-dicarboxylate compounds containing oxalate ligand have been reported so far 12,13 . In this work, we reported a novel Zn(II) complex [Zn 2 (Hpdc) 2 (Ox)(H 2 O) 4 ] with 3D-supermolecular structure based on hydrogen bonding and π-π stacking interactions.…”

“…This kind of ligand combines the advantages of both organic multicarboxylic acid and hetero aromatic compound. Pyridine-2,5-dicarboxylic acid (pdc) is one of such typical examples due to the chelating ability of carboxylic acid and heterocyclic nitrogen group in the chelating mode to catch various metal ions to be amenable to the formation of extended coordination complex structures [2][3][4][5][6][7][8][9][10][11] . However, few examples of metal-pyridine-2,5-dicarboxylate compounds containing oxalate ligand have been reported so far 12,13 .…”