Switchable supramolecular helices for asymmetric stereodivergent catalysis

Despite recent developments on the design of dynamic catalysts, none of them have been exploited for the in-situ control of multiple stereogenic centers in a single molecular scaffold. We report herein that it is possible to obtain in majority any amongst the four possible stereoisomers of an amino alcohol by means of a switchable asymmetric catalyst built on supramolecular helices. Hydrogen-bonded assemblies between a benzene-1,3,5-tricarboxamide (BTA) achiral phosphine ligand coordinated to copper and a chiral BTA comonomer are engaged in a copper-hydride catalyzed hydrosilylation and hydroamination cascade process. The nature of the product stereoisomer is related to the handedness of the helices and can thus be directed in a predictable way by changing the nature of the major enantiomer of the BTA comonomer present in the assemblies. The strategy allows all stereoisomers to be obtained one-pot with similar selectivities by conducting the cascade reaction in a concomitant manner, i.e. without inverting the handedness of the helices, or sequentially, i.e. by switching the handedness of the supramolecular helices between the hydrosilylation and hydroamination steps. Supramolecular helical catalysts appear as a unique and versatile platform to control the configuration of molecules or polymers embedding several stereogenic centers.

Chiral Truxene-Based Self-Assembled Cages: Triple Interlocking and Supramolecular Chirogenesis

Incorporating chiral elements in host-guest systems currently attracts much attention because of the major impact such structures may have in a wide range of applications, from pharmaceuticals to materials science and beyond. Moreover, the development of multi-responsive and -functional systems is highly desirable since they offer numerous benefits. In this context, we describe herein the construction of a metal-driven self-assembled cage that associates a chiral truxene-based ligand and a bis-ruthenium complex. The maximum separation between both facing chiral units in the assembly is fixed by the intermetallic distance within the lateral bis-ruthenium complex (8.4 Å). The resulting chiral cavity was shown to encapsulate polyaromatic guest molecules, but also to afford a chiral triply interlocked [2]catenane structure. The formation of the latter occurs at high concentration, while its disassembly could be achieved by the addition of a planar achiral molecule. Interestingly the planar achiral molecule exhibits induced circular dichroism signature when trapped within the chiral cavity, thus demonstrating the ability of the cage to induce supramolecular chirogenesis.

Enhanced Host-Guest Association and Fluorescence in Copolymers from Copper Salphen Complexes by Supramolecular Internalization of Anions

We describe the synthesis, crystallographic characterization of a new Cu-Salphen compound and its use as a host Lewis-acid against guest anions in two versions: a) free molecule, b) copolymerized with methyl methacrylate:n-butyl acrylate (1 : 4-wt.) as protective co-monomers. Higher contents in Cu-Salphen yielded larger and more homogeneous polymer sizes. Polymer size together with glass transitions, heat capacity, thermal degradation, guest-saturation degrees and host-guest species distribution profiles from spectrophotometric titrations explained growths of up to 630-fold in K₁₁ and 180000-fold in K₁₂ for the host's binding site attributable to a solvophobic protection from the macromolecular structure. Spectrofluorimetry revealed blue-shifted×13-16 larger luminescence for Cu-Salphen in the polymers (λ_em =488-498 nm) than that of the non-polymerized counterpart (λ_em =510-543 nm) and "turn-on" blue-shifted enhanced fluorescence upon guest association. We propose a cooperative incorporation of the guests occurring from the outer medium toward internally protected binding site pockets in the random coil polymer conformations.

Induction and rationalization of supramolecular chirality in a highly flexible Zn(II)porphyrin dimer: structural, spectroscopic and theoretical investigations

