A Redox-Reconfigurable, Ambidextrous Asymmetric Catalyst

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.

Synthesis and Characterization of Photoswitchable Dithienylethene-Based Chiral Bisoxazoline Compounds with Bidirectional Visible-Light Control

In this paper, we have successfully synthesized dithienylethene-based chiral bisoxazoline ligands with bidirectional photoswitching capabilities under visible light irradiation and proposed a strategy for adjusting the conjugation system length in sensitizer groups. The detailed experimental procedures and the characterization data are presented in the main text and the Supporting Information. Despite their moderate photoswitching rates, these ligands provide a promising approach towards developing fully visible light-responsive chiral catalysts.

Asymmetric Binary Acid Catalysis: Switchable Enantioselectivity in Enantioselective Conjugate Hydride Reduction

The exchange of the metal ion from Zr(IV) to Fe(III) leads to a switch in the enantioselectivity of binary acid-catalyzed conjugate hydride reductions. In the presence of Hantzsch ester, γ-indolyl β,γ-unsaturated α-keto esters could be reduced to the desired (S)- or (R)-products, respectively, with good to excellent enantioselectivity (up to 98% ee).

Valence-variable Catalysts for Redox-controlled Switchable Ring-opening Polymerization

As a representative class of sustainable polymer materials, biodegradable polymers have attracted increasing interest in recent years. Despite significant advance of related polymerization techniques, realizing high sequence-control and easy-handling in ring-opening (co)polymerizations still remains a central challenge. To this end, a promising solution is the development of valence-variable metal-based catalysts for redox-induced switchable polymerization of cyclic esters, cyclic ethers, epoxides, and CO₂ . Through a valence-determined electron effect, the switch between different catalytically active states as well as dormant state contributes to convenient formation of polymer products with desired microstructures and various practical performances. This redox-controlled switchable strategy for controlled synthesis of polymers is overviewed in this Review with a focus on potential applications and challenges for further studies.

Synthesis, Biophysical Interaction of DNA/BSA, Equilibrium and Stopped-Flow Kinetic Studies, and Biological Evaluation of bis(2-Picolyl)amine-Based Nickel(II) Complex

Reaction of bis(2-picolyl)amine (BPA) with Ni(II) salt yielded [(BPA)NiCl2(H2O)] (NiBPA). The Ni(II) in NiBPA bound to a BPA ligand, two chloride, and one aqua ligands. Because most medications inhibit biological processes by binding to a specific protein, the stopped-flow technique was used to investigate DNA/protein binding in-vitro, and a mechanism was proposed. NiBPA binds to DNA/protein more strongly than BPA via a static quenching mechanism. Using the stopped-flow technique, a mechanism was proposed. BSA interacts with BPA via a fast reversible step followed by a slow irreversible step, whereas NiBPA interacts via two reversible steps. DNA, on the other hand, binds to BPA and NiBPA via the same mechanism through two reversible steps. Although BSA interacts with NiBPA much faster, NiBPA has a much higher affinity for DNA (2077 M) than BSA (30.3 M). Compared to NiBPA, BPA was found to form a more stable BSA complex. When BPA and NiBPA bind to DNA, the Ni(II) center was found to influence the rate but not the mechanism, whereas, for BSA, the Ni(II) center was found to change both the mechanism and the rate. Additionally, NiBPA exhibited significant cytotoxicity and antibacterial activity, which is consistent with the binding constants but not the kinetic stability. This shows that in our situation, biological activity is significantly more influenced by binding constants than by kinetic stability. Due to its selectivity and cytotoxic activity, complex NiBPA is anticipated to be used in medicine.

Stimuli Responsive Asymmetric Catalysis by Triggered Pseudo-Enantiomeric Proligand Release

Complex stimuli responsive systems are synthetic analogues of natural cell environments, and the basis for molecular machines and computing. A dual psuedo-enantiomer system was concieved, where the combination of two...

: Helical Poly(quinoxaline-2,3-diyl)s Bearing 4-(Dipropylamino)pyridin-3-yl Pendants as Chirality-Switchable Nucleophilic Catalysts for the Kinetic Resolution of Secondary Alcohols

Helically chiral poly(quinoxaline-2,3-diyl)s bearing 4-(dipropylamino)pyridin-3-yl pendants at the 5-position of the quinoxaline ring (PQXdpap) exhibited high catalytic activities and moderate to high selectivities (up to s = 87) in the acylative kinetic resolution of secondary alcohols. The solvent-dependent helical chirality switching of PQXdpap between pure toluene and a 1:1 mixture of toluene and 1,1,2-trichloroethane enabled the preparation of either compound of a pair of enantiomerically pure alcohols (>99% ee) from a single catalyst.

