The Ascent of Supramolecular Polymers and Gels in Asymmetric Catalysis

Abstract

Supramolecular polymers (SPs) and gels, formed by the spontaneous assembly of small molecules through various types of noncovalent interactions, are attractive materials for many applications. Their modularity also offers many opportunities in asymmetric catalysis that have been tackled in the last two decades and more intensively in the last one. In this review, strategies adopted to develop efficient asymmetric catalysts supported on SPs and gels are first categorized according to the chiral or achiral nature of the monomers used for their construction and second to their ability to be commuted into different states. Catalytic SPs have been described for which enantioselectivity stems mostly from the molecular chirality located next to the reactive group, or at opposite ends of the spectrum, exclusively from the chiral environment provided by the supramolecular helices. New paradigms revealed by these systems include (i) the organization of catalytic sites at the periphery of modular and well-structured 1D assemblies, (ii) the possibility to conduct asymmetric reactions with a sub-catalytic amount of chiral inducers and even in the absence of chiral monomers, and (iii) the development of a new class of switchable asymmetric catalysts.

Keywords: asymmetric catalysis; chirality; supramolecular gels; supramolecular polymers; synthesis.

Figure 1

a) Example of an asymmetric reaction involving the constituting monomers of a supramolecular gel.^{[ 63 ]} b) Schematic representations of the four strategies used to construct catalytically active chiral SPs and SCPs. The main chain of the chiral SP corresponds to a helical arrangement of the monomers as represented as a dotted helix highlighted in blue and red for P (right‐handed) and M (left‐handed) helices, respectively. SMSB: SMSB. S&S: “S&S”.

Figure 2

Chemical structure of PV6 a), SEM image of a gel of PV6 in toluene b), and reaction scheme of the 1,4‐conjugate addition c). The SEM image is reproduced from Ref. [98] with permission from the Royal Society of Chemistry.

Figure 3

Chemical structures of L‐Pro‐L‐Glu and L‐Pro‐D‐Glu a) and SEM image of the L‐Pro‐L‐Glu xerogel formed from CH₃CN/CHCl₃ b). TBDM: tert‐butyldimethylsilyl. The SEM image is reproduced from Ref. [100], https://doi.org/10.1021/acsomega.8b00852, under the terms of the Creative Commons CC BY license, https://creativecommons.org/licenses/.

Figure 4

Chemical structures of PVC12 and SucV8 a) and SEM image of the PVC12 hydrogel b). Catalytic performance of PVC12, SucV8 and their mixture in the Mannich reaction c). The SEM image is reproduced from Ref. [108] with permission from the Royal Society of Chemistry.

Figure 5

Self‐aldol reaction of glycolaldehyde catalyzed by a glutamine derivative.

Figure 6

Two‐component hydrogel and schematic representation of the stacks formed by G2. Adapted from Ref. [115] with permission from the American Chemical Society.

Figure 7

Chemical structure of L‐Pro‐NDI and schematic representation of its assembly into a supramolecular nanotube. Adapted from Ref. [119] with permission from the Royal Society of Chemistry.

Figure 8

Chemical structures of the BTA monomers investigated in the model aldol reaction a). Cryo‐TEM images of BTA‐a (5 × 10^─4 M) in water after temperature treatment; the scale bar represents 100 nm (bottom left, b). Reproduced from Ref. [120] with permission from Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim.

Figure 9

Chemical structure of L‐HDGA and schematic representation of its assembly into multi‐walled nanotubes a). Adapted from Ref. [124] with permission from the American Chemical Society. b) AFM images of L‐HDGA nanotubes loaded with Bi(OTf)₃ (Bi:L‐HDGA ratio of 1/50). The size of the image is 5 × 5 µm²; the size of the enlarged part is 1 × 1 µm². Reproduced from Ref. [125] with permission from the American Chemical Society. c) Reactions catalyzed by L‐HDGA metal hybrids with the highest observed selectivities.

Figure 10

a) Chemical structure of L‐PhgC₁₆ and D‐PhgC₁₆ enantiomers and schematic representation of their assembly into helical nanoribbons. CD spectra of the assemblies formed in a 4/6 methanol/water solvent mixture. b) Catalytic performance of the copper hybrid nanoribbons in the Diels‐Alder reaction. Reproduced from Ref. [126] with permission from Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim.

Figure 11

a) Schematic representation of the assemblies formed between HDGA and the Rh(II) precursor. Best catalytic performance of the nanosphere and nanotube assemblies in the rhodium‐catalyzed cyclopropanation reaction. b) SEM image of the assembly formed by mixing equimolar amounts of the Rh(II) precursor and HDGA. Inset: photograph of the assemblies formed in water (coloration is due to the Tyndall effect). c) AFM image of the assembly formed with a Rh(II) precursor/HDGA ratio of 1/100. Reproduced from Ref. [134] with permission from Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim. Copyright 2018.

Figure 12

a) Chemical structures of the chiral BTA ligands and of the chiral BTA co‐monomer. b) Schematic representation of the chiral SP formed by BTA m‐P(S),(S) coordinated to rhodium. c) Catalytic performance of the BTA ligands in the rhodium‐catalyzed hydrogenation reaction.

Figure 13

Chemical structure of zw‐TPPS₄ (zw stands for zwitterionic), its assembly to chiral J‐aggregate upon vortex stirring, and the implementation of the hybrid assemblies formed in the presence of isoindoline as a catalyst for a Diels‐Alder reaction.