A highly flexible pyrrole-bridged Zn(II)porphyrin dimer has been successfully utilized as an efficient host which enables an accurate determination of the absolute configuration directly for a large number of chiral amino alcohols and 1,2-diols. The addition of substrates resulted in the formation of 1 : 1 sandwich complexes which, after the addition of excess substrates, produced 1 : 2 host-guest complexes. In principle, the 1 : 2 host-guest complexes can be stabilized in three possible conformations, viz. exo-exo, exo-endo, and endo-endo based on how a substrate binds to the metal. The endo-endo conformation is stabilized by two strong interligand H-bonds [O-H⋯O(H)] between encapsulated diols which thereby interlock the stereochemistry. In the absence of such interligand H-bonding interactions, exo-endo binding is preferred as it is indeed observed for amino alcohols which show weaker CD couplets due to the free movement of substrates. The sandwich complexes with amino alcohols show a more intense CD couplet compared to the diols due to the stronger binding of the amine functionality (-NH₂) towards a Zn-atom over an alcoholic moiety (-OH). The CD amplitude showed linear dependence with a binding constant for the 1 : 1 sandwich complex upon varying the substrates. Spectroscopic investigations, single crystal X-ray structure determination of four such host-guest complexes and DFT studies have enabled us to rationalize systematically the origin of optical activity unambiguously in the 1 : 1 and 1 : 2 host-guest complexes, which lead to an absolute stereochemical determination of a large number of chiral substrates. The larger vertical and horizontal flexibility of a diethyl pyrrole spacer induces stronger binding of the substrates to form the 1 : 1 complex with a much larger torsional angle along with intense CD couplets. In contrast, a rigid dibenzothiophene-bridged tweezer, due to its limited horizontal and vertical flexibility, facilitates 1 : 2 complexation more as compared to the highly flexible pyrrole-bridged host which results in stronger binding of the substrate with the intense CD couplet for the former.

Hydrogen-Bonding Interactions Trigger Induction of Chirality via Formation of a Cyclic Dimer

A rationalization for the chirality transfer mechanism in the supramolecular host-guest assemblies of an achiral Zn(II) porphyrin dimer (host) and a series of chiral diamines and diamino esters (substrates) via cyclic dimer formation has been reported for the first time. Stepwise formations of 2:2 host-guest cyclic dimers and 1:2 host-guest monomeric complexes have been observed via intermolecular assembling and disassembling processes. A large bisignate CD couplet was observed for the cyclic dimer, whereas the monomeric complexes exhibited negligible CD intensity. Crystallographic characterizations demonstrate that the strong intermolecular H bonding in cyclic dimers is responsible for their stability over the linear chain, which thereby display high-intensity bisignate CD couplets. In order to minimize the steric crowding within the host-guest assembly, the cyclic dimer switches its helicity toward the conformer having less steric hindrance. The cyclic scaffold is oriented according to the pre-existing chirality of the substrate in both the solid and solution phases: the substrates having R chirality display a negative CD couplet, whereas the substrates with S chirality display a positive couplet. Opposite signs for the CD couplets between R and S substrates suggest that the stereographic projection at the chiral centers solely dictates the overall helicity of the cyclic dimer. DFT studies further support the experimental observations.

Induction, control, and rationalization of supramolecular chirogenesis using metalloporphyrin tweezers: a structure-function correlation

Supramolecular chirogenesis is one of the most rudimentary topics in the interdisciplinary sciences and essentially deals with various natural processes and innovative modern technologies. A comprehensive and rigorous understanding of such phenomenon is necessary to have a clear insight into the fundamental mechanisms and the various controlling factors, which would eventually lead to a range of practical applications of chiral supramolecular science. Metalloporphyrin tweezers have been extensively employed for such chirogenic processes due to their exciting physicochemical and tunable spectral properties, large stabilities, easily available synthetic protocols, and excellent abilities to form molecular assemblies. During the last few decades, various metalloporphyrin tweezers have been developed and considerably utilized by several research groups for assigning the absolute configuration to a variety of chiral diamines, conjugates of primary and secondary amines, amino alcohols, secondary alcohols, α-chiral carboxylic acids, etc. Our group has been at the forefront in trying to establish the structure-property correlation in this important area of interdisciplinary research. A brief account of our systematic investigation for understanding the underpinning mechanism of chirality induction and control at the molecular level over the last few years is presented in this Perspective article. The comprehensive understanding of such mechanistic details will be helpful in understanding various natural processes and designing modern technologies for various chirogenic functions in the fields of molecular sensors, nanotechnology, and supramolecular chemistry.

