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

Abstract

Controlling the properties of supramolecular assemblies requires unveiling the specific interactions between their components. In the present work, the catalytic properties and structure of co‐assemblies composed of a benzene‐1,3,5‐tricarboxamide (BTA) ligand coordinated to copper (the soldier) and seven enantiopure BTAs (the sergeants) have been determined. Whatever the sergeant, the enantioselectivity of the reaction is directly proportional to the optical purity of the supramolecular helices. More strikingly, the role played by the sergeant in the co‐assembly process differs significantly: from almost pure intercalator (when it is incorporated in the stacks of the soldier and generates long homochiral helices) to pure chain capper (when it leads to the formation of partly helically biased and short assemblies). The former situation leads to optimal enantioselectivity for the catalytic system under study (58 % ee) while the latter situation leads to very low selectivity (8 % ee). The successful rationalization of this high and unexpected difference is crucial for the development of more efficient catalysts and more elaborate supramolecular systems.

Rationalizing the Extent of the “Sergeants‐and‐Soldiers” Effect in Supramolecular Helical Catalysts: Effect of Copper Coordination

“…CD and UV‐Vis analyses of copper‐free S&S‐type mixtures are shown in Figure 1a together with those previously recorded for copper‐containing mixtures [47] . All mixtures contain the same quantity of BTA P (5.8 mM), a concentration that ensures the formation of long hydrogen‐bonded helical stacks [60] . The fraction of BTA ( S )‐Cha introduced in the sample, denoted as fs 0 , was varied between 0.1 % and 52 %.…”

“…The radius of both cylinders and spheres was fixed (11±1 Å) because this value is consistent with the radius obtained by fitting the SANS curves in C 7 D 8 of the individual components of the mixtures [59] . The scattering length densities (SLD) for the spheres were calculated from the atomic bound coherent scattering lengths (value of 0.506×10 −6 Å −2 for BTA ( S )‐Cha ) [60] . The SLD for the cylinders were calculated by averaging the molecular SLD according to the composition of the cylinders.…”

“…The composition of the BTA helices can also be deduced from FTIR analyses since helical co‐assemblies and BTA Cha dimers have drastically different signatures (Figure S7) [58–61] . It also allows to get the fs s values for solutions that contain aggregated helices and for which SANS analyses cannot be fitted precisely.…”

“…Our group investigated S&S‐type helical coassemblies composed of an achiral BTA phosphine ligand (the “soldier”) and an enantiopure BTA derived from amino ester (the “sergeant”) as scaffold for asymmetric rhodium‐ [56,57] and copper‐catalyzed reactions [58–61] . When a sufficient amount of “sergeant” is incorporated into the assemblies, homochiral helices are generated that yield the optimal selectivity for the catalytic system under consideration.…”

“…When a sufficient amount of “sergeant” is incorporated into the assemblies, homochiral helices are generated that yield the optimal selectivity for the catalytic system under consideration. We actually found that the enantioselectivity of the copper‐catalyzed hydrosilylation of 4‐nitroacetophenone (NPnone) is linearly related to the optical purity of the helices coordinated to [Cu(OAc) 2 ⋅H 2 O], as probed by circular dichroism (CD) spectroscopy [60] . This means that for a given S&S combination, the selectivity of the reaction can in principle be predicted by a single CD measurement.…”