Emergence of Homochiral Benzene-1,3,5-tricarboxamide Helical Assemblies and Catalysts upon Addition of an Achiral Monomer

Abstract

Chirality amplification refers to the ability of a small chiral bias to fully control the main chain helicity of polymers and assemblies. Further implementation of functional chirally-amplified helices as switchable asymmetric catalysts, chiral sensors and circularly-polarized light emitters will require a greater control of the energetics governing these chirality amplification effects. In this work, we report on the counterintuitive ability of an achiral molecule to suppress conformational defects in supramolecular helices thus leading to the emergence of homochirality in a system containing a very small chiral bias. We focus our investigation on supramolecular helices composed of an achiral benzene-1,3,5-tricarboxamide (BTA) ligand, coordinated to copper, and an enantiopure BTA co-monomer. Amplification of chirality as probed by varying the amount (sergeants-and-soldiers effect) or the optical purity (diluted majority-rules effect) of the enantiopure co-monomer, are modest in this initial system. However, both effects are hugely enhanced upon addition of a second achiral BTA monomer leading to a perfect control of the helicity either by means of a remarkably low amount of sergeants (0.5%) or a small bias from a racemic mixture of enantiopure co-monomers (10% e.e.). Such an enhancement in the amplification of chirality is only achieved by mixing the three components, i.e. the two achiral and the enantiopure co-monomers, highlighting a synergistic effect upon co-assembly of the three monomers. Investigation of the role of the achiral additive by multifarious analytical techniques supports its ability to stabilize the helical co-assemblies and suppress helix reversals i.e. conformational defects. Implementation of these helical copper precatalysts in the hydrosilylation of 1-(4-nitrophenyl)ethanone confirms that the effect of the achiral BTA additive is also operative under the conditions of the catalytic experiment. A highly enantioenriched product (90% e.e.) is produced by a supramolecular catalyst operating with ppm levels of chiral species. Scheme 1 Schematic representation of the concept. Supramolecular stacks are composed of a first soldier (green) and a low amount of sergeants (blue) (I) or a mixture of enantiopure monomers slightly biased from the racemic mixture (II). The stacks consist of a nearly equal amount of left and right-handed helical parts. Upon addition of a second soldier (orange), these helices become homochiral (III and IV). This soldier acts as a "rigidifier" by stabilizing the helical co-assemblies and removing helix reversals as indicated by the helix reversal penalty (HRP) values. This concept also applies for the construction of helical catalysts in which the first soldier bears a catalytic site. Scheme 2 Left: chemical structures of the BTA monomers used in this study. Right: representation of the helical co-assemblies, preferentially right-handed, formed by mixing l1-BTA, [Cu(OAc)2•H2O], (S)-BTA and a-BTA (S&S type mixture). For a tentative molecular representation of the concept sho...

Dissecting the Role of the Sergeants in Supramolecular Helical Catalysts: From Chain Capping to Intercalation

“…Those concerning BTA Leu , BTA Nle and BTA Phe as sergeants are examined into more details in the following as they are representative of the trend exhibited by the other sergeants (vide infra). For BTA Leu , the CD intensity increases gradually until the fraction of sergeant is ≈40 % and then reaches a plateau corresponding to a g value of 7.7×10 −4 (Figure 1 b) close to the one recorded for the best S&S‐type mixtures (g max ≈7.0×10 −4 ) [8e] . This value is considered as the maximal value that can be reached for this type of helical assemblies and will be referred to as the homochiral state in the following discussion.…”

“…This signal is absent from solutions containing only the soldier (because it is achiral) or containing only the sergeant (because it has no absorption band in this region). Such an induced CD signal (ICD) is indicative of the extent of chirality induction of the soldier by the sergeant [8e] . The corresponding CD intensities, under the form of the Kuhn anisotropy factor (g 295 , see the caption of Figure 1), offer the possibility to compare the degree of helicity of the different co‐assemblies.…”

“…Disk‐like molecules decorated with hydrogen bonding moieties, such as benzene‐1,3,5‐tricarboxamides (BTAs) and their derivatives, [9] are unique in regards to their ability to form remarkably stable supramolecular helices by a combination of hydrogen bonding and aromatic interactions. It is classically established in these helical systems that soldiers and sergeants co‐assemble through a cooperative mechanism [10] yielding very long one‐dimensional helices which are one‐handed at the condition that a few percent of sergeants are present in the co‐assemblies [8e, 11] . We previously investigated helical co‐assemblies embedding achiral BTA ligands (coordinated to Rh [8a] or Cu) [8b] and enantiopure BTAs as asymmetric catalysts.…”

Effect of Stereochemistry on Chirality and Gelation Properties of Supramolecular Self‐Assemblies

“…However, it is still difficult to predict self‐assembled supramolecular chirality based on a known peptide sequence that contains more than one chiral center [15–17] . Recently, some intriguing investigations into the chiral amplification phenomenon have been reported [36–38] . For example, Xu and co‐workers investigated the self‐assembly chirality of three pairs of short tetrapeptide enantiomers and found that the handedness of the supramolecular nanofibrils was determined by the chirality of the hydrophilic Lys head at the C terminus, whereas the morphological handedness could not be changed by altering the molecular chirality of hydrophobic Ile residues [16] …”

Defects and defect engineering in Soft Matter

“…[505][506][507][508][509] Conformational defects have further been investigated in the context of chirality amplification in helical 1D supramolecular polymers. 510 Amplification of chirality is generally referred to as the ability of a small chiral bias to fully control or dictate the helicity of polymers and assemblies, 511 and was recently used to prepare functional helical nanostructures as asymmetric catalysts, 512 or chiral stationary phases. 513 Giuseppone and coworkers have used supramolecular polymers in the gel-state to combine the electronic properties of organic materials exhibiting metallic behavior, with the mechanical properties of soft self-assembled systems.…”