Enantioselectivity switch in asymmetric Michael addition reactions using phosphonium salts

Efficient access to two enantiomers of one chiral compound is critical for the discovery of drugs. However, it is still a challenging problem owing to the difficulty in obtaining two enantiomers of one chiral catalyst. Here, we report a general method to obtain both enantiomeric products via fine tuning the hydrogen-bonding interactions of phosphonium salts. Amino acid derived phosphonium salts and dipeptide derived phosphonium salts exhibited different properties for controlling the transition state, which could efficiently promote the Michael addition reaction to give opposite configurations of products with high yields and enantioselectivities. Preliminary investigations on the mechanism of the reaction and applications of the products were also performed.

A Switchable Catalyst Duo for Acyl Transfer Proximity Catalysis and Regulation of Substrate Selectivity

Enzymes are encoded with a gamut of information to catalyze a highly selective transformation by selecting the proper reactants from an intricate mixture of constituents. Mimicking biological machinery, two switchable catalysts with differently sized cavities and allosteric control are conceived that allow complementary size-selective acyl transfer in an on/off manner by modulating the effective local concentration of the substrates. Selective activation of one of two catalysts in a mixture of reactants of similar reactivity enabled upregulation of the desired product.

Helical polymer self-assembly and chiral nanostructure formation

This review surveys recent progress towards robust chiral nanostructure fabrication techniques using synthetic helical polymers, the unique inferred properties that these materials possess, and their intricate connection to natural, biological chirality.

Copper catalysis with redox-active ligands

Copper catalysis finds applications in various synthetic fields by utilizing the ability of copper to sustain mono- and bielectronic elementary steps. Further to the development of well-defined copper complexes with classical ligands such as phosphines and N-heterocyclic carbenes, a new and fast-expanding area of research is exploring the possibility of a complementing metal-centered reactivity with electronic participation by the coordination sphere. To achieve this electronic flexibility, redox-active ligands can be used to engage in a fruitful “electronic dialogue” with the metal center, and provide additional venues for electron transfer. This review aims to present the latest results in the area of copper-based cooperative catalysis with redox-active ligands.

Enantiodivergence by minimal modification of an acyclic chiral secondary aminocatalyst

The development of enantiodivergent catalysis for the preparation of both enantiomers of a chiral compound is of importance in pharmaceutical and bioorganic chemistry. With the design of a class of reactive and stereoselective organocatalysts, acyclic chiral secondary amines, a method for achieving the enantiodivergence is developed simply by changing the secondary N-i-Bu- to N-Me-group within the catalyst architecture while maintaining the same absolute configuration of the catalysts, which modulates the catalyst conformation. This catalyst-controlled enantiodivergent method not only enables challenging asymmetric transformations to occur in an enantiodivergent manner but also features a high level of stereocontrol and broad scope that is demonstrated in eight different reactions (90 examples), all delivering both enantiomers of a range of structurally diverse products including hitherto less accessible, yet important, compounds in good yields with high stereoselectivities.

Enantiodivergent methods, which to access both enantiomers of the same compound, are of importance in drug synthesis. Here, the authors show that by simply changing a NiBu- to a NMe-group in readily available amine organocatalysts, high stereocontrol and broad scope are achieved in eight asymmetric reactions.

Dynamic Control of Chiral Space Through Local Symmetry Breaking in a Rotaxane Organocatalyst

We report on a switchable rotaxane molecular shuttle that features a pseudo-meso 2,5-disubstituted pyrrolidine catalytic unit on the axle whose local symmetry is broken according to the position of a threaded benzylic amide macrocycle. The macrocycle can be selectively switched (with light in one direction; with catalytic acid in the other) with high fidelity between binding sites located to either side of the pyrrolidine unit. The position of the macrocycle dictates the facial bias of the rotaxane-catalyzed conjugate addition of aldehydes to vinyl sulfones. The pseudo-meso non-interlocked thread does not afford significant selectivity as a catalyst (2-14 % ee), whereas the rotaxane affords selectivities of up to 40 % ee with switching of the position of the macrocycle changing the handedness of the product formed (up to 60 % Δee).