Supramolecular Chirogenesis Induced by Platinum(II) Tweezers with Excellent Environmental Tolerance

Supramolecular chirogenesis has emerged as an effective strategy to access symmetry breaking in artificial systems. However, the chirogenic signals suffer from high susceptibility toward environmental variations. An effective strategy has been developed to address this issue by constructing platinum(II)-based tweezer/guest complexes stabilized by two-fold donor-acceptor and Pt^II -Pt^II metal-metal interactions. Upon guest encapsulation, the two pincers on the achiral Pt^II tweezer undergo a stereospecific twist to minimize steric repulsion, thus locking tweezer/guest complexes into the preferred chiral conformations. The induced chiroptical effects display outstanding solvent and temperature tolerance, ascribed to the balance between electrostatic and desolvation effects for the involved non-covalent interactions. Moreover, hierarchical and multi-component supramolecular assembly of tweezer/guest complexes provide a convenient way to modulate chirogeneic signals for their intensities.

Acrylic Polymers Containing a Nickel Salphen Complex: An Approach to Supramolecular and Macromolecular Systems

The synthesis, characterization and crystallographic analysis is reported of a new Nickel Salphen complex and its radical copolymerization with n-butyl acrylate and methyl methacrylate to produce novel host macromolecules with tunable association against guest anions. Spectrophotometric titrations of the complex and of the polymers revealed that a supramolecular regulation of guest-binding accessibility was enabled by the number of Ni-Salphen units per chain. The latter content in turn, determined the chain size and molecular weight uniformity upon polymerization, and likely increased the strength in interchain/intrachain non-covalent interactions over the nickel center and the acrylic domains. The study also showed that incorporation of the monomer into the acrylic polymer backbone opened the possibility for the nickel binding site to gain access to host:guest stoichiometric discrimination, switching from 1 : 1 (major) and 1 : 2 (minor) both coexisting for the host when in the free form, to mostly 1 : 2 when in the polymerized version.

Synthesis of a Novel Zn-Salphen Building Block and Its Acrylic Terpolymer Counterparts as Tunable Supramolecular Recognition Systems

In this work, we present the synthesis of a novel Zn-Salphen complex containing an allyl group, which was used as building block in the further preparation of a new family of functional terpolymers. These polymers were obtained through radical co-polymerization with methyl metacrylate (MMA) and n-butyl acrylate (nBuA) in different ratios. The supramolecular recognition behavior of each polymer was evaluated via potentiometric measurements against selected anions in aqueous media. Interestingly, this proof of concept study shows that these systems were selective against only fluoride (F⁻) or both, fluoride and acetate (OAc⁻), by tailoring the relative content of Zn-Salphen monomer, thus making them a promising starting point for modular design of chemical sensors through straightforward synthetic approaches.

Ligand libraries for high throughput screening of homogeneous catalysts

This review describes different approaches to construct ligand libraries towards high throughput screening of homogeneous metal catalysts.

Room temperature chiral reorganization of interfacial assembly of achiral double-decker phthalocyanine

Chiral reorganization with amplification of the Cotton effect is achieved at room temperature and atmospheric pressure in the solid-state.

The stereoselectivity of bipyrrolidine-based sequential polydentate ligands around Ru(ii)

The Ru(ii) coordination chemistry of the sequential hexadentate, tetradentate and the novel hybrid pentadentate ligands assembled around the chiral bipyrrolidine core and including bipyridyl and pyridyl periphery units is described. The bipyridine group exhibited priority in binding over the bipyrrolidine group, which led to a diastereomer mixture in the case of the hexadentate ligand 1. Employing only monopyridyl or a combination of monopyridyl and bipyridyl peripheral groups restored the chiral induction ability to the bipyrrolidine core resulting in predetermined chiral-at-metal complexes for the ligands 3 and 4.