Figure 14

a) Chemical structure of BTA^BA and its assembly into biased helical assemblies (–)‐PBTA^BA and (+)‐PBTA^BA thanks to SMSB. b),c) Photographs of the suspensions obtained upon cooling and rotary stirring of the heated solutions of BTA^BA . SEM images of the air‐dried suspensions corresponding to P‐and M‐biased supramolecular helical assemblies. d) Catalytic performance in the copper‐catalyzed Diels‐Alder reaction. Figures a)‐c) are adapted from Ref. [139] https://doi.org/10.1038/s41467‐019‐11840‐3, under the terms of the Creative Commons CC BY license https://creativecommons.org/licenses/.

Figure 15

a) Chemical structure of the BTA monomers and schematic representation of their coassembly into “S&S” type SCPs. “Sergeants” in blue and red induce the preferential formation of right‐handed and left‐handed helices, respectively (represented by the dotted lines connecting the rings). The dots on the rings represent the amide functions (three per monomer). b) Catalytic performance of the S&S‐type copolymers in the rhodium‐catalyzed hydrogenation reaction. c) Plot of the enantioselectivity as a function of the fraction of the “sergeant” (BTA Ile) present in the mixture. The fraction of “sergeants” is equal to the concentration in “sergeants” divided by the total concentration in BTA monomers; in that case, [BTA Ile]/([BTA m‐P]+[BTA Ile]). This definition is valid throughout this review and abbreviated as fs. d) Schematic representation of the postulated structures leading to highly (structure A) and poorly (structure B) selective catalytic systems.

Figure 16

a) Chemical structure of the BTA monomers (see also Figure 15a). b) Catalytic performance in the copper‐catalyzed hydrosilylation reaction. c) Plot of the enantiomeric excess in NPnol versus the Kuhn anisotropy factor extracted from CD analyses of the “S&S” type coassemblies. d) Schematic representation of the role of the “sergeant” as intercalator or chain stopper depending on its structure. Chiral defects are represented by the reversal of the handedness of the helical coassemblies.

Figure 17

a) Chemical structure of the BTA monomers BTA (S)‐Leu and BTA (S)‐Eth. b)‐d) Catalytic performance in the copper‐catalyzed hydrosilylation reaction for the “S&S” type coassemblies for two different concentrations in BTA P.

Figure 18

a) Reaction scheme and conditions for probing the influence of achiral additives on the extent of the S&S effect displayed by BTA helical catalysts. b) Molecular structures of the tested achiral BTA additives and schematic representation of the BTA helical catalyst embedding the three types of monomers (BTA ligand, “sergeant,” and additive). c) Plot of the ee in NPnol as a function of the fraction of “sergeants” for the S&S‐type helical catalyst composed of BTA P and BTA Cha in the presence and absence of a‐BTA. d) Plot of the ee in NPnol as a function of the fraction of “sergeants” for the S&S‐type catalyst composed of BTA P and BTA Cha in the presence and absence of different achiral BTA additives. e) Best‐performing S&S‐type helical catalyst in the copper‐catalyzed hydrosilylation reaction. f) Representation and composition of the best‐performing supramolecular helical catalyst for the hydrosilylation reaction. g) Schematic representation of the role of the achiral BTA additive, a‐BTA.

Figure 19

a) Reaction scheme and conditions for the copper‐catalyzed hydroamination with BTA helical catalysts. b) Plot of the ee in DBA as a function of the fraction of “sergeants” for the S&S‐type helical catalyst composed of BTA P^CF3 and BTA (S)‐Leu in the presence and absence of a‐BTA. c) Schematic representation of the structure of the helical BTA terpolymer used for the hydroamination reaction as well as the benefits brought by the presence of a‐BTA.

Figure 20

a) Schematic representation of the strategy used to reversibly dissociate supramolecular helical catalysts together with the relationship between their length and their selectivity toward the model hydrosilylation reaction. b) Reaction scheme for the successive transformation of several equivalents of NPnone and the selectivity expected for each run. c) Plot of the obtained enantioselectivity in NPnol for the different runs.

Figure 21

a) Schematic representation of the strategy used to switch the handedness and thus the selectivity of BTA‐based helical catalysts. b) Plot of the net helicity, that is, the optical purity of the supramolecular helices, as a function of the ee in “sergeants” for the mixtures composed of BTA P and both enantiomers of BTA Cha. c) Catalytic results obtained for the transformation of an equimolar mixture of NPnone and PPnone for BTA helical catalysts without and with commutation of the selectivity during the transformation.

Figure 22

a) Schematic representation of the strategy used to switch the handedness of the BTA helical catalyst used in the cascade hydrosilylation/hydroamination reaction. Composition of the supramolecular ter‐ and tetrapolymer. b) Catalytic results obtained in the copper‐catalyzed hydrosilylation/hydroamination cascade transformation of VPnone with the BTA helical catalysts without and with commutation of the selectivity during the cascade transformation. Catalytic conditions: BTA monomers shown in a) P(3,5‐(CF₃)₂‐C₆H₃)₃ dimethoxymethylsilane (DMMS) for both hydrosilylation and hydroamination, O‐pivaloyl‐N‐dibenzylhydroxylamine (amine transfer reagent) for hydroamination, toluene, 313 K. c) HPLC traces of APnol.