Supramolecular chirality transformation driven by monodentate ligand binding to a coordinatively unsaturated self-assembly based on C 3-symmetric ligands

Monodentate ligand binding is facilitated by supramolecular chirality transformations from propeller-shaped chirality into single-twist chirality by altering the self-assembly of C 3-symmetric chiral ligands. The C 3-symmetric chiral ligands (Im R 3Bz and Im S 3Bz) contain three chiral imidazole side arms (Im R  and Im S ) at the 1,3,5-positions of a central benzene ring. Upon coordination to zinc ions (Zn2+), which have a tetrahedral coordination preference, the C 3-symmetric chiral ligands assemble, in a stepwise manner, into a propeller-shaped assembly with a general formula (Im( R or S ) 3Bz)4(Zn2+)3. In this structure each Zn2+ ion coordinates to the three individual imidazole side arms. The resulting assembly is formally coordinatively unsaturated (coordination number, n = 3) and capable of accepting monodentate co-ligands (imidazole: ImH2) to afford a coordinatively saturated assembly [(ImH2)3(Im R 3Bz)4(Zn2+)3]. The preformed propeller-shaped chirality is preserved during this transformation. However, an excess of the monodentate co-ligand (ImH2/Zn2+ molar ratio of ∼1.7) alters the propeller-shaped assembly into a stacked dimer assembly [(ImH2) m (Im R 3Bz)2(Zn2+)3] (m = 4-6) with single-twist chirality. This switch alters the degree of enhancement and the circular dichroism (CD) pattern, suggesting a structural transition into a chiral object with a different shape. This architectural chirality transformation presents a new approach to forming dynamic coordination-assemblies, which have transformable geometric chiral structures.

Modulation of catalyst enantioselectivity through reversible assembly of supramolecular helices

The enantioselectivity displayed by a supramolecular copper catalyst involved in successive reactions can be modulated through reversible assembly of the helices supporting the catalytic centres.

Modulation of the coordination geometries of NCN and NCNC Rh complexes for ambidextrous chiral catalysts

A chirality switch between novel NCN pincer Rh complexes and a related double cyclometalated NCNC Rh complex containing secondary amino groups is described.

Stereodivergent Catalysis

This review covers diastereo- and enantiodivergent catalyzed reactions in acyclic and cyclic systems using metal complexes or organocatalysts. Among them, nucleophilic addition to carbon-carbon and carbon-nitrogen double bonds, α-functionalization of carbonyl compounds, allylic substitutions, and ring opening of oxiranes and aziridines are considered. The diastereodivergent synthesis of alkenes from alkynes is also included. Finally, stereodivergent intramolecular and intermolecular cycloadditions and other cyclizations are also reported.

Stereodivergent synthesis with a programmable molecular machine

It has been convincingly argued that molecular machines that manipulate individual atoms, or highly reactive clusters of atoms, with Ångström precision are unlikely to be realized. However, biological molecular machines routinely position rather less reactive substrates in order to direct chemical reaction sequences, from sequence-specific synthesis by the ribosome to polyketide synthases, where tethered molecules are passed from active site to active site in multi-enzyme complexes. Artificial molecular machines have been developed for tasks that include sequence-specific oligomer synthesis and the switching of product chirality, a photo-responsive host molecule has been described that is able to mechanically twist a bound molecular guest, and molecular fragments have been selectively transported in either direction between sites on a molecular platform through a ratchet mechanism. Here we detail an artificial molecular machine that moves a substrate between different activating sites to achieve different product outcomes from chemical synthesis. This molecular robot can be programmed to stereoselectively produce, in a sequential one-pot operation, an excess of any one of four possible diastereoisomers from the addition of a thiol and an alkene to an α,β-unsaturated aldehyde in a tandem reaction process. The stereodivergent synthesis includes diastereoisomers that cannot be selectively synthesized through conventional iminium-enamine organocatalysis. We anticipate that future generations of programmable molecular machines may have significant roles in chemical synthesis and molecular manufacturing.

A high-speed network of nanoswitches for on/off control of catalysis

Copper(i) ion translocation is the key for fast and reliable communication between networked devices in the catalytic machinery.

Bimetallic gold(i) complexes of photoswitchable phosphines: synthesis and uses in cooperative catalysis

The first photoswitchable bimetallic gold catalysts based on an azobenzene backbone have been synthesized and their catalytic properties have been investigated.

A complementary pair of enantioselective switchable organocatalysts

A pair of enantioselective switchable bifunctional catalysts are shown to promote a range of conjugate addition reactions in up to 95 : 5 e.r. and 95% conversion. Each catalyst can be switched OFF using conditions that switch the other catalyst ON. Catalyst ON : OFF ratios of up to 98 : 2 and 1 : 99 were achieved, with a ratio of reaction rates of up to 16 : 1 between the ON and OFF states, maintained over complete ON-OFF-ON and OFF-ON-OFF cycles. However, simultaneous operation of the catalyst pair in the same reaction vessel, which in principle could allow product handedness to be switched by simple E-Z isomerisation of the catalyst pair, was unsuccessful. In this first generation complementary pair of enantioselective switchable organocatalysts, the OFF state of one catalyst inhibits the ON state of the other.

