Single-Handed Helical Poly(quinoxaline-2,3-diyl)s Bearing Achiral 4-Aminopyrid-3-yl Pendants as Highly Enantioselective, Reusable Chiral Nucleophilic Organocatalysts in the Steglich Reaction

Abstract

Helically chiral poly(quinoxaline-2,3-diyl)s bearing 4-aminopyrid-3-yl pendants were synthesized as new helical-polymer-based chiral nucleophilic organocatalysts. The obtained chiral nucleophilic polymer catalysts exhibited high catalytic activity, enantioselectivity, and reusability in asymmetric Steglich rearrangement of oxazolyl carbonate to C-carboxyazlactone. The polyquinoxaline-based, helically chiral DMAP catalyst mediated intramolecular acyl transfer selectively, by contrast with known small-molecule-based chiral organocatalysts, which also mediate intermolecular acyl transfers.

Helicity Induction and Its Static Memory of Poly(biphenylylacetylene)s Bearing Pyridine N‐Oxide Groups and Their Use as Asymmetric Organocatalysts

“…The application of optically active helical polymers for asymmetric catalysis has also attracted growing interest, because a one‐handed helical chirality can significantly contribute to the enantioselectivity due to a one‐handed helical arrangement of the catalytically active, chiral/achiral units along the one‐handed helical polymer backbone . Up to now, a variety of helical polymer‐based asymmetric catalysts have been developed mainly by introducing catalytically active, chiral/achiral units as components in or along artificial helical polymers that involve polyacetylenes, poly(methacrylate)s, polyisocyanates, polyisocyanides, and poly(quinoxaline‐2,3‐diyl)s . In these cases, the use of optically active monomers is essential to control their helical handedness except for poly(methacrylate)s with a bulky pendant prepared by helix‐sense‐selective polymerization using chiral catalysts or initiators .…”

Helical Poly(quinoxaline‐2,3‐diyl)s Bearing 1,2,3‐Triazole Pendants: Synthesis by CuAAC and Use as Reusable Abnormal NHC Ligands in Gold Catalysis

“…Because of the rigid rod‐like structure without folding or aggregation, most PQX bearing alkoxy side chains show high solubility toward most organic solvents even if the polymerization degree reaches a thousand, but become insoluble in some solvents such as water, methanol, acetonitrile, and pentane. Taking advantage of the single‐handed helical structure, PQX has been used as a highly enantioselective chiral catalyst in transition metal catalysis and organocatalysis with the introduction of catalytically active pendants such as diarylphosphino, bipyridyl, and 4‐aminopyridyl groups. Although the focus has been mostly on their use as chiral catalysts, the merits of using PQX as a scaffold for catalysts are not limited to its ability to provide a highly efficient asymmetric reaction environment.…”

“…We have already established that PQX‐based catalysts are recoverable and reusable by very simple experimental operations. For instance, PQX bearing 4‐aminopyridyl pendants was easily recovered by adding acetonitrile after asymmetric Steglich rearrangement, in which the PQX was well dissolved in toluene at low temperature and reused at least 11 times without any decrease in enantioselectivity and catalyst activity . The switchable solubility can be claimed as an additional key feature of the PQX‐based catalyst.…”

The Steglich Rearrangement of 2‐Oxindole Derivatives Promoted by Anion‐based Nucleophilic Catalysis

“…The Lewis base‐catalysed Steglich rearrangement involving O ‐ to C ‐acyl transfer in oxazolyl carbonates (and related O ‐acylated benzofuran‐ and ‐oxindole derivatives) has been used as an effective method for generating chiral all‐carbon quaternary stereocentres and as a platform to both demonstrate and evaluate the ability of chiral catalysts to bring about enantioselective acylation . A wide variety of neutral chiral nucleophilic catalysts have been reported to promote the rearrangement, including ( inter alia ) derivatives of 4‐pyrrolidinopyridine (PPY), N,N ‐dimethylaminopyridine (DMAP), chiral phosphines, amidines, isothioureas, N ‐heterocyclic carbenes, chiral thioureas (in conjunction with DMAP), helical poly(quinoxaline‐2,3‐diyl)s bearing achiral aminopyridines, and most recently ‐ a modified DMAP‐ N ‐oxides …”