Highly Efficient Chirality Transfer from Diamines Encapsulated within a Self-Assembled Calixarene-Salen Host

A calix[4]arene host equipped with two bis-[Zn(salphen)] complexes self-assembles into a capsular complex in the presence of a chiral diamine guest with an unexpected 2:1 ratio between the host and the guest. Effective chirality transfer from the diamine to the calix-salen hybrid host is observed by circular dichroism (CD) spectroscopy, and a high stability constant K2,1 of 1.59×10(11)  M(-2) for the assembled host-guest ensemble has been determined with a substantial cooperativity factor α of 6.4. Density functional calculations are used to investigate the origin of the stability of the host-guest system and the experimental CD spectrum compared with those calculated for both possible diastereoisomers showing that the M,M isomer is the one that is preferentially formed. The current system holds promise for the chirality determination of diamines, as evidenced by the investigated substrate scope and the linear relationship between the ee of the diamine and the amplitude of the observed Cotton effects.

Dynamic induction of enantiomeric excess from a prochiral azobenzene dimer under circularly polarized light

The ability to photoinduce enantiomeric excess from the chirality of circularly polarized light (CPL) is pertinent to the study of the origin of homochirality in biomolecules. Such CPL-induced reactions, including both chirality generation and formation of partial enantiomeric imbalance, from nonchiral starting compounds have been known, however, only for the conversion of diarylolefins into chiral helicenes. In this study we synthesized three different prochiral molecules, each featuring a pair of photoisomerizable phenylazo moieties arranged symmetrically upon the phenyl rings of an sp3-hybridized carbon atom (1), the phenyl rings of [2.2]paracyclophane (2), and the ortho positions of a phenyl ring bearing a naphthyl unit (3), and then investigated the possibility of photoinducing enantiomeric excess under CPL. Irradiation of 1-3 with light induced E ↔ Z photoisomerizations of their azobenzene moieties, giving mixtures of their EE, EZ, and ZZ isomers in the photostationary state (PSS). Among these regioisomers, the EZ forms are chiral and existed as racemic mixtures of R and S stereoisomers. Upon CPL irradiation of 3, circular dichroism (CD) revealed enantiomeric enrichment of one of the EZ stereoisomers; furthermore, irradiation with r- or l-CPL gave CD signals opposite in sign, but with equal intensity, in the PSS. In contrast, 1 and 2 did not give any detectable induced CD upon CPL irradiation. These experimental results can be explained by considering the different Kuhn anisotropy factors (g) of the (R)-EZ and (S)-EZ stereoisomers of 1-3, assuming that the origin of the enantiomeric excess is the enantio-differentiating photoisomerization from EZ stereoisomers to nonchiral EE or ZZ regioisomers by r- or l-CPL. In short, we demonstrate the simultaneous induction of chirality and enantiomeric excess from a prochiral azobenzene dimer via a chiral regioisomer formed in situ upon CPL irradiation.

Supramolecular chirality in self-assembled soft materials: regulation of chiral nanostructures and chiral functions

Supramolecular chirality, which arises from the nonsymmetric spatial arrangement of components in the self-assembly systems, has gained great attention owing to its relation to the natural biological structures and the possible new functions in advanced materials. During the self-assembling process, both chiral and achiral components are possible to form chiral nanostructures. Therefore, it becomes an important issue how to fabricate these molecular components into chiral nanostructures. Furthermore, once the chiral nanostructure is obtained, will it show new functions that simple component molecule could not? In this research news, we report our recent development in the regulation of chiral nanostructures in soft gels or vesicle materials. We have further developed several new functions pertaining to the soft gel materials, which single chiral molecules could not perform, such as the chiroptical switch, chiral recognition and the asymmetry catalysis.

Supramolecular catalysis. Part 2: artificial enzyme mimics

The design of artificial catalysts able to compete with the catalytic proficiency of enzymes is an intense subject of research. Non-covalent interactions are thought to be involved in several properties of enzymatic catalysis, notably (i) the confinement of the substrates and the active site within a catalytic pocket, (ii) the creation of a hydrophobic pocket in water, (iii) self-replication properties and (iv) allosteric properties. The origins of the enhanced rates and high catalytic selectivities associated with these properties are still a matter of debate. Stabilisation of the transition state and favourable conformations of the active site and the product(s) are probably part of the answer. We present here artificial catalysts and biomacromolecule hybrid catalysts which constitute good models towards the development of truly competitive artificial enzymes.