The development of asymmetric catalysis in the past half century has been the history of discovery of small-molecule-based chiral catalyst structures with fixed molecular chirality. Establishment of several privileged structural motifs such as binaphthyl is particularly noteworthy. In stark contrast, utilization of chiral macromolecular structures as catalyst scaffolds has gained little attention, even though the unique characteristics of polymer structures such as the huge steric effect and dynamic conformational change may open up new possibilities of chiral catalysts. Poly(quinoxaline-2,3-diyl)s (PQX hereafter) are a relatively new class of helical macromolecules, being synthesized through living polymerization of 1,2-diisocyanobenzenes. Notably, they adopt stable helical structures that are able to undergo dynamic helix inversion. We have pursued studies to endow new molecular functions to PQX to take advantage of the chirality-switchable nature of the helical macromolecular scaffolds by incorporating various functional groups at the vicinity of the helical main chain. The helical structures are switched sharply by changing surrounding circumstances such as solvents and guest molecules, which interact with the polymer scaffold through nonbonding molecular forces. Catalytic use of PQXs including PQXphos, PQXbpy, and PQXdmap show high enantioselectivities, high reusability, and high catalyst activity in several different transition-metal-catalyzed and organocatalytic reactions. Use of PQX derivatives as chiral catalysts allows utilization of weak nonbonding interaction between PQX and chiral organic molecules as a source of chirality in asymmetric synthesis. For instance, achiral PQX-based catalysts bearing no chiral group form single-handed helical conformation upon dissolution in chiral solvents such as limonene or in achiral solvents to which chiral additives such as protected amino acids are added. Such amplification of weak chiral nonbonding interaction has been made possible by virtue of the dynamic nature of the macromolecular chiral scaffold. Moreover, classical asymmetric amplification, in which the output enantiopurity is higher than the input one, can also be achieved.

Dynamic helical macromolecular catalysts for catalytic asymmetric amplification

ACS Spring 2022

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