Redox-Responsive Molecular Systems and Materials

Redox reactions can alter the electronic, optical, and magnetic properties of molecules and their ensembles by adding or removing electrons. Here, the developments made over the past 10 years using molecular events are discussed, such as assembly/disassembly, transformation of ensembles, geometric changes, and molecular motions that are designed to be redox-responsive. Considerable progress has occurred in the application of these events to the realization of electronic memory, color displays, actuators, adhesives, and drug delivery. In these cases, systems behave in either a highly or a poorly correlated manner depending on the number of redox-active units involved, based on the method of integration. One of the great advantages of redox-responsive devices and materials is that they have the potential to be readily integrated into existing electronic technologies.

Dynamic control over catalytic function using responsive bisthiourea catalysts

Responsive molecular motor-based bisthiourea organocatalysts were used in the enantioselective Henry reaction to achieve efficient dual stereocontrol in the presence of an external base.

Reagent-controlled enantioselectivity switch for the asymmetric fluorination of β-ketocarbonyls by chiral primary amine catalysis

A swap of fluorination reagents led to a switch of enantioselectivity in a chiral primary amine catalyzed asymmetric α-fluorination of β-ketocarbonyls.

A reagent-controlled enantioselectivity switch was uncovered in the asymmetric α-fluorination of β-ketocarbonyls by a chiral primary amine catalyst. By a simple swap of fluorination reagents, both enantiomers of the quaternary fluorination adducts could be obtained with good yields and high enantioselectivity. Mechanistic studies disclosed dual H-bonding and electrostatic stereocontrolling modes for the catalysis.

Enantiodivergent Steglich rearrangement of O-carboxylazlactones catalyzed by a chirality switchable helicene containing a 4-aminopyridine unit

Chirality-switchable, 4-aminopyridine-based, pseudo-enantiomeric helicenes can catalyze enantiodivergent Steglich rearrangement in up to 91% ee (R) and 94% ee (S), respectively.

A pseudo-enantiomeric pair of optically switchable helicenes containing a catalytic 4-N-methylaminopyridine (MAP) bottom unit and a C₂-symmetric, (10R,11R)-dimethoxymethyl-dibenzosuberane top template was synthesized. They underwent complementary photoswitching at 290 nm (P/M′, <1/>99) and 340 nm (P/M′, 91/9) and unidirectional thermo-rotation at 130 °C (P/M′, >99/<1). They were utilized to catalyze enantiodivergent Steglich rearrangement of O- to C-carboxylazlactones, with formation of either enantiomer with up to 91% ee (R) and 94% ee (S), respectively.

Asymmetric Autocatalysis as a Relay for Remote Amplification of Chirality of Target Molecules Used as Triggers

Nearly racemic target molecules are enantiomerically enriched through an asymmetric autocatalytic relay for a remote amplification of chirality. Target alkynols with very low initial ee act as chiral triggers for asymmetric amplification of the Soai autocatalyst, which in turn enables the formation of the same alkynols with greater enantiomeric purity. Additionally, the stereochemical correlation between the trigger/target and autocatalyst molecules is discussed.

Refoldable Foldamers: Global Conformational Switching by Deletion or Insertion of a Single Hydrogen Bond

Small changes in the structure of a foldamer may lead to gross changes in conformational preference. We show that the simple insertion or deletion of a single hydrogen bond by changes in pH or by photochemical deprotection is sufficient to refold a helical oligomer, interconverting M and P screw-sense preference. As a consequence of the switch, information may be transmitted to a remote catalytic site, selectively directing the formation of either of two enantiomeric products by a reaction involving 1,22-remote intermolecular asymmetric induction.

Catalysts Encapsulated in Molecular Machines

Smart catalysts offer the control of chemical processes and sequences of transformations, and catalysts with unique catalytic behavior can afford chiral products or promote successive polymerization. To meet advanced demands, the key to constructing smart catalysts is to incorporate traditional catalytic functional groups with trigger-induced factors. Molecular machines with dynamic properties and particular topological structures have typical stimulus-responsive features. In recent years, scientists have made efforts to utilize molecular machines (molecular switches, rotaxanes, motors, etc.) as scaffolds to develop smart catalysts. This Minireview focuses on the achievements of developing catalysts encapsulated in molecular machines and their remarkable specialties. This strategy is believed to provide more potential applications in switchable reactions, asymmetric synthesis, and processive catalysis.

Reversible modulation of the activity of thiourea catalysts with anions: a simple approach to switchable asymmetric catalysis

A simple and straightforward approach to switchable asymmetric catalysis is presented, based on the interactions of thiourea catalysts